IDENTIFICATION OF VIRULENCE-ASSOCIATED CHARACTERISTICS IN CLINICAL ISOLATES OF YERSINIA-ENTEROCOLITICA LACKING CLASSICAL VIRULENCE MARKERS

Yersinia enterocolitica is an important enteric pathogen which has wel l-defined virulence determinants that allow the bacteria to become est ablished in their hosts and overcome host defenses. A number of strain s obtained from patients,vith diarrhea, however, lack these genes. Acc ordingly, the mechanisms by which they cause disease are uncertain. Mo st of these isolates belong to biotype 1A. Strains of this biotype are also frequently isolated from a variety of nonclinical sources, such as food, soil, water, and healthy animals, and there is evidence that some of these strains are avirulent. In this study we investigated 111 strains of Y. enterocolitica biotype 1A, 79 from symptomatic humans a nd 32 from nonclinical sources, for virulence-associated characteristi cs. DNA hybridization studies showed that none of the strains carried sequences homologous with pYV, the similar to 70-kb Yersinia virulence plasmid. Some strains hybridized with DNA probes for one of the follo wing chromosomal virulence-associated genes: ail (7.2%), myfA (11.7%), ystA (0.9%), and ystB (85%). In addition, 33 strains (29.7%) produced an enterotoxin that was reactive in infant mice. However, the frequen cies of these virulence-associated properties in clinical and nonclini cal isolates were similar. Clinical isolates invaded HEp-2 cells and C hinese hamster ovary cells to a significantly greater extent than nonc linical strains (P less than or equal to 0.002). In addition, clinical strains colonized the intestinal tracts of perorally inoculated mice for significantly longer periods than nonclinical isolates (P less tha n or equal to 0.01). Light and electron microscopic examination of tis sue culture cells incubated with invasive yersiniae revealed that the bacteria invaded selected cells in large numbers but spared others, su ggesting that biotype-1A strains of Y. enterocolitica may invade cells by a novel mechanism. These results indicate that some clinical isola tes of Y. enterocolitica which lack classical virulence markers may be able to cause disease via virulence mechanisms which differ from thos e previously characterized in enteropathogenic Yersinia species.