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Yersinia pestis, the causative agent of plague, has been responsible for at least three pandemics. During the last pandemic, which started in Hong Kong in 1894, the microorganism colonized new, previously unscathed geographical areas where it has become well established. The aim of this longitudinal study was to investigate the genetic stability of Y. pestis strains introduced into a new environment just under a century ago and to follow the epidemiology of any new genetic variant detected. In the present study, 187 strains of Y. pestis isolated between 1939 and 1996 from different regions of Madagascar and responsible mainly for human cases of bubonic and pneumonic plague were studied. Our principal genotyping method was rRNA gene profiling (ribotyping), which has previously been shown to be an effective scheme for typing Y. pestis strains of different geographical origins. We report that all studied Y. pestis strains isolated in Madagascar before 1982 were of classical ribotype B, the ribotype attributed to the Y. pestis clone that spread around the world during the third pandemic. In 1982, 1983, and 1994, strains with new ribotypes, designated R, Q, and T, respectively, were isolated on the high-plateau region of the island. Analysis of other genotypic traits such as the NotI genomic restriction profiles and the EcoRV plasmid restriction profiles revealed that the new variants could also be distinguished by specific genomic and/or plasmid profiles. A follow-up of these new variants indicated that strains of ribotypes Q and R have become well established in their ecosystem and have a tendency to spread to new geographical areas and supplant the original classical strain.

Several atypical sucrose-negative Yersinia strains, isolated from clinical samples and sometimes associated with symptoms, proved to have full virulence potential in in vitro and in vivo testings. DNA-relatedness studies revealed that they were authentic Yersinia enterocolitica strains. Therefore, atypical sucrose-negative Yersinia isolates should be analyzed for their virulence potential.

Yersinia pestis is the causative agent of plague, a disease which has caused the deaths of millions of people and which persists now in endemic foci. The rRNA gene restriction patterns (i.e., ribotypes) of 70 strains of Y. pestis, isolated on the five continents over a period of 72 years, were determined by hybridization with a 16S-23S rRNA probe from Escherichia coli. The combination of the EcoRI and EcoRV patterns resulted in the elucidation of 16 ribotypes. Two of them (B and O) characterized 65.7% of the strains studied, while the 14 other ribotypes were found in no more than three strains each. A relationship was established between biovars and ribotypes: strains of biovar Orientalis were of ribotypes A to G, those of biovar Antiqua were of ribotypes F to O, and those of biovar Medievalis were of ribotypes O and P. Great heterogeneity in rRNA restriction patterns was found among strains isolated in Africa; this heterogeneity was less pronounced among Asian isolates and was completely absent from the American strains. Pulsed-field gel electrophoresis was performed on the DNAs of some strains, but it appeared that different colonies from the same strain displayed different pulsed-field gel electrophoresis patterns and therefore that this technique was not suitable for comparison of Y. pestis isolates. In contrast, the ribotypes of individual colonies within a given strain were stable and were not modified after five passages in vivo. A clear correlation between the history of the three plague pandemics and the ribotypes of the strains could be established.