Deletion mutants in the lpxM gene in two Yersinia pestis strains, the live Russian vaccine strain EV NIIEG and a fully virulent strain, 231, synthesise a less toxic penta-acylated lipopolysaccharide (LPS). Analysis of these mutants revealed they possessed marked reductions in expression and immunoreactivity of numerous major proteins and carbohydrate antigens, including F1, Pla, Ymt, V antigen, LPS, and ECA. Moreover, both mutants demonstrated altered epitope specificities of the antigens as determined in immunodot-ELISAs and immunoblotting analyses using a panel of monoclonal antibodies. The strains also differed in their susceptibility to the diagnostic plague bacteriophage L-413C. These findings indicate that the effects of the lpxM mutation on reduced virulence and enhanced immunity of the Y. pestis EV DeltalpxM is also associated with these pleiotropic changes and not just to changes in the lipid A acylation.
Vaccine. 2009 Apr 6;27(16):2240-50.

Two isogenic sets of Yersinia pestis strains were generated, composed of wild-type strains 231 and I-1996, their non-polar pH 6(-) mutants with deletions in the psaA gene that codes for its structural subunit or the whole operon, as well as strains with restored ability for temperature- and pH-dependent synthesis of adhesion pili or constitutive production of pH 6 antigen. The mutants were generated by site-directed mutagenesis of the psa operon and subsequent complementation in trans. It was shown that the loss of synthesis or constitutive production of pH 6 antigen did not influence Y. pestis virulence or the average survival time of subcutaneously inoculated BALB/c naïve mice or animals immunized with this antigen.
J Med Microbiol. 2009 Jan;58(Pt 1):26-36.

Increased interest in the pathogenic potential of Yersinia pestis has emerged because of the potential threats from bioterrorism. Pathogenic potential is based on genetic factors present in a population of microbes, yet most studies evaluating the role of specific genes in virulence have used a limited number of strains. For Y. pestis this issue is complicated by the fact that most strains available for study in the Americas are clonally derived and thus genetically restricted, emanating from a strain of Y. pestis introduced into the United States in 1902 via marine shipping and subsequent spread of this strain throughout North and South America. In countries from the former Soviet Union (FSU), Mongolia, and China there are large areas of enzootic foci of Y. pestis infection containing genetically diverse strains that have been intensely studied by scientists in these countries. However, the results of these investigations are not generally known outside of these countries. Here we describe the variety of methods used in the FSU to classify Y. pestis strains based on genetic and phenotypic variation and show that there is a high level of diversity in these strains not reflected by ones obtained from sylvatic areas and patients in the Americas.
Clin Microbiol Rev. 2004 April; 17(2): 434–464. doi: 10.1128/CMR.17.2.434-464.2004.

Guinea-pigs were infected subcutaneously or by respiratory challenge with plasmid-containing (pPst+pCad+pFra+) Yersinia pestis strain 358 and its pPst-pCad+pFra+, pPst+pCad+pFra- and pPst- pCad+pFra- derivatives, grown in vitro at 28°C or at 37°C. Lack of plasmid pPst did not lead to an increase in LD50 with either route of challenge. When the virulence of the four Y. pestis strains grown at the two temperatures was compared, the LD50 values of those grown at 37°C were lower. Respiratory challenge with cultures grown at 37°C mimics the man-to-man pneumonic plague cycle. The average LD50 values decreased c. two-fold and 10-fold for pPst+ and pPst- Y. pestis variants, respectively. The data suggest that historical epidemic outbreaks of pneumonic plague in the human population residing in the Caucasus region where there are natural plague foci in common voles may have been caused by pPst- Y. pestis strains.
J. Med. Microbiol. 1996; 45 (6): 440-444.

Insertion mutagenesis with the help of the plasmid pFS23 was used to generate Yersinia pestis fra mutants. The results of pFra- strain production under non-selective conditions suggested that such Y. pestis variants may be generated in natural plague foci at high frequency and may participate in supporting the epizootic process. Present data suggest that the reduction of virulence in Fra- strains reported by the majority of investigators was connected with the use of Y. pestis variants carrying additional unidentified mutations. It was shown that the loss of the ability to produce capsular antigen (FI) alone or in combination with absence of murine toxin production did not lead to an increase in LD50 absolute values. Simultaneous loss of these two virulence determinants did not influence the duration of survival of the infected animals. However, absence of only FI antigen production in the infecting strain resulted in prolonged survival of the infected animals. Conversion of plague infection from acute to chronic form is probably dependent on the animal host species and the Y. pestis parent strain subjected to mutagenesis.
J. Med. Microbiol. 1995; 42 (4): 264-268.