The established Yersinia pestis biovars are characterized by typical patterns of I-CeuI restriction fragment length polymorphism.

The genomes of three main biovars of Yersinia pestis were subjected to restriction fragment length polymorphism analysis using I-CeuI endonuclease. I-CeuI which is encoded by a mobile intron in Chlamydomonas engamenans recognizes a 25-bp site in the ribosomal RNA rrl gene and cuts DNA of most representatives of Enterobacteriaceae into seven fragments corresponding to the presence of seven rrn-operons. Glycerol-positive Y. pestis strains (biovars antiqua and mediaevalis) contain seven ribosomal operons which can be recognized by I-CeuI endonuclease. However, glycerol-negative strains of Y. pestis biovar orientalis expose only six restriction sites for I-CeuI. The restriction fragment length polymorphism patterns obtained with I-CeuI make it possible to distinguish between three biovars of Y. pestis. Use of another rare cutting restriction enzyme, Bln/I, permits differentiation between pigment-adsorbing and avirulent non-pigment-adsorbing Y. pestis. Still, due to homologous recombination between the two copies of IS 100 insertion sequence bracketing the pgm-locus, the mechanism of deletions in the pgm-locus seems to be confined only to strains of biovars antiqua and mediaevalis, and can be different in Y. pestis strains of biovar orientalis. The I-CeuI restriction patterns of two Yersinia strains isolated within a ten-year period in the port of St. Petersburg and originally identified as Y. pseudotuberculosis 01 turned out to be related to typical representatives of Y. pestis biovar antiqua. These strains could be exported from the same source or circulate among Rattus norvegicus population of the port as non-pigment-adsorbing avirulent immunogenic clone.