ABSTRACT. The existence of apathogenic strains of Yersinia pseudotuberculosis (Yp) has not so far been reported. Recently, the authors characterized new serogroups and a new subgroup in Yp, that is, O9, O10, O12, O13, O14 and O1c, and the pathogenicity of these new strains was of interest. A total of 137 strains of serogroups O1c, O6, O7, O9, O10, O11, O12, O13 and O14 of Yp were investigated for their pathogenicity in vivo and in vitro. Although catalase activity and the inv gene were detected in all strains except those of groups O13 and O14, only a few strains, from serogroups O6 and O10 caused severe infection in mice. The remaining strains caused no mortality or severe infection even when they grew in limited tissues of infected mice. All the strains of Yp not possessing the virulence plasmid pYV caused no severe infection in mice. It is evident that less pathogenic Yp exists and that not only pathogenic but also less pathogenic Yp organisms exist in the same serogroup. -KEY WORDS: new serogroup, non-pathogenic strain, Yersinia pseudotuberculosis.

The biological properties of non-melibiose-fermenting (NMF) strains of Yersinia pseudotuberculosis investigated.These strains were clearly distinguished from representative melibiose-fermenting (MF) strains of Y. pseudotuberculosis O3 by their pathogenicity in mice, sensitivity to some phages, production of catalase, restriction endonuclease analysis of virulence plasmid DNA with BamHI, detection of specific yersinia outer-membrane proteins with SDS-PAGE, antigenicity of the outer-membrane proteins and neutrophil resistance to phagocytosis. The pathogenicity of NMF strains was clearly less than that of MF strains. In addition, the resistance of NMF strains to phagocytosis and catalase activity was evidently weaker than that of MF strains. These results suggested that the difference of pathogenicity was due to the ability of catalase production. Although the relationship between the above characteristics and melibiose-fermentation was not analysed, the pathogenicity of Y. pseudotuberculosis O3 strains can probably be predicted by testing melibiose-fermentation and catalase production.

Thirty strains of Yersinia pseudotuberculosis were isolated from rabbits (17 strains), wild rats (9 strains) and house rats (4 strains) in China between 1990 and 1993. The biochemical properties of these isolates were identical with those of Y. pseudotuberculosis and no special characteristics were found in these strains. Serologically, serogroups 4b and 5b were identical to isolates found in Japan, and a new serogroup lc and unclassified strains have also been detected. The existence of virulence-associated properties were different among strains. The pYV plasmid was detected from 6 strains of 30 isolates. This report documents the presence of Y. pseudotuberculosis in China, providing important epidemiological information.