Genotypic alterations associated with avirulence in streptomycin-resistant Pasteurella pestis.

Brubaker, Robert R. (Fort Detrick, Frederick, Md.) and Michael J. Surgalla. Genotypic alterations associated with avirulence in streptomycin-resistant Pasteurella pestis. J. Bacteriol. 84:615-624. 1962.-Avirulence in Pasteurella pestis, as indicated by the existence of a new genotype, was associated with the genetic loss of Ca(++) dependence rather than with the usual concomitant loss of ability to produce virulence antigens. Strains of the new type exhibited all four of the known virulence factors of P. pestis but were avirulent for mice and guinea pigs (ld(50) > 10(8) cells). In addition to producing virulence antigens, the new strains formed rough colonies at 37 C when grown on Ca(++)-deficient medium, as opposed to typical avirulent strains which formed smooth colonies on this medium and failed to produce virulence antigens. Growth of the new strains in a corresponding Ca(++)-deficient broth medium was slow, as compared with that of typical avirulent strains, but was more rapid than that of virulent strains. The new strains were obtained (i) at 26 C by selecting for resistance to streptomycin or (ii) at 37 C by isolation from rough colonies growing on a Ca(++)-deficient agar medium plated with either a virulent streptomycin-sensitive or -resistant culture. Usually, isolates obtained at 26 C exhibited a marked reduction in ability to form colonies upon primary plating at 37 C. Of 108 streptomycin-resistant isolates obtained from six virulent strains, 20% were of the new type. In addition, five isolates were of reduced virulence (mouse ld(50) approximately 10(4) cells) but differed in being Ca(++)-dependent; they possessed all of the known virulence properties. Certain streptomycin-resistant strains failed to give rise to avirulent colonies when plated on a Ca(++)-deficient agar medium containing streptomycin. The significance of this phenomenon, which was not exhibited by the five strains of reduced virulence, is discussed, as are some apparent differences between isolates from a glycerol-fermenting strain and similar isolates obtained from five nonglycerol-fermenting strains.