MUTATION TO RHAMNOSE UTILIZATION IN PASTEURELLA PESTIS ,

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Abstract
Sixteen strains of P. pestis, consisting of virulent, avirulent, glycerol-utilizing, and nonutilizing strains, are unable to utilize rhamnose as a C source but give rise to rare rhamnose utilizing mutants (R+) in old cultures on a modified Endo''s rhamnose agar medium. As demonstrated with strain A1122B1, mutants appear at a constant number per day over a period from 7 to 12 days of incubation and at a decreasing number per day thereafter from an initial inoculum free of R+, while the parent population, which reaches maximum growth within 2 days, then undergoes rapid death and lysis. Since 6 days of incubation are required for the detection of small numbers of R+ added to a large parent population, a mutation rate to R+ of 2.6 x 10-11 was calculated on the basis that all mutant colonies appearing during the 7th and 8th days of incubation occurred during the period of positive growth of the culture. Mutation to rhamnose utilization is not associated with any changes which would be required for the conversion of P. pestis to P. pseudotuberculosis. Because of the rarity of this mutat and the fact that P. pestis R+ requires approximately 4 days to produce a positive reaction on Endo''s agar as compared to 2 days required by P. pseudotuberculosis, rhamnose utilization as determined on Endo''s rhamnose medium is deemed a reliable criterion for distinguishing between these 2 organisms.