Dissociation in Pseudomonas aeruginosa colonies

Abstract

Dissociation in Pseudomonas aeruginosa should be taken into consideration in establishing a typing method of the bacterium. The results of a study on the dissociation of the bacterium are reported in this paper.Nine strains of Pseudomonas aeruginosa were used for the purpose of isolating their dissociants. Of them, five had been developed from a single cell. Two kinds of colonial dissociants were isolated on an agar plate from each of the nine strains; that is a large, flat, and moist colony and a small, round, and convex colony. The former was designated as type la and the latter, as type sm. Investigation was carried out to find any differences between the two types, la and sm, in frequency of appearance under cultural conditions, cell morphology revealed by electron microscopy, antigenic structure, and sensitivities to bacteriophages, pyocines, and antibiotics.1) In broth type sm formed a pellicle which was not dispersed on shaking the tube. Electron microscopic examination showed that some of the cells of type sm were linked to one another by a string-like material. On the other hand, type la formed a homogeneous turbid culture in broth. It rarely formed a pellicle, which was easily dispersed on shaking the tube. Such string-like material as present in the cells of type sm was not observed in those of type la by electron microscopy.2) There could not be found any difference between the two types, la and sm, in serological specificity on the thermostable O antigen in the agglutinin-absorption test.3) Bacteriophage-sensitivity tests were performed on the dissociants derived from each of the five strains, using eight kinds of temperate phage preparations. As a result, remarkable difference were found in this sensitivity between the two types derived from each of three strains out of the five.4) Pyocine-sensitivity tests were performed on the dissociants of types sm and la derived from each of the seven strains, using fourteen kinds of pyocine preparations. There was a difference in this sensitivity between the two types derived from each of three strains out of the seven.5) Antibiotic-sensitivity tests were performed on the dissociants derived from each of the five strains. The results indicated no major difference between the dissociants of the given strains.6) The frequency of appearance of the different type arising from each of the two types, sm and la, derived from each of the five strains was investigated. Three different conditions of cultivation were employed: nutrient broth at 37C overnight; nutrient broth under shaking at 37C for 20 hours; and nutrient agar slant, first at 37C overnight and then left at 4C for fifteen days. The frequency of appearance of the different type that had arisen from each of the two types was 0.1% or less in most cases and 10% or more in a few cases.From the results described above, it was concluded that the bacteriophage typing and the pyocine typing should be applied carefully to the characterization of Pseudomonas aeruginosa, since there were remarkable differences among the dissociants in sensitivity to phages and pyocines. The serological typing using thermostable O antigen is considered to be a most reliable method for the characterization of Pseudomonas aeruginosa, if it is performed with the cautions mentioned in the text, because there was found no qualitative differences in thermostable antigen between type sm and la.