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I would like to raise an exciting issue that has become very hot recently and has been of great interest to me for a long time: the possibility of synthesizing a living cell from its components. My story starts in 1964. I spent then my first sabbatical with Prof. Harold Morowitz at Yale University. Let me first remind you that these were the early sixties, times of revolutionary ideas. It was the time of planning the trip to the moon. We had entertained the idea, promoted by Morowitz, to synthesize a living cell from its components and turn this project into an American National Goal instead of going to the moon. We started by reconstituting the mycoplasma membrane from its solubilized components. Despite early promising results and a rather provocative paper (1.usa.gov/1MhD04c) we published in the Proceedings of the National Academy of Sciences on mycoplasma membrane reconstitution, we failed to progress in this direction. Clearly, we were much ahead of the times.

This brief historical development of the biology of the mycoplasmas begins with their discovery in 1898 to the present. Mycoplasmas are the smallest free-living microorganisms and for years were thought to be viruses because they passed through the usual bacterial filters. They lack a cell wall, are widespread in nature and many are animal, plant and human pathogens. The extensive use of cell cultures in the last fifty years and their frequent contamination with mycoplasmas, together with their possession of the smallest genome of any free-living organism, has drawn enormous attention to these organisms and has revealed considerably more about their biology.

The significant genome compaction in mycoplasmas was made possible by adoption of a parasitic lifestyle. During their evolution and adaptation to a parasitic mode of life the mycoplasmas have developed various genetic systems enabling their attachment to host tissues as well as a highly plastic set of variable surface proteins. The generation of a versatile surface coat through high-frequency phase and size variation provides the organism with a useful tool for immune system avoidance, allowing the mycoplasmas to escape antibody attack, explaining why these minute organisms are such successful parasites.

Laboratory diagnosis of mycoplasma infections is hampered by the difficulty or total failure to cultivate the organisms in vitro, and by the frequently weak and poorly specific serological response of the host. DNA probes consisting of cloned ribosomal RNA genes, cDNA to mycoplasmal rRNA, synthetic 16S rRNA oligonucleotide sequences, or cloned mycoplasmal protein genes, have been developed and applied as diagnostic tools in a variety of human and animal mycoplasma infections. These included primary atypical pneumonia caused by Mycoplasma pneumoniae, urogenital infections associated with M. genitalium and Ureaplasma urealyticum, and infections with M. fermentans, M. penetrans or M. pirum--mycoplasmas recently incriminated as cofactors in AIDS. DNA probes were also designed to aid in diagnosis of mycoplasma diseases of farm and laboratory animals, and the hard-to-diagnose mycoplasma infections of cell cultures. Sensitivity of mycoplasma detection by the different probes ranged between 10(3) and 10(6) colony-forming units, a level which may not be sufficiently high for use in a clinical laboratory. The introduction of PCR has pushed aside the previously developed DNA probes, by providing faster and much more sensitive tests. The sensitive level of a PCR test can be as low as a single organism, enabling detection of mycoplasmas in patients treated with antibiotics and in asymptomatic patients. PCR becomes positive prior to serological response and is also effective in immunocompromised hosts. PCR was shown to be most valuable in detection and identification of the non-culturable plant and insect mycoplasma-like organisms (MLOs). Nevertheless, false-negative PCR results are rather common due to inhibitors of the PCR reaction in the clinical specimen, while false-positive results may occur due to contamination of the reagents with target DNA. In conclusion, the PCR procedure is still too complex to be carried out in a routine diagnostic laboratory. PCR prepackaged quality-controlled diagnostic kits are now in the process of rapid development. Once these kits become available, and at a reasonable cost, PCR will certainly take its place as a major diagnostic tool in the routine diagnosis of mycoplasma infections.

Many pathogenic mycoplasmas are surface parasites, adhering to the epithelial linings of the respiratory and urogenital tracts. Since mycoplasmas lack cell walls their plasma membrane comes in close contact with that of their host, allowing exchange of components between the two membranes and possibly fusion. The tight association of the parasite with its host is illustrated in scanning electron micrographs of Mycoplasma pneumoniae and M. gallisepticum adhering to human red blood cells. Specialized structure at the tips of the mycoplasma cells appear to function as attachment organelles. Our main aim has been to chemically define the receptors on the host cell and the binding sites on the mycoplasma cells responsible for adhesion. Glycophorin (the major sialoglycoprotein of human red blood cells) serves as the main or sole receptor for M. gallisepticum whereas M. pneumoniae binds to additional receptors on human red blood cells. Trypsin treatment of M. pneumoniae cells abolishes their ability to attach to human red cells, suggesting the protein nature of the binding sites. M. pneumoniae membranes solubilized by detergents were subjected to affinity chromatography on glycophorin-Sepharose so that membrane components with high affinity for glycophorin could be isolated. The fraction isolated consisted of several proteins (relative molecular mass 25 000 and 45 000). The binding of this fraction to red cells was relatively low but appeared to be specific, as it was inhibited by glycophorin but not by its hydrophobic moiety. The possibility is discussed that the exposure of the binding sites on the mycoplasma cell surface is influenced by the electrochemical ion gradient across the membrane.

Summary The Mycoplasma pneumoniae tuf gene, encoding the elongation factor protein Tu, was cloned and sequenced. The nucleotide sequence of the mycoplasmal gene showed about 60% homology to the sequences of tuf genes of other prokaryotes, yeast mitochondria and Euglena gracilis chloroplasts, and about 75% similarity was found when comparing the deduced amino acid sequences of the various Tu proteins. The relatively low G+C content (40%) of the M. pneumoniae DNA was reflected in a low G+C content (44.6%) of the tuf gene, and in a preferential use of adenine and uracil at the third position of codons, yet codon usage analysis revealed the presence of almost all of the codons of the genetic code in the mycoplasmal gene. Southern blot hybridization of digested DNAs of 11 Mollicutes species with the entire M. pneumoniae tuf gene and with its 5′ part suggested the presence of one copy only of this gene in the representative species of the Mollicutes. In this respect, the Mollicutes resemble Gram-positive bacteria and differ from the Gram-negative bacteria, which carry two copies of the tuf gene.

Mycoplasma identification and laboratory diagnosis of mycoplasmal infections has been based on the classical bacteriological tests, including morphology, cultural characteristics, physiological and serological properties (47). While these tests still occupy a major part in mycoplasmal diagnostics, new tests based on the molecular analysis of genomic DNA, ribosomal RNAs, cell proteins, and lipids appear to push aside the classical tests, and one may expect that the molecular tests will shortly become the prevailing tests in mycoplasma identification, particularly with regards to the slow-growing and/or extremely fastidious species. Systematic and detailed description and evaluation of the classical procedures used in mycoplasma identification can be found in the two volumes of Methods in Mycoplasmology (49; 64), while the newer molecular methods are included in the two more recent volumes of Molecular and Diagnostic Procedures in Mycoplasmology (50; 65). A succinct practical review of current methods applied in routine laboratory diagnosis of mycoplasmal infections, with emphasis on infections in humans has recently been published (69).

SUMMARY The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors’ chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

Publisher Summary The more recent development of the polymerase chain reaction (PCR) enables the amplification of target DNA in the specimen, increasing the sensitivity of the tests undertaken for diagnosis of infections by several orders of magnitude. Positive hybridization data, and in the case of PCR, demonstration in gels of the amplified target sequence, may suffice to indicate the presence of the infectious agent. The first DNA probes applied to diagnosis of mycoplasma infections consisted of cloned ribosomal RNA genes, such as the recombinant plasmid pMC5. Another class of DNA probes consisting of chromosomal segments specific for a certain mollicute species is discussed. These segments are derived from a genomic library of the specific mycoplasm. PCR tests are several orders of magnitude more sensitive than those based on direct hybridization with a DNA probe. The first major issue in the development of a PCR-based test concerns the selection of the appropriate target sequences for amplification. The sequence to be amplified can be chosen from a published mycoplasmal gene sequence or from a randomly cloned DNA fragment demonstrated to be specific for the mycoplasma to be detected. The methodology of target sequence amplification and optimization of the PCR test conditions as well as the identification of the PCR products are presented.

Publisher Summary This chapter outlines a polymerase chain reaction (PCR) technique for detection of mycoplasma infection. Universal primers that amplify a conserved region of the 16S ribosomal RNA gene found in all prokaryotes. The use of these primers in a PCR assay allows the detection of any bacterial infection in cell cultures. The method described in this chapter allows for 10-15 samples to be prepared and assayed in 48 hours with consistent results.

This chapter discusses the use of cloned genomic DNA fragments as probes for the detection and identification of mollicutes. The probes are prepared by shotgun cloning of genomic DNA segments and selection of those segments that recognize relatively stable chromosomal regions discriminatory for the specific mollicute species. For fast and highly efficient construction of a genomic library to be used for the selection of DNA probes, the bacteriophage λGEM-12 with Xho Ihalf-site arms is used. The purified phage DNA are used directly as a DNA probe or restriction segments of the phage DNA insert are subcloned into a plasmid vector. Plasmid vectors have been routinely employed in the selection of DNA probes. The availability of genetically engineered commercial bacteriophage vectors provides an alternative way for selecting DNA probes. The procedure is very rapid, extremely efficient, and requires only small amounts of genomic DNA. The main advantage of this cloning strategy is that it makes genomic DNA fractionation and phosphorylation unnecessary. Within two rounds of plaque purification, a phage that does not contain highly conserved genes or other common sequences of a related mollicute species can be easily obtained.

This chapter provides an overview of molecular properties of mollicutes. The lack of cell walls is used to separate mycoplasmas from other bacteria in a class named mollicutes. The trivial terms mycoplasmas or mollicutes have been used interchangeably to denote any species included in mollicutes, the trivial names ureaplasmas, entomoplasmas, mesoplasmas, spiroplasmas, acholeplasmas, asteroleplasmas, and anaeroplasmas are used for members of the corresponding genus. The molecular characterization of the uncultured plant and insect mycoplasma-like organisms has provided strong experimental support for their inclusion in the class mollicutes. The mollicutes vary in shape from spherical or pear-shaped structures to branched or helical filaments. Mollicutes are widespread in nature as parasites of humans, mammals, reptiles, fish, arthropods, and plants. Based on their ability to metabolize carbohydrates, the mollicutes are divided into fermentative and non-fermentative organisms. Members of the fermentative group produce acids from carbohydrates, decreasing the pH of the growth medium. Most of the non-fermentative mollicutes, and some fermentative species, possess the arginine dihydrolase pathway.

The mycoplasmas, lacking a cell wall and intracytoplasmic membranes, have only one type of membrane, the plasma membrane. The ease with which this membrane can be isolated and the fact that controlled alterations in its composition are intraducibie have made mycoplasma membranes most effective and popular tools in biomembrane research. The aim of this chapter is to introduce the reader to the mycoplasmas and provide a historical background of the various phases in mycoplasma membrane research. Focus will be on the advantages of mycoplasma membranes as models in studying structure and function of biological membranes, and on the development of concepts that have arisen since the beginning of mycoplasma membrane research, about 30 years ago.

Using Southern blotting analysis with labelled mycoplasmal ribosomal RNA as probe, two fragments (1 Kb and 5 Kb) were detected in an EcoR I digest of Mycoplasma capricolum DNA. This analysis revealed that the 5 Kb fragment carries both 16S rRNA sequences and the entire 23S rRNA gene of this mycoplasma. The 1 Kb fragment contains 16S rRNA sequences only. The 5 Kb EcoR I fragment has been cloned and used to characterize the structure of rRNA cistrons in various Mycoplasma strains. These experiments clearly demonstrate a substantial homology of Mycoplasma capricolum rRNA sequences with the E. coli rRNA cistron on one hand, and with Mycoplasma mycoides subsp. capri and Acholeplasma laidlawii on the other hand. This analysis also reveals two rRNA cistrons in Mycoplasma mycoides subsp. capri and Acholeplasma laidlawii whereas one rRNA cistron is present in Mycoplasma capricolum.

The 5' region of the rRNA operon, rrnA, of M. capricolum was cloned. Sequence analysis revealed two tRNA genes, tRNA(leu) and tRNA(lys), upstream to the promoter of the rRNA operon. The in vivo transcription start sites of the rRNA operon and of the tRNA genes were mapped. The same promoters used by M. capricolum RNA polymerase are also recognized by E. coli RNA polymerase both in vivo and in vitro. We find that high levels of ppGpp in E. coli, resulting from amino acid starvation or from spoT mutation, activate rather than repress the transcription of the mycoplasma rrnA operon.

DNA probes specific for Mycoplasma gallisepticum and M. synoviae were selected from genomic libraries prepared in the pUC13 vector. The probes hybridized with the DNA of a wide spectrum of strains within each homologous species, but did not react with the heterologous species or with DNA from any other avian mycoplasma or bacteria tested. Experimental infection and contact exposure of chickens to M. gallisepticum served as models to test the effectiveness of the DNA probe in diagnosis as compared with serological and culture detection methods carried out in parallel. A correlation was generally found between the level of M. gallisepticum in tracheal swabs and the effectiveness of the probe, although a predictably reactive level of mycoplasmas was not always detected. Treatment of clinical specimens with acetylcysteine to disrupt mucus improved the detection rate. Dot-blot hybridization with probe pMG4 enabled positive identification of M. gallisepticum at an early stage of infection, prior to the development of a serological response in the infected chicken. Results are obtainable within 4 days of sampling, much more rapidly than culture, and also in clinical specimens from which mycoplasma isolation is impossible, such as carcasses. The results indicate that the use of DNA probes for the early and rapid detection of M. gallisepticum infection is feasible; a development which can replace laborious culture techniques and less effective serological methods, and thus reduce the time required for diagnosis.

The small size of the mollicute genome considerably restricts the amount of genetic information available to the organisms. This is reflected in the relatively small number of cell proteins synthesized, the lack of many biosynthetic pathways and the marked dependence on exogenous nutrients for growth. The protein synthesizing machinery of mollicutes resembles that of eubacteria and is sensitive to the same antibiotics, except for rifampicin, to which RNA polymerases of mollicutes appear resistant. The mollicute ribosomes are built of 50 S and 30 S subunits and contain about 50 different proteins and 5 S, 16 S and 23 S rRNA, as in eubacteria. However, the 5 S rRNA in mollicutes appears shorter (107-112 nucleotides) than in eubacteria (116-120 nucleotides). We hybridized restriction endonuclease-digested DNA from a variety of Mycoplasma, Ureaplasma, Acholeplasma and Spiroplasma species with nick-translated probes consisting of defined portions of the rrnB rRNA operon of Escherichia coli and the rRNA operon of M. capricolum. The results suggest the presence of only one or two sets (operons) of rRNA genes in the genome of Mollicutes, a number falling considerably below that of the eubacteria examined so far but resembling that found in archaebacteria. Our data also indicate a marked nucleotide sequence homology along the rrnB rRNA operon of E. coli and the rRNA operons of the various mollicutes, indicating that the rRNA genes in mollicutes are linked in the classical prokaryotic fashion 16 S-23 S-5 S. Each mollicute appeared to possess, on its genome, different flanking sequences adjacent to the rRNA operon(s), resulting in species-specific hybridization patterns.(ABSTRACT TRUNCATED AT 250 WORDS)

Species of the Acholeplasmataceae differ from species of the Mycoplasmataceae and Spiroplasmataceae in many respects, including lack of a nutritional requirement for sterol, ability of most species to synthesize saturated fatty acids and polyterpenes from acetate, and several other properties related to lipid metabolism and to the incorporation and location of lipids in the cell membrane. Acholeplasma species have also been found to differ from Mycoplasma species in possessing a nicotinamide adenine dinucleotide-dependent lactate dehydrogenase that is specifically activated by fructose 1,6-diphosphate and in containing superoxide dismutase, as well as glucose-6-phosphate and 6-phosphogluconate dehydrogenases. In addition, reduced nicotinamide adenine dinucleotide oxidase activity is located in the cell membrane of Acholeplasma species and is associated with the soluble cytoplasmic fraction of Mycoplasma and Spiroplasma species. Finally, significant differences exist between the nucleic acids of the Acholeplasmataceae and the Mycoplasmataceae. The genome molecular weight for Acholeplasma species is about 1.0 × 109, compared with about 5.0 × 108 for species of the Mycoplasmataceae. Moreover, a recent comparison of ribosomal ribonucleic acid oligonucleotide catalogs has demonstrated that Acholeplasma species are more closely related phylogenetically to two clostridial species than to the Mycoplasma and Spiroplasma species tested. Because the characteristics of species of the Acholeplasmataceae differ in major respects from those of other families of the Mollicutes, we propose elevation of the family Acholeplasmataceae to the rank of a new order, Acholeplasmatales. We provide a description of the proposed taxon, the second order of the class Mollicutes.

The influence of the physical state of the membrane lipid matrix on the vertical disposition of membrane proteins was studied with Acholeplasma laidlawii. Changes in membrane fluidity were brought about by altering the fatty acid composition of membrane lipids, by changing the growth temperature, by aging of cultures and by inducing changes in the membrane lipid-to-protein ratio through treatment with chloramphenicol. The lactoperoxidase-mediated iodination technique was used to label membrane proteins exposed to the aqueous surroundings. The degree of exposure of the iodine-binding sites of membrane proteins on the external surface of intact cells was found to undergo significant changes on varying growth conditions, but the changes could not be consistently correlated with changes in membrane fluidity, nor were they discernible on iodination of isolated membranes.

Mycoplasmas, unique among the prokaryotes in requiring cholesterol for growth, are most useful tools for studying the transfer of cholesterol from serum lipoproteins to the cell membrane, the localization of cholesterol in the membrane and its role as a regulator of membrane fluidity.

The DNAs of four Mycoplasma and one Acholeplasma species were found to contain methylated bases. All of the five species contained 6-methyladenine (m6Ade), the methylated base characteristic of prokaryotic DNA. The extent of methylation of adenine residues in the mycoplasmal DNA ranged from 0.2% in Mycoplasma capricolum to about 2% in Mycoplasma arginini and Mycoplasma hyorhinis with intermediate methylation values for Mycoplasma orale and Acholeplasma laidlawii DNAs. About 5.8% of the cytosine residues in M. hyorhinis DNA were methylated also. Analysis of cell culture DNA for the presence of m6Ade as a means for detection of contamination by mycoplasmas, and the phylogenetic implications of the finding of methylated bases in mycoplasmal DNAs are discussed.

Polymorphism in mycoplasma strains was observed by Southern blot hybridization of the digested mycoplasma DNAs with the elongation factor (EF-Tu) gene tuf of Escherichia coli. The hybridization patterns revealed genotypic heterogeneity among Mycoplasma gallisepticum strains and a remarkable degree of homogeneity among Mucoplasma pneumoniae strains isolated from pneumonia patients. The distinction among M. gallisepticum strain clusters achieved by the tuf gene probe corresponded exactly with that obtained with the rRNA gene probe pMC5. The tuf gene probe may thus be added as another effective tool in the taxonomy of Mollicutes and in epidemiological surveys of mycoplasma infections.

Attachment of M. pneumoniae to glass was quantitated in an experimental system enabling the settling down of [3H]palmitic acid-labeled cells onto glass cover slips. Attachment of mycoplasmas suspended in buffer increased with temperature, decreased with higher ionic strength, and showed a maximum at about pH 5.5. The findings suggest a participation of ionic bonds in the attachment process. Trypsin did not detach glass-bound mycoplasmas, and treatment of the cells with glutaraldehyde did not reduce their attachment to glass, suggesting that membrane components other than proteins may be involved in the attachment. Low concentrations (up to 20 mg/ml) of bovine serum albumin buffer. However, during the next few hours, attachment increased far above the bovine serum albumin control. This marked increase was reduced by more than half in the presence of chloramphenicol. Increased attachment was also observed when glucose (0.1 to 2 mg/ml) was added to the bovine serum albumin-containing buffer. The findings suggest different mechanisms for the attachment in protein-free buffer and in growth medium or glucose-containing bovine serum albumin buffer, respectively. The latter apparently requires metabolic activity of the mycoplasmas.

The peripheral membrane protein fraction released by washing Acholeplasma laidlawii membranes with low-ionic strength buffers contained about 50 % of the total membrane-bound ribonuclease and deoxyribonuclease activities. The ATPase, NADH oxidase and p -nitrophenylphosphatase activities remained bound to the membrane even when EDTA was added to the wash fluids, and thus appear to belong to the integral membrane protein group. Serving as a marker for peripheral membrane proteins, the membrane-bound ribonuclease activity was solubilized by bile salts much more effectively than the integral membrane-bound enzymes. On the other hand, the solubilized ribonuclease showed a much lower capacity to reaggregate with other solubilized membrane components to membranous structures. Yet, most of the ribonuclease molecules which were bound to the reaggregated membranes could not be released by low-ionic strength buffer. The reaggregated membranes differed from the native membranes in the absence of particles on their fracture faces obtained by freeze cleaving, and by their much higher labeling by the [ 125 I]lactoperoxidase iodination system. These results suggest that most of the proteins are exposed on the reaggregated membrane surfaces, with very little, if any, protein embedded in its lipid bilayer core. Enzyme disposition in the A. laidlawii membrane was studied by comparing the activity of isolated membranes with that of membranes of intact cells after treatment with pronase or with an antiserum to membranes. The data indicate the asymmetrical disposition of these activities, the ATPase and NADH oxidase being localized on the inner membrane surface, while the nucleases are exposed on the external membrane surface.

Pathogenic mycoplasmas rarely invade the tissues or bloodstream. Their adherence to epithelial cell surface, the first stage in disease, involves protein binding sites on the mycoplasmal cell membrane and receptors on the host cell membrane. Strong evidence indicates that Mycoplasma gallisepticum and Mycoplasma pneumoniae adhere with the aid of sialic acid residues on host cells, but the data do not preclude participation by other host-cell membrane components. Several studies indicate that these mycoplasmas adhere by blebs or terminal structures; others suggest that binding occurs via other cell areas. Scanning electron microscopy suggests tight interaction between these mycoplasmas and red blood cell membranes, causing imprints resembling those from interaction of viruses with red blood cells. Because sialoglycoproteins are major sites for attachment of M. pneumoniae to respiratory epithelium and red blood cells, glycophorin--the major sialoglycoprotein of human red blood cells--was the ligand used in affinity chromatography for isolation of the binding sites specific for sialic acid receptors from M. pneumoniae membranes solubilized by detergents. The fraction eluted with 0.2% sodium dodecylsulfate from the glycophorin-Sepharose column, highly enriched with two proteins, exhibited high binding capacity to glycophorin-Sepharose beads and lower binding capacity to human red blood cells. The latter capacity was nearly abolished by glycophorin, but not by its hydrophobic moiety.

The biological properties and special characteristics of the human T mycoplasmas have been reviewed and summarized. The T mycoplasmas are distinguished from all other known mycoplasmas by their production of urease, and, therefore, by their ability to hydrolyze urea. This singular property significantly sets the T mycoplasmas apart from all other members of the order Mycoplasmatales. In consideration of this distinguishing property, it is reasonable to propose establishing a new, separate genus in the family Mycoplasmataceae in which to classify the T mycoplasmas isolated from man and lower animals. The name Ureaplasma is proposed for this new genus, which at present contains a single human species of at least eight different serotypes. The name Ureaplasma urealyticum is proposed for this new species. The type strain of U. urealyticum is human strain 960-(CX8), serotype VIII (Black); it has been deposited in the American Type Culture Collection as ATCC 27618.

The synthesis of stable RNA in Mycoplasma capricolum was studied by [32P] labeling of cellular RNA of cells grown in a partially-defined medium in the presence or absence of an amino acid mixture supplement. The results indicate that M. capricolum employs the same stringent control mechanism used by E. coli cells, as judged by a decreased synthesis of stable RNA and accumulation of 5'-triphosphoguanosine-3'-diphosphate (pppGpp) and 5'-diphosphoguanosine-3'-diphosphate (ppGpp) in response to amino acid starvation. In addition, the results suggest that precursors of stable RNA accumulate and an intracellular pool of the precursors exists at all times under the growth conditions used by us. These findings may be interpreted to reflect a slow rate of RNA processing in M. capricolum.

The conservation and duplication of the tuf gene encoding the elongation factor EF-Tu were used to define phylogenetic relationships among eubacteria. When the tufA gene of Escherichia coli was used as a probe in hybridization experiments, duplicate tuf genes were found in gram-negative bacteria from three major phyla: purple bacteria, bacteroides, and cyanobacteria. Only a single copy of tuf was found in gram-positive bacteria, including mycobacteria and mycoplasmas. Gram-positive clostridia were found to carry two copies of tuf.

A specific DNA probe (pMG4) was used to diagnose M. gallisepticum (MG) infection in field outbreaks in chicken and turkey breeder flocks in Israel. Dot-blot hybridisation of tracheal swab suspensions with the probe enabled positive identification of MG as early as 4 days after sampling, even in flocks at an early stage of infection when no other specific indications of infection were available.

Membranes of Mycoplasma hominis cells from cultures progressing from the mid to the end of the logarithmic phase of growth became richer in protein, poorer in phospholipids and cholesterol, heavier in density, and more viscous as determined by EPR. The membrane-bound ATPase activity declined steeply. Electrophoretic analysis failed to show marked changes in membrane protein composition on aging, apart from an increase in the staining intensity of one protein band (Mr approximately 130 000) concomitant with a decrease in the staining intensity of several minor protein bands of high molecular weight. To test for possible changes in the disposition of the various membrane proteins on aging of cultures, a comparison was made of the susceptibility of membrane proteins of intact cells and isolated membranes to trypsinization and lactoperoxidase-mediated iodination. The iodination values and the percent of membrane protein released by trypsinization of intact cells were similar in cells from cultures of different ages, indicating no significant changes in the organization of the proteins on the outer surface. On the other hand, trypsinization and iodination of isolated membranes were found to be most markedly affected by the culture age, indicating significant changes in the organization of the proteins on the inner membrane surface. Thus, the iodination values of isolated membranes decreased by almost two fold, while the percentage of protein released from the membrane by trypsin increased from 28% to 50% during the experimental period. It is suggested that aging in M. hominis cultures is accompanied by a continuous increase in the packing density of the protein molecules on the inner surface of the cell membrane.

Mycoplasma hominis and Acholeplasma laidlawii were grown in media in which a fraction of human serum lipoproteins provided the sole source of cholesterol. Increasing levels of very low density lipoproteins had an inhibitory effect on the growth of the organisms. Low and high density lipoproteins in all concentrations proved to be excellent sources of cholesterol. Both organisms were able to limit the amount of cholesterol taken up and to preferentially incorporate free cholesterol despite an excess of esterified cholesterol in the medium. When similar levels of free cholesterol were provided by low density or high density lipoproteins, the organisms incorporated from 20–45% more cholesterol from the former. This preference for cholesterol from low density lipoproteins partially supports the theory that the low density lipoproteins act as a donor while the high density lipoproteins are a scavenger of cholesterol.

SummaryThe cell membrane ofMycoplasma gallisepticumwas isolated by lysing the cells with digitonin. Chemical and density-gradient analyses and electron microscopy showed the isolated membranes to be relatively free of cytoplasmic contaminants. The density of the membranes exceeded that of other mycoplasma membranes, indicating a higher protein content. Small vesicular extensions seen in the sectioned membranes were interpreted as empty blebs.The isolated membranes, but not the cytoplasmic fraction, elicited in chickens the production of growth-inhibiting, agglutinating and haemagglutination-inhibition antibodies toM. gallisepticumin titres resembling those obtained by injection of whole cells. The peak of the serological response varied with the serological test employed. The rapid slide-agglutination test became positive as early as 3 days after the first injection of only 50 μg. of membrane protein. The haemagglutination-inhibition antibody titre reached its peak at about 10 days after the first injection, while that of the growth-inhibiting antibodies was reached only at about 25 days. The addition of adjuvant to the membrane antigen did not improve the production of the growth-inhibiting antibodies in chickens, but it produced some improvement in rabbits. Our results support the thesis that the chief immunogens ofM. gallisepticumreside in the cell membrane of this organism.

Summary A microattachment assay for quantitating adherence of radiolabelled Mycoplasma pneumoniae to human WiDr cell culture monolayers is described. Preincubating the WiDr cell monolayers with a protein-rich extract of M. pneumoniae inhibited the subsequent attachment of radiolabelled organisms. Competitive attachment inhibition provided a quantitative procedure to determine M. pneumoniae -binding components in the extract. The microattachment assays also measured attachment inhibition by the sialoglycoconjugates ceruloplasmin, orosomucoid and gangliosides, indicating that these reagents may be structural analogues of the mammalian cell receptor. Attachment of virulent M. pneumoniae strains to glutaraldehyde-treated monolayers was reduced approximately 60% and showed a different temperature dependence compared with untreated cells. These results suggest that maximal attachment of virulent M. pneumoniae may require two or more different receptors and binding components.

Purified deoxyribonucleic acids (DNAs) of representative strains of nine established serovars of the human mycoplasma Ureaplasma urealyticum and of five strains of Mycoplasma pneumoniae were digested with a variety of restriction endonucleases. The cleavage patterns obtained by electrophoresis of the digestion products separated the nine U. urealyticum serovars into two definite clusters, one consisting of serovars 1, 3, and 6 and the other consisting of serovars 2, 4, 5, 7, 8, and 9, in agreement with previous results obtained by electrophoresis of cell proteins and DNA-DNA hybridization. Although the five M. pneumoniae strains differed in virulence and adherence capacity, they exhibited similar DNA cleavage patterns, indicating a high degree of genetic homogeneity. The enzymes KpnI, XhoI, BamHI, and PstI, which possess guanine-plus-cytosine-rich recognition sequences, cleaved the DNA of U. urealyticum (guanine-plus-cytosine content, about 28 mol%) into a small number of fragments, whereas SmaI, with the recognition sequence CCC/GGG, failed to cleave U. urealyticum DNA into visibly distinct fragments. The same restriction enzymes produced multiband cleavage patterns with M. pneumoniae DNA, which has a guanine-plus-cytosine content of about 40 mol%. Restriction endonucleases EcoRI, HindIII, HpaI, and XbaI, which have recognition sequences rich in adenine plus thymine, produced multiband patterns with the DNAs of both U. urealyticum and M. pneumoniae. We concluded that the cleavage patterns of mycoplasmal DNAs digested with restriction endonucleases provide a means for determining genetic relatedness among mycoplasmas.

SUMMARY: Electrophoretic patterns of digestion products of Acholeplasma and Mycoplasma DNA by restriction endonucleases were compared. The patterns of Acholeplasma axanthum strains isolated from a variety of hosts and habitats differed markedly from each other, indicating considerable genotypic heterogeneity among strains included in this species. Heterogeneity was less marked among the Acholeplasma oculi strains tested, and was minimal among strains of the avian pathogen Mycoplasma gallisepticum. Strains of Mycoplasma genitalium isolated from the urethra of patients with non-gonococcal urethritis and from the urethra of an experimentally infected chimpanzee yielded identical cleavage patterns, indicating a high degree of genetic homogeneity of these strains. The data support the notion that mycoplasma species of strict host and tissue specificity exhibit marked genetic homogeneity. The advantages and deficiencies of the use of DNA cleavage patterns for classification purposes are discussed.

1. 1. The solubilization and fractionation of the outer cytoplasmic membrane components of Proteus mirabilis were studied as a first step towards the reconstitution of the membranes from their solubilized components. 2. 2. Membrane phospholipids were specifically labeled by growing the organisms with radioactive oleic acid. The lipopolysaccharide component of the outer membrane was effectively labeled by adding radioactive galactose to the medium. Less than 5% of the label derived from galactose was found in the outer membrane phospholipids and none in the protein. 3. 3. Sodium dodecyl sulfate effectively solubilized all three components of the outer membrane while sodium deoxycholate and Triton X-100 solubilized almost all the phospholipid, but only about half of the protein and lipopolysaccharide. The cytoplasmic membranes of P. mirabilis were much more susceptible to solubilization by the three detergents tested. Treatment of the outer membrane with aqueous butan-l-ol separated the phospholipid into the butanol phase, while most of the protein and lipopolysaccharide was found in the aqueous phase. 4. 4. Electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate separated the lipopolysaccharide into two bands. Both bands were stained with the periodic acid-Schiff reagent, but only the slower migrating one stained with Coomassie blue. Filtration of the solubilized outer membranes through a Sephadex G-200 column containing sodium dodecyl sulfate separated the two lipopolysaccharide types. Our results support and supplement those of Gmeiner (Eur. J. Biochem. (1975) 58, 621–626) in showing that P. mirabilis produces two lipopolysaccharide types, one having long O-antigenic side chains, rich in lysine and amino sugars, while the other one is poorer in 0-antigenic chain components and more hydrophobic in character.

DNA probes were applied to detect spiroplasmas and uncultivable mycoplasma-like organisms (MLOs) in infected plants and insects. The probes consisted of pMC5, a plasmid carrying the RNA genes of Mycoplasma capricolum and pRA1, a plasmid recovered from Spiroplasma citri. Southern blot hybridization of pMC5 with digested DNAs of periwinkle plants infected with S. citri, or with various MLOs, yielded 2 heavy and several weaker bands. The heavy hybridization bands were shown to represent rRNA genes of the plant chloroplasts, indicating significant nucleotide sequence homology between the mycoplasmal rRNA genes and those of plant chloroplasts. Some of the weaker hybridization bands, not revealed in DNA of healthy plants, appeared to represent rRNA gene sequences of the infectious agent. Use of the spiroplasma plasmid as a probe enabled the detection of S. citri in infected plant material and in hemolymph of infected leafhoppers at a high sensitivity level.

Pathogenic mycoplasmas adhere to and colonize the epithelial lining of the respiratory and genital tracts of infected animals. An experimental system suitable for the quantitative study of mycoplasma adherence has been developed by us. The system consists of human erythrocytes (RBC) and the avian pathogen Mycoplasma gallisepticum , in which membrane lipids were labeled. The amount of mycoplasma cells attached to the RBC, which was determined according to radioactivity measurements, decreased on increasing the pH or ionic strength of the attachment mixture. Attachment followed first-order kinetics and depended on temperature. The mycoplasma cell population remaining in the supernatant fluid after exposure to RBC showed a much poorer ability to attach to RBC during a second attachment test, indicating an unequal distribution of binding sites among cells within a given population. The gradual removal of sialic acid residues from the RBC by neuraminidase was accompanied by a decrease in mycoplasma attachment. Isolated glycophorin, the RBC membrane glycoprotein carrying almost all the sialic acid moieties of the RBC, inhibited M. gallisepticum attachment, whereas asialoglycophorin and sialic acid itself were very poor inhibitors of attachment. Only part of the 125 I-labeled glycophorin bound to mycoplasmas could be removed by neuraminidase or by exchange with unlabeled glycophorin. It is suggested that glycophorin, representing the isolated major RBC receptor for M. gallisepticum , binds to the mycoplasmas both specifically, through its sialic acid moieties, and nonspecifically, through its exposed hydrophobic polypeptide moiety.

1. The crude envelope preparation obtained by sonication of Proteus mirabilis cells in the presence of lysozyme was separated into outer and cytoplasmic membrane fractions by sucrose density gradient centrifugation. The outer membrane fraction accounted for about two thirds of the dry weight of the envelope preparation. 2. In thin sections, the outer and cytoplasmic membrane fractions were shown to consist of vesicles bounded by a single trilaminar membrane, but those of the outer membrane were considerably smaller and were frequently open, forming C-shaped structures. The cytoplasmic membrane vesicles were cleaved by freeze fracturing to expose fracture faces studded with particles, while the outer membrane fragments resisted cleavage. 3. The outer membrane fraction consisted of protein (similar to 40%), lipopolysaccharide (similar to 36%) and lipid (similar to 18%) and had a density of about 1.22 g/cm3. The cytoplasmic membrane fraction consisted mostly of protein (similar to 56%) and lipid (similar to 38%), had a density of about 1.16 g/cm3, and contained almost all the NADH oxidase, succinate and D-lactate dehydrogenase activities of the crude envelope preparation. 4. Electrophoresis in polyacrylamide gels containing sodium dodecylsulfate revealed over 20 polypeptide bands in the cytoplasmic membrane fraction and only 6-7 in the outer membrane fraction. The outer membrane electrophorogram was dominated by a major band (mol. wt 40 000) which was resolved into two bands when electrophoresed in an acidic gel system. Amino acid analysis revealed a higher content of polar amino acids in the protein moiety of the outer membrane.