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The pathogenesis of bacillary dysentery can be studied at different levels of integration of the cellular components that constitute the colonic mucosal barrier. We considered the interaction of Shigella flexneri in three experimental systems that provide complementary information and a scheme of events occurring in human colorectal mucosa as Shigella invasion proceeds. Interaction of S. flexneri with individual epithelial cells shows a series of events in which the bacterium, upon contact with the cell surface, releases a set of Ipa proteins (i.e. invasins) through a specialized, activable, type-III secretory apparatus (i.e. Mxi/Spa). Via a complex signaling process, these invasins cause major rearrangements of the subcortical cytoskeletal network which allow bacterial entry by a macropinocytotic event. Then the bacterium lyses its phagocytotic vacuole and initiates intracytoplasmic movement, due to polar assembly of actin filaments caused by a bacterial surface protein, IcsA. This allows very efficient colonization of the host cell cytoplasm and passage to adjacent cells via protrusions which are engulfed by a cadherin-dependent process. However, when invasive Shigella are deposited on the apical side of polarized monolayers of human colonic cells, they appear unable to invade, indicating that bacteria need to reach the subepithelial area to invade the epithelium. In this system, it has been shown that transepithelial signaling caused by apical bacteria induces adherence and transmigration of basal polymorphonuclears (PMN), thus disrupting the monolayer permeability and facilitating bacterial invasion. LPS accounts for a large part of this transepithelial signalization to PMN. Such a process could account for invasion in intestinal crypts. Finally, models of infection, such as the rabbit ligated intestinal loop show that initial bacterial entry occurs essentially via M cells of the follicular associated epithelium. It then causes apoptosis of macrophages located in the follicular dome, inducing release of IL-1 beta which, in turn, initiates inflammation, leading to destabilization of the epithelial structures as modeled above. These data can now be used to understand the mechanisms of mucosal protection against bacillary dysentery.

In a French multicenter study, six verocytotoxin-producing Escherichia coli strains were isolated from the stools of 6 of 69 children suffering from hemolytic-uremic syndrome. All strains belonged to serotype O103:H2, a serotype commonly associated with diarrhea in weaned rabbits in France. To determine whether the strains from humans and rabbits were genetically related, they were compared by analyzing their esterase electropherotypes and the restriction fragment length polymorphisms of the ribosomal DNA regions. A common clonal origin of these pathogenic strains was suggested by their identical esterase electropherotypes and their identical ribotypes, in addition to their identical serotypes. However, strains from humans, which are cytotoxic for HeLa cells through the production of verocytotoxin type 1, do not show adhesion in vitro to HeLa 229 cells and cannot infect rabbits. On the other hand, strains from rabbits do not carry the verocytotoxin type 1 gene, are not cytotoxic for Hela cells, and adhere to ileal villi and HeLa 229 cells because of the expression of their 32-kDa adhesin. Our results therefore identify a clone of verocytotoxin-producing E. coli O103:H2 as a potential agent of hemolytic uremic syndrome in France. They further suggest that clones from humans and rabbits probably have a common origin but that adaptation to the two species occurred by different mechanisms. Thus, they eliminate the hypothesis that the species is horizontally transmitted between rabbits and humans.

The human interface with the microbial world has so far largely been considered through the somewhat restrictive angle of host-pathogen interactions resulting in disease. It has consequently largely ignored the daily symbiosis with the microbiota, an ensemble of symbiotic microorganisms engaged in a commensal, and for some of them mutualistic, interaction. This microbiota heavily populates essential surfaces such as the oral and intestinal cavity, the upper respiratory tract, the vagina, and the skin. Host response to the pathogens is characterized by quick recognition combined with strong innate (i.e., inflammatory) and adaptive immune responses, causing microbial eradication often at the cost of significant tissue damage. Response to the symbiotic microbiota is characterized by a process called tolerance that encompasses a complex integration of microbial recognition and tightly controlled innate (i.e., physiological inflammation) and adaptive immune responses. This dichotomy in host response is critical at the gut mucosal surface that is massively colonized by a diverse population of bacteria. The host is therefore permanently facing the challenge of discriminating among symbiotic and pathogenic bacteria in order to offer an adapted response. This asks the fundamental existential question: "to be or not to be… a pathogen." This review has attempted to consider this question from the host angle. What do host mucosal sensing systems see in the bacteria to which they become exposed to establish proper discrimination? A new facet of medicine resides in the dysfunction of this complex balance that has likely forged the complexity of the immune system.

Shigella invades epithelial cells by inducing cytoskeletal reorganization localized at the site of bacterial-host cell interaction. During entry, the Shigella type III secretion apparatus allows the insertion of a pore that contains the IpaB and IpaC proteins into cell membranes. Insertion of this complex is thought to allow translocation of the carboxy-terminus moiety of IpaC, but also of other Shigella effectors, such as IpaA, into the cell cytosol. IpaC triggers actin polymerization and the formation of filopodial and lamellipodial extensions dependent on the Cdc42 and Rac GTPases. IpaA, on the other hand, binds to the focal adhesion protein vinculin and induces depolymerization of actin filaments. IpaA and the GTPase Rho are not required for actin polymerization at the site of bacterial contact with the cell membrane, but allow the transformation of the IpaC-induced extensions into a structure that is productive for bacterial entry. Rho is required for the recruitment at entry foci of ezrin, a cytoskeletal linker required for Shigella entry, and also of the Src tyrosine kinase. The Src tyrosine kinase activity, which is required for Shigella-induced actin polymerization, also appears to be involved in a negative regulatory loop that downregulates Rho at the site of entry.

Interaction of Shigella flexneri with epithelial cells includes contact of bacteria with the cell surface and release of Ipa proteins through a specialized type III secreton. A complex signaling process involving activation of small GTPases of the Rho family and c-src causes major rearrangements of the subcortical cytoskeleton, thereby allowing bacterial entry by macropinocytosis. After entry, shigellae escape to the cell cytoplasm and initiate intracytoplasmic movement through polar nucleation and assembly of actin filaments caused by bacterial surface protein IcsA, which binds and activates neuronal Wiskoff-Aldrich syndrome protein (N-WASP), thus inducing actin nucleation in an Arp 2/3-dependent mechanism. Actin-driven motility promotes efficient colonization of the host cell cytoplasm and rapid cell-to-cell spread via protrusions that are engulfed by adjacent cells in a cadherin-dependent process. Bacterial invasion turns infected cells to strongly proinflammatory cells through sustained activation of nuclear factor-kappaB. A major consequence is interleukin (IL)-8 production, which attracts polymorphonuclear leukocytes (PMNs). On transmigration, PMNs disrupt the permeability of this epithelium and promote its invasion by shigellae. At the early stage of infection, M cells of the follicle-associated epithelium allow bacterial translocation. Subsequent apoptotic killing of macrophages in a caspase 1-dependent process causes the release of IL-1beta and IL-18, which accounts for the initial steps of inflammation.

The gastrointestinal tract has a particularly complex relationship with the bacterial world. On the one hand it has its own indigenous microflora composed of a vast number of different bacterial species that are most densely represented in the large bowel. These organisms generally live symbiotically with the host without causing enteric disease. Furthermore, many organisms in the environment are ingested in food and water, yet do not colonise the intestinal tract in the face of local host defence mechanisms and the resident microflora (see Chapters 1 and 2). However, a few bacterial species can establish infection and cause disease following ingestion. Interestingly, it is likely that most of them have evolved from the nonpathogenic intestinal microflora of animals through acquisition of one or more groups of virulence factors. The genetic organisation and biochemical mechanisms involved in the functions of these virulence factors share common themes, yet overall, the pathophysiological characteristics of the major enteric infections are distinct. In general, the first step in the infectious process requires adherence or contact with the surface epithelium. From there, bacteria either remain in the lumen and secrete toxins and factors that modulate epithelial cell function, or invade and survive in the mucosal tissue.

Few studies provide data on the global morbidity and mortality caused by infection with Shigella spp.; such estimates are needed, however, to plan strategies of prevention and treatment. Here we report the results of a review of the literature published between 1966 and 1997 on Shigella infection. The data obtained permit calculation of the number of cases of Shigella infection and the associated mortality occurring worldwide each year, by age, and (as a proxy for disease severity) by clinical category, i.e. mild cases remaining at home, moderate cases requiring outpatient care, and severe cases demanding hospitalization. A sensitivity analysis was performed to estimate the high and low range of morbid and fatal cases in each category. Finally, the frequency distribution of Shigella infection, by serogroup and serotype and by region of the world, was determined. The annual number of Shigella episodes throughout the world was estimated to be 164.7 million, of which 163.2 million were in developing countries (with 1.1 million deaths) and 1.5 million in industrialized countries. A total of 69% of all episodes and 61% of all deaths attributable to shigellosis involved children under 5 years of age. The median percentages of isolates of S. flexneri, S. sonnei, S. boydii, and S. dysenteriae were, respectively, 60%, 15%, 6%, and 6% (30% of S. dysenteriae cases were type 1) in developing countries; and 16%, 77%, 2%, and 1% in industrialized countries. In developing countries, the predominant serotype of S. flexneri is 2a, followed by 1b, 3a, 4a, and 6. In industrialized countries, most isolates are S. flexneri 2a or other unspecified type 2 strains. Shigellosis, which continues to have an important global impact, cannot be adequately controlled with the existing prevention and treatment measures. Innovative strategies, including development of vaccines against the most common serotypes, could provide substantial benefits.This article presents a review of the literature published between 1966 and 1997 on Shigella infection. The purpose of the review is to provide data on the global morbidity and mortality caused by the infection and to plan strategies of prevention and treatment. The data obtained from this literature were used to calculate the number of Shigella infection cases and the associated mortality occurring worldwide each year, by age and by clinical category. The burden of Shigella infection was also estimated by serogroup and serotype. A sensitivity analysis was performed to estimate the high and the low range of morbid and fatal cases in each category (mild cases remaining at home, moderate cases requiring outpatient care and severe cases demanding hospitalization). The result of the calculations and analysis revealed that the annual number of Shigella infections throughout the world was estimated to be 164.7 million. 163.2 million occurred in developing countries, with 1.1 million deaths, and 1.5 million occurred in industrialized countries. More than half of the episodes and death affects children under 5 years of age. In comparing developing countries against industrialized countries, the median of isolates are S. flexneri (60% vs. 16%), S. sonnei (15% vs. 77%), S. dysenteriae (6% vs. 1%), and S. boydii (6% vs. 2%). The predominant serotype of S. flexneri in developing countries is 2a, followed by 1b, 3a, 4a, and 6, while in industrialized countries most isolates are S. flexneri 2a and unspecified type 2 strains.

The pathogenesis of shigellosis is characterized by the capacity of the causative microorganism, Shigella, to invade the epithelial cells that compose the mucosal surface of the colon in humans. The invasive process encompasses several steps which can be summarized as follows: entry of bacteria into epithelial cells involves signalling pathways that elicit a macropinocitic event. Upon contact with the cell surface, S. flexneri activates a Mxi/Spa secretory apparatus encoded by two operons comprising about 25 genes located on a large virulence plasmid of 220 kb. Through this specialized secretory apparatus, Ipa invasins are secreted, two of which (IpaB, 62 kDa and IpaC, 42 kDa) form a complex which is itself able to activate entry via its interaction with the host cell membrane. Interaction of this molecular complex with the cell surface elicits major rearrangements of the host cell cytoskeleton, essentially the polymerization of actin filaments that form bundles supporting the membrane projections which achieve bacterial entry. Active recruitment of the protooncogene pp 60c-src has been demonstrated at the entry site with consequent phosphorylation of cortactin. Also, the small GTPase Rho is controlling the cascade of signals that allows elongation of actin filaments from initial nucleation foci underneath the cell membrane. The regulatory signals involved as well as the proteins recruited indicate that Shigella induces the formation of an adherence plaque at the cell surface in order to achieve entry. Once intracellular, the bacterium lyses its phagocytic vacuole, escapes into the cytoplasm and starts moving the inducing polar, directed polymerization of actin on its surface, due to the expression of IcsA, a 120 kDa outer membrane protein, which is localized at one pole of the microorganism, following cleavage by SopA, a plasmid-encoded surface protease. In the context of polarized epithelial cells, bacteria then reach the intermediate junction and engage their components, particularly the cadherins, to form a protrusion which is actively internalized by the adjacent cell. Bacteria then lyse the two membranes, reach the cytoplasmic compartment again, and resume actin-driven movement.

A key step in the pathogenesis of shigellosis is the capacity of the causative bacteria, shigellae, to invade colonic and rectal epithelial cells in humans. This invasive process encompasses several steps: entry into epithelial cells by induction of a macropinocytic event caused by secreted Ipa proteins. the bacterium then escapes from the vacuole and reaches the cytoplasmic compartment in which it divides rapidly and becomes motile via the expression of a surface protein, IcsA, whose polar localization achieves directed polymerization of actin filaments that push the bacterial body forward. Bacteria then engage the inner face of the cellular membrane in the junctional area and form protrusions allowing their passage into the adjacent cell. Lysis of the double membrane eventually allows access to the cytoplasmic compartment of the adjacent cell, thus providing the bacterium with a very efficient mechanism of epithelial colonization.

Vaccination against diarrhoeal diseases has been set up as a primary goal for developing countries. Recent progress in understanding the molecular and cellular bases of these infections and the function of the mucosal immune system opens the way to new strategies of vaccination. Mucosal surfaces, particularly the intestine, appear as suitable targets for a new generation of vaccines among which live attenuated vectors are promising. However, we are still a long way from mastering all the relevant parameters of such approaches.

Development of vaccines against enterobacterial species responsible for enteric infections sounds like an unrealistic project. On the other hand, based on our growing understanding of the pathogenesis of infections caused by the major species (i.e. Salmonella typhi, Shigella flexneri and Shigella dysenteriae 1, Enterotoxigenic Escherichia coli) and on our better characterization of the immunological parameters of mucosal protection, it is likely that a limited number of vaccines controlling diseases such as typhoid fever, bacillary dysentery and cholera-like E. coli infections will become available. There are three major problems that still need to be solved: the variety of antigens and antigenic specificities, the nature of protective antigens and, provided that these prerequisites are fulfilled, the presentation of the vaccine and the immunization route. With the exception of typhoid fever, which has a systemic phase that probably makes immunization attempts by purified Vi antigen successful, experimental strategies rely very much on induction of a mucosal immunity. Either subunit vaccines, or genetically manipulated strains which attenuated virulence are currently considered.

Shigella flexneri ist das am besten untersuchte atiologische Agens der bakteriellen Ruhr, einer Erkrankung, an der jahrlich weltweit mindestens 500 000 Menschen sterben [11]. Wahrend die Shigellose in gemasigten Klimazonen sporadisch oder in epidemischer Form auftritt, ist die von S. flexneri verursachte Erkrankung in subtropischen und tropischen Regionen endemisch.

Development of vaccines against enterobacterial species responsible for enteric infections sounds like an unrealistic project. On the other hand, based on our growing understanding of the pathogenesis of infections caused by the major species (i.e. Salmonella typhi, Shigella flexneri and Shigella dysenteriae 1, Enterotoxigenic Escherichia coli) and on our better characterization of the immunological parameters of mucosal protection, it is likely that a limited number of vaccines controlling diseases such as typhoid fever, bacillary dysentery and cholera-like E. coli infections will become available. There are three major problems that still need to be solved: the variety of antigens and antigenic specificities, the nature of protective antigens and, provided that these prerequisites are fulfilled, the presentation of the vaccine and the immunization route. With the exception of typhoid fever, which has a systemic phase that probably makes immunization attempts by purified Vi antigen successful, experimental strategies rely very much on induction of a mucosal immunity. Either subunit vaccines, or genetically manipulated strains which attenuated virulence are currently considered.

A cohort of 50 initially asymptomatic seropositive patients have been followed for five years. Various clinical evolution have been observed: 21 remain stable, 29 underwent complications that take then to AIDS, 11 died. In any case, invariability of the MRI anomalies seems had to be imputed to primary infection scars.

Shigella flexneri, a Gram negative bacillus, causes bacillary dysentery, an ulcerative disease of the human colon, by invading intestinal epithelial cells. Entry into epithelial cells occurs via an induced phagocytic process which involves the actino-myosin complex. The host-cell receptor and the transmembrane signal which initiate reorganization of the cytoskeleton are under study. Binding to integrins has recently been demonstrated in related models such as the entry of Yersinia pseudotuberculosis and Bordetella pertussis into cells. Bacterial genes necessary to achieve entry are located on five contiguous loci covering 30 kb on a 220 kb virulence plasmid in S. flexneri. Locus 2 has been particularly studied. Six genes organized as an operon encode highly immunogenic proteins among which IpaB (62 kD) and IpaC (48 kD) are the invasins of this microorganism which subsequently grows very rapidly within infected cells due to its capacity to lyse the membrane bound phagocytic vacuole. Once free within the cytoplasm, bacteria interact again with the cell cytoskeleton. They first express Olm (organelle like movement), a phenotype reflecting intracellular movement along actin stress cables. They subsequently express lcs (intracellular spread), a phenotype by which intracellular bacteria induce nucleation and polymerization of actin followed by accumulation of this material at one end of the bacillus. This process causes rapid random movement leading to the formation of protrusions which allow passage to adjacent cells. A combination of these two movements achieves bacterial colonization of the epithelium.

Shigella flexneri, a Gram negative bacillus, causes bacillary dysentery, an ulcerative disease of the human colon, by invading intestinal epithelial cells. Entry into epithelial cells occurs via an induced phagocytic process which involves the actino-myosin complex. The host-cell receptor and the transmembrane signal which initiate reorganization of the cytoskeleton are under study. Binding to integrins has recently been demonstrated in related models such as the entry of Yersinia pseudotuberculosis and Bordetella pertussis into cells. Bacterial genes necessary to achieve entry are located on five contiguous loci covering 30 kb on a 220 kb virulence plasmid in S. flexneri. Locus 2 has been particularly studied. Six genes organized as an operon encode highly immunogenic proteins among which IpaB (62 kD) and IpaC (48 kD) are the invasins of this microorganism which subsequently grows very rapidly within infected cells due to its capacity to lyse the membrane bound phagocytic vacuole. Once free within the cytoplasm, bacteria interact again with the cell cytoskeleton. They first express Olm (organelle like movement), a phenotype reflecting intracellular movement along actin stress cables. They subsequently express Ics (intracellular spread), a phenotype by which intracellular bacteria induce nucleation and polymerization of actin followed by accumulation of this material at one end of the bacillus. This process causes rapid random movement leading to the formation of protusions which allow passage to adjacent cells. A combination of these two movements achieves bacterial colonization of the epithelium.

The pathogenesis of bacillary dysentery involves invasion of the human colonic mucosa (1, 2). Invasion encompasses entry of the bacterium into epithelial cells, followed by intracellular multiplication, intra and intercellular spread and host cell killing (3). These bacteria then gain access to the lamina propria of the colonic mucosa in which a severe inflammatory reaction causes abscesses and ulcerations. The first step reflects the ability of the bacterium to induce its own phagocytosis by cells which are non professional phagocytes. It can be studied in vitro by infection of monolayers of mammalian cells such as Henle or HeLa cells (1, 4, 5). The second step requires bacteria to survive within tissues and to elicit inflammation and tissue destruction. It can be studied in assays such as the Sereny test (6), and the rabbit ligated ileal loop (7). A modification of the tissue culture assay allows quantitation of the microorganisms capacity to invade cells, multiply intracellularly and spread to adjacent cells, thus causing a cytopathic effect visualized as clear plaques on a confluent monolayer (8). Cell invasion by Shigella has recently been extensively reviewed (3, 9).

The in vivo induction of T cell-mediated immunity was studied by infecting mice with two genetically closely related mutants from Listeria monocytogenes, differing only with respect to the secretion of an active SH-dependent hemolysin. It is shown that even minute doses of hemolytic bacteria capable of growing in host tissues easily induced the expression of T cell-mediated immunity, as estimated by the level of delayed sensitivity, adoptive protection and long-lasting immunological memory. On the contrary, nonhemolytic bacteria unable to multiply in host tissues totally failed to initiate the expression of T cell-mediated immunity in vivo. This failure was even observed when mice were repeatedly infected by high doses of nonhemolytic bacteria, allowing to maintain a significant amount of viable bacteria for several days in host tissues. These results mean that the presence of viable bacteria at a significant level in the host is not sufficient per se to induce detectable T cell clonal expansion in the in vivo setting, implying that the process of bacterial growth inside macrophages is required to initiate in vivo the expression of T cell-mediated immunity.

Twenty-six antibiotics belonging to thirteen different chemical families had been tested on seven Enterobacteriaceae harbouring a virulence plasmid (Shigella sonnei, S. flexneri, S. dysenteriae, Escherichia coli (two plasmids). Yersinia and Salmonella dublin). Novobiocin and rifampicin were found to be most efficient eliminating three plasmids of which two come from Shigella. Clindamycin, cotrimoxazole, nifurzide, ciprofloxacin, and tilbroquinol were also efficient, but at lower rate. Four virulence plasmids (from the three Shigella sp and Y. pestis) were eliminated by one or several antibiotics. The frequency of elimination was low (at best 10% bacteria per generation). The plasmid pWR105 from S. sonnei was the less stable.

Virulent and nonvirulent isolates of avian Escherichia coli were tested for the presence of aerobactin genes by colony hybridization with a specific gene probe constructed from plasmid pABN1 (A. Bindereif and J. B. Neilands, J. Bacteriol. 153:1111-1113, 1983). Positive hybridization with the gene probe was highly correlated with virulence, as measured by the 50% lethal dose of the strains for chicks. Evidence for the expression of aerobactin genes in the virulent strains was obtained by demonstrating their susceptibility to cloacin DF13, which binds to the same receptor that binds aerobactin, and their ability to produce aerobactin, as revealed by cross-feeding the E. coli mutant WO987 (aroB fepA iuc iut+), which is unable to synthesize but capable of taking up aerobactin. We suggest that the production of aerobactin is involved in the virulence of avian septicemic E. coli.

Invasive Escherichia coli is a "Shigella-like" microorganism which causes a dysenteric syndrome through invasion of the human colonic epithelium. Representative strains of different serotypes were studied in order to determine whether plasmids are involved in their virulence. All invasive E. coli strains, irrespective of serotype, were found to harbour a large plasmid of approximately 140 Mdal. Spontaneous variants of serotypes O143 and O124 had lost this plasmid and had become avirulent, i.e. could neither penetrate into HeLa cells nor produce a keratoconjunctivitis in guinea-pigs. pWR110, a Tn5-labelled virulence plasmid of Shigella flexneri, was transferred into these avirulent variants, thus restoring their virulence and demonstrating that S. flexneri and invasive E. coli share a common extrachromosomal control of their ability to penetrate into cells.

Resistance of Plasmodium falciparum to antimalarial drugs is a growing worldwide problem in endemic areas. This is particularly obvious with chloroquine resistance which has been spreading throughout Africa since 1978. Initially localized on the Eastern coast of the continent, it has recently reached the Atlantic coast although Western Africa is as yet unaffected. This resistance has most probably arisen in the autochtonous population under the selective pressure of insufficient curative treatments as a result of self medication. It is most prevalent in children of urban areas but is often revealed in non immune expatriates. In addition, moderately resistant parasites in expatriates under chemoprophylaxis may lead to an insidious clinical form with a low or even negative parasitemia which raises diagnostic problems. Prophylactic and therapeutic schemes must take this new situation into account.

The high frequency of meningococcal infections in patients with congenital deficiency of a component of the complement terminal pathway emphasizes the critical role of this system in host resistance against Neisseriaceae. We report the observation of a subacute septicaemia due to N. meningitidis serogroup Y. This girl had an acquired deficit of the C3 fraction of complement due to a high titre of C3 nephritic factor (C3NeF). There was no evidence of partial lipodystrophy or biological symptoms of glomerular disease. The meningococcal infection revealed this biological abnormality.

Twelve recent cases of Plasmodium falciparum malaria presented with unusual clinical and laboratory features. All patients had regularly been taking adequate doses of amino-4-quinolines as prophylaxis. In most cases the symptoms were mild as compared with those in a group of 20 control patients with typical malaria. More surprisingly, parasitaemia was either negative or very weakly positive (less than 1000 infested erythrocytes per mm3). All cases occurred in people returning from areas in which resistance of P. falciparum to these drugs is endemic. Diagnostic problems could be solved in most patients by new immunological techniques relying on the detection of parasitized erythrocytes and plasmodium antigens using a battery of monoclonal antibodies. A physiopathological model explaining why parasitaemia is negative in such cases is suggested.

In an attempt to determine factors of predictive value in HIV (human immuno-deficiency virus) seropositive patients, particular attention was payed to symptoms indicating early involvement of the central nervous system (CNS). A cohort of healthy carriers was thus constituted. Follow-up will be carried out every six month including clinical, biological as well as CNS imaging by NMR. Among the first 15 of them, abnormalities could be observed in 4 individuals. Lesions consisted in nodules of high signal in T2 which were localized either in the white matter or thalamic nuclei. No relationship could be demonstrated between the existence of these lesions and various criteria such as age, sex, risk factors and T4 cells count. Such lesions appeared similar to the localizations observed in multiple sclerosis or to the scars of limited vascular accidents. The nature of these lesions is not clear. They certainly indicate early involvement of CNS after primary infection by the HIV virus. They may either represent sequellae of this primary infection or early alterations announcing developing encephalopathy.

Central Nervous System (CNS) involvement, whether primary by the Human Immunodeficiency Virus--HIV--itself, or secondary (toxoplasmosis or lymphoma) is remarkably frequent in AIDS, in 40 to 70% of cases, depending upon the author. In order to study the natural history of this illness, a cohort of 25 asymptomatic seropositive patients have been established. Every 6 months these patients undergo biological and clinical examinations, as well as Magnetic Resonance brain scans. After two examinations at a 6 month's interval, the first results are reported. Out of these 25 cases, 9 present anomalies: One patient with diffuse cerebral atrophy and 8 others with high signal intensity areas on T2 weighted sequences, like those of the Multiple Sclerosis. No relationship could be demonstrated between the existence of these lesions and various criteria such as age, sex, risk factors and T4 cells count. The nature of these lesions is not clear. They certainly indicate early involvement of the CNS after primary infection by the HIV virus. They may either represent scars of the primary infection or early alterations announcing developing encephalopathy.

Nine isolates of Klebsiella pneumoniae belonging to capsular serotypes K1 and K2 were assayed for virulence in mice. Virulent isolates (50% lethal dose of less than 10(3) microorganisms) and avirulent isolates (50% lethal dose of over 10(6) microorganisms) were selected. Supplementation of a defined minimal medium with transferrin markedly reduced the growth of avirulent strains but had no significant effect on the growth of virulent strains. All isolates produced enterochelin, but only production of aerobactin could be correlated with virulence. The genes encoding aerobactin and its receptor protein were located on a 180-kilobase plasmid. They were cloned into the mobilizable vector pSUP202. Homology was demonstrated with the aerobactin operon of the Escherichia coli plasmid pColV-K30. Transfer of the recombinant plasmid pKP4 into an avirulent recipient enhanced virulence by 100-fold. These experiments demonstrated that aerobactin is an essential factor of pathogenicity in K. pneumoniae.

Shigella flexneri M90T (invasive) and BS176 (noninvasive) are typical nonfimbriated organisms that do not bind to or activate phagocytic cells. We demonstrate that S. flexneri M90Tp and BS176p, obtained by transformation of the strains named above with the cluster of genes encoding type 1 (mannose-specific) fimbriae of Escherichia coli, express the functional fimbriae, as shown by electron microscopy, by binding of antifimbria antibodies and by yeast cell aggregation. The transformants, but not the parental strains, bound to human granulocytes and mouse peritoneal macrophages. This binding was inhibited by methyl alpha-D-mannoside but not by methyl alpha-D-galactoside. The bound bacteria induced oxidative burst activation and degranulation of the granulocytes in vitro. With mouse peritoneal macrophages, the binding of the fimbriated bacteria induced degranulation in vitro. Injection of the bacteria into mouse peritoneum also induced degranulation of the macrophages in vivo; no such effect was observed with the nonfimbriated strains. The bound fimbriated transformants were effectively killed by the human granulocytes in vitro in the absence of opsonins or after opsonization with human anti-S. flexneri antiserum. The nonfimbriated strains were killed only after opsonization. These results provide further evidence for the role of type 1 fimbriae in lectin-mediated nonopsonic phagocytosis.

Mucin glycoproteins are secreted in large amounts by the intestinal epithelium and constitute an efficient component of innate immune defenses to promote homeostasis and protect against enteric pathogens. In this study, our objective was to investigate how the bacterial enteropathogen Shigella flexneri , which causes bacillary dysentery, copes with the mucin defense barrier. We report that upon in vitro infection of mucin-producing polarized human intestinal epithelial cells, virulent S. flexneri manipulates the secretion of gel-forming mucins. This phenomenon, which is triggered only by virulent strains, results in accumulation of mucins at the cell apical surface, leading to the appearance of a gel-like structure that favors access of bacteria to the cell surface and the subsequent invasion process. We identify MUC5AC, a gel-forming mucin, as a component of this structure. Formation of this gel does not depend on modifications of electrolyte concentrations, induction of trefoil factor expression, endoplasmic reticulum stress, or response to unfolded proteins. In addition, transcriptional and biochemical analyses of infected cells reveal modulations of mucin gene expression and modifications of mucin glycosylation patterns, both of which are induced by virulent bacteria in a type III secretion system-dependent manner. Thus, S. flexneri has developed a dedicated strategy to alter the mucus barrier by targeting key elements of the gel-forming capacity of mucins: gene transcription, protein glycosylation, and secretion.

International audience; Shigella infection is characterized by the induction of acute inflammation, which is responsible for the massive tissue destruction of the intestinal mucosa. A murine model would be a valuable tool for gaining a better understanding of the physiopathology of shigellosis and the host immune response to Shigella infection, but adult mice do not develop disease upon oral inoculation. We therefore attempted to develop a model of infection in newborn mice. Four-day-old mice inoculated with 50 microl of 5 x 10(9) invasive wild-type Shigella flexneri 5a were susceptible to bacterial infection, but mice inoculated with the non-invasive strain BS176 were not. Histologically, 4-day-old mice infected with the invasive strain presented intestinal lesions and inflammation similar to those described in patients with shigellosis. Moreover, cytokine and chemokine responses consistent with inflammation were observed. Lower bacterial inocula induced less severe intestinal damage. In contrast, 5-day-old mice inoculated with either the invasive or the non-invasive strain were not infected. We have thus established a mouse model that is suitable for the study of the pathogenesis of intestinal Shigella infection.

International audience; An efficient synthesis of the methyl glycoside of a branched octasaccharide representative of Shigella flexneri serotype 2a O-specific polysaccharide is described. The synthesis is based on the use of the trichloroacetimidate methodology, and involves the condensation of a pentasaccharide acceptor and a trisaccharide donor as the key step.The target octasaccharide was synthesized efficiently according to a strategy based on the condensation of a pentasaccharide acceptor and a trisaccharide donor as the key step.