Forty-two cultures of pseudomonas comprising 28 clinical isolates from a pseudo-outbreak on a Special-Care Baby Unit and 14 reference strains, including 9 type strains, of various Pseudomonas species, were characterized by one-dimensional sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) of whole-cell proteins. The protein patterns were highly reproducible and were used as the basis for a numerical analysis which divided the strains into 9 phenons. Two of the 28 clinical isolates were identified by biochemical tests as P. pickettii and their identification was confirmed by SDS–PAGE as they fell in the same phenon as the type strain of the species. The remaining 26 isolates, which could not be identified on phenotypic tests, fell in the same phenon as three reference strains of ‘P. thomasii’. The protein patterns provided the first clear evidence that P. pickettii and ‘P. thomasii’ were separate taxa and that the ‘outbreak’ was polymicrobial in origin, in line with the probable aqueous source of contamination. We conclude that high-resolution SDS–PAGE of proteins provides an effective method of identifying and differentiating pseudomonads, especially where this cannot be done adequately using conventional biochemical tests.
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