Development of Streptomycin-resistant Variants of Meningococcus

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Abstract
Plates were prepared with streptomycin in concs. from 10 to several thousand [gamma]/ml. of media, and each was inoculated with meningococci. The plates were examined after 24, 48, and 72 hrs.'' incubation. Normal colonies developed on streptomycin concs. of 10-20 [gamma]/ml., but not when concs. were above 40 [gamma]/ml. On higher concs. variant colonies called A and B developed. A variant appeared in approx. equal numbers on all cones, of the drug, but the numbers of colonies per plate varied among strains. It is probable that A arises from streptomycin-resistant mutants. Variant A differed from the original culture in size and color of colony and in being streptomycin-resistant in vitro and in vivo. It multiplies on any medium which supports meningococcal growth. It is as virulent for mice as the original culture and produces infection against which no dose of streptomycin affords protection. The B variant colonies all required streptomycin for reproduction in vitro and in vivo. Variant B was not virulent for mice. However, mice inoculated intraperit. with mucin suspensions and treated with 500 to 5,000 [gamma] of streptomycin developed fatal meningococcal sepsis. Meningococci could be recovered from hearts'' blood in cultures made on streptomycin-containing media but not in duplicate cultures on streptomycin-free media. The dependence of B variants on streptomycin for growth persists after multiplication within the body of an infected host. The origin of the B variant is hard to explain except as current mutation. The numbers of colonies developing from equivalent inocula were always greatest on cones. from 100 to 400 [gamma]/ml. whether the seedings were made from a parent strain or from a type B subculture taken from a high or low conc. of streptomycin. The results indicated that the B variants were alike genetically and that the higher reproductive rates at those cones. reflected a functional response to the drug rather than a differential induction of the variants at diff. concs. The variation in color and size of their colonies in relation to the conc. of the drug was further evidence that streptomycin directly affected the physiology of the bacterial cells.

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