Distribution of Pseudomonas syringae pathovars into twenty-three O serogroups.

Serological reactions of Pseudomonas syringae and Pseudomonas viridiflava were studied by Ouchterlony double diffusion. A total of 55 polyclonal antisera, containing anti-lipopolysaccharide (anti-LPS) precipitating antibodies, were cross-tested against antigenic suspensions of 51 strains. Twenty-three O serogroups were defined, primarily on the reaction of the type strains. Two families of O serogroups showed antigenic crossreactivities (PHA, MOP1, MOP2, MOP3, HEL1, HEL2, and SYR1; PERSAVTOM1, PERSAVTOM2, DEL, POR, and SYR2). Ten O serogroups showed a clearcut specificity: APTPIS, TAB, VIR1, VIR2, VIR3, SYR3, SYR4, SYR5, HUS, and LAC. The last serogroup (RIB) contained strains with rough colony morphology and side chain-deficient LPSs, as evidenced by sodium dodecyl sulfate-polycrylamide gel electrophoresis. The LPS basis of the O serogroups was demonstrated by immunoblotting. Serological reference strains were designated for all of the O serogroups and correspondence was established between the O serogroups studied and seven previous serogroups (L. T. Pastushenko and I.D. Simonovich, Mikrobiol, Zh. 41:222-229 and 330-339, 1979). A total of 355 strains of P. syringae (sensu lato) belonging to 15 pathovars, not including pathovar syringae, were typed into the 23 described O serogroups. O serogroups were assigned after double-diffusion reactions, with each strain compared with serological references. The utility of O serogrouping to study P. syringae pathovar structure and diversity is discussed.