The isolation and characteristics of variant plague microbes not producing the capsular antigen

: Fifteen stable variants of Yersinia pestis strains exhibiting different degrees of virulence for white mice and guinea pigs were obtained. Multiple passages of the organisms at 37 degrees C in a fluid nutrient medium containing antiplague agglutinating serum were found to be the most efficient method for obtaining noncapsular forms of the plague agent. Acquisition of the Fra(-) phenotype both by wild and laboratory strains was not associated with a loss of the high-molecular plasmid by cells but was probably a result of mutational alterations in the plasmid genes.