Heat-stable somatic antigens of a group of unclassified fluorescent pseudomonads (UFP).

(i) Isolates belonging to a group of unclassified fluorescent pseudomonads (UFP) are similar to Pseudomonas aeruginosa not only in cultural behaviour but also in basic serological properties of their somatic antigens. Living bacteria and cultures subjected to prolonged heating at 100 degrees C or above agglutinated readily in homologous O serum, whereas after exposure to 60 degrees C, ethanol, saturated sodium chloride and formalin the cells of both species became practically inagglutinable. Surface factors inhibiting the agglutination of living bacteria in O sera being absent, the slide technique was chosen as a routine method for the serological grouping of UFP. (ii) The O antigens of UFP were different in serological specificity from those of P. aeruginosa: the two organisms were related only by minor "common" antigens detectable with bacteria heated at 130 degrees C. One hundred and ten out of 115 UFP isolates were classified into 17 serological groups; O groups 2, 5, 10, 12 and 16 were each further divided into two subgroups. Five isolates reacted in several sera or were unstable. (iii) Serological grouping of UFP is reproducible and adequate for the tracing of isolates and may be helpful for a rapid differentiation of the organism from P. aeruginosa.