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Stable small-colony variants of Proteus species were isolated and characterised. They differed from typical organisms in their morphology, biochemical properties and drug resistance. They were found most frequently in culture media of low osmolarity. Their growth rate was depressed at high concentrations of salts, which had only a small effect on typical organisms. This may be important in the routine isolation of proteus spp. and in testing of disinfectants.

The mutagenicity of some novel 2-phenyl-imidazopyridines was investigated by the Ames test. Those with nitro substituents, and 2 related nitrophenylpurines, were frame shift mutagens without liver activation. Similar compounds without nitro groups had no activity. The mutagenic compounds were more active against TA1538 than against TA98, and were also active against TA100, a base-pair substitution mutant. This unusual pattern of activity is similar to that of 2-nitrosofluorene and related compounds. Calculations of molecular size showed a relationship between the 2 groups of compounds, suggesting similar modes of action. The more potent mutagens were toxic at high concentrations. Many of the non-mutagenic compounds were toxic, depending upon their substituents. In general, the imidazol[4,5-c]pyridines were more biologically active than the imidazo [4,5-b]pyridines.

The presence of R-plasmid RP1 increased the adhesion of chemostat-grown iron- and carbon-limited Proteus mirabilis to the surfaces of various medical prostheses and to glass. Similar results were obtained with iron-limited Pseudomonas aeruginosa and anaerobically-grown Escherichia coli. Changes in the surface properties of P. mirabilis indicated that the R-plasmid-mediated increase in negative charge was one of the factors that promoted adhesion.

The presence of R-plasmid RP1, as well as the conditions of growth, affected the surface hydrophobicity of a clinical isolate of Proteus mirabilis. However, results depended upon the method of assessment. Stationary phase plasmid-containing cells appeared to be less hydrophobic than plasmid-free cells when hydrophobicity was measured by the contact angle method, but more hydrophobic when measured by bacterial adherence to hydrocarbons or hydrophobic interaction chromatography. Cells growing in a chemostat differed in hydrophobicity from stationary phase cells and results varied with the growth rate. Plasmid-mediated effects were greatest in iron-depleted cells, and differences between plasmid-containing and plasmid-free cells were virtually eliminated by pre-treatment with antiserum.

The bactericidal action of non-immune whole blood on Proteus mirabilis was increased when the bacteria contained the R-plasmid RP1. This effect was due mainly to increased phagocytosis. Iron-depleted stationary-phase cells were more sensitive than carbon-depleted cells. The contribution of serum was usually negligible but was increased during a minor non-specific infection. Most plasmid-containing phenotypes were more sensitive than were those without plasmids but there were considerable differences between stationary and exponentially-growing cells. The R-plasmid-mediated increase in sensitivity to phagocytosis may be due in part to the presence of additional glycosylated proteins in the outer membrane.

The minimal nutritional requirements of Escherichia coli have been quantitatively determined in batch culture for cells with (R+) and without (R-) the R plasmid RP1. In these conditions R+ cells have a greater requirement than R- for several nutrients, particularly Mg2+, K+, Fe2+ and PO43-. The maximum growth rate in a simple salts medium was the same for R+ and R- cells. At low concentrations of phosphate, the specific growth rate of R+ cells differed from that found for R- cells. The R plasmid was stable in simple salts medium, irrespective of the nutrient ultimately depleted by growth, but, on storage, R+ cells survived for a shorter time than R- cells.

The lytic and bactericidal actions of polymyxin B on whole cells and spheroplasts of Pseudomonas aeruginosa varied markedly with the suspending media, and there was little correlation between them. Relative rates of lysis of these preparations and also of Bacillus megaterium protoplasts suggested that polymyxin causes progressive damage to the cytoplasmic membrane, such that membrane permeability towards various ions increased as follows: K(+) > Na(+) > NO(3) (-) > Cl(-), Ca(2+), H(2)PO(4) (-)/HPO(4) (2-). Impermeant compounds, such as NaCl and sucrose, protected whole cells against lysis but not against death. It is suggested that lysis of whole cells by polymyxin is a secondary effect, resulting from entry of solutes normally excluded by the cytoplasmic membrane and the fragility of the damaged outer membrane. Because the degree of lysis varies with the external solutes, it should be treated with caution as a descriptor of polymyxin activity.