Twitching motility is a mode of flagella-independent surface translocation exhibited by Pseudomonas aeruginosa and other bacteria on solid media. All species exhibiting it carry thin pili, usually polar. This work shows that only PAO and K strains of P. aeruginosa with retractile (PSA) pili were able to move in this way, those with no pili or non-retractile pili remaining stationary. Specific agents such as anti-pilus serum, which prevents otherwise functional pili from retracting, also prevented twitching motility.

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Representative plasmids for most incompatibility groups in Escherichia coli K-12 were transferred to a "bald" strain to compare transfer frequencies for liquid and solid media. Standard broth matings were used for a liquid environment, but for solid surface mating, conjugation was allowed to take place on nutrient plates before washing off the cells for transconjugant selection on plates containing appropriate drugs. Plasmids that determine rigid pili transferred at least 2,000X better on plates than in broth. Some plasmids that determine thick flexible pili transferred 45 to 470x better, whereas others transferred equally well in both environments, as did plasmids of the I complex, which determine thin flexible pili. These results clearly distinguished a number of surface mating systems where most plasmids were derepressed for transfer and determined conjugative pili constitutively. The temperature-independent IncH2 plasmid R831b transferred best on plates, but other IncH plasmids transferred equally well in broth. This inconsistency led to the reclassification of R831b as IncM.

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SUMMARYFour Pseudomonas aeruginosa bacteriophages with non-contractile tails were thought to be pilus-dependent because they did not lyse pilus-less host mutants. They were, therefore, subjected to a series of tests based on the properties of known pilus phages. The lytic spectra of the phages and their efficiency of adsorption to various host mutants were compared with those of the RNA pilus phage PP7 and the tailed pilus phage PO4. Electron microscopy was used to locate the sites of adsorption of the candidate phages on sensitive host cells, and on cells of host mutants with non-retractile pili. In all respects, the four isolates behaved like known pilus phages, and it was concluded that they too were pilus-dependent. A detailed model for the adsorption process of one of the isolates (M6) is proposed on the basis of high-resolution electron micrographs.