In Vitro Resistance of the Genus Bacteroides to Streptomycin

Abstract

Strains of Bacteroides were classified as B. funduliformis if pleomorphic (4) and if not B. fragilis (8). Streptomycin sensitivity was detd. in unsealed 0.1% Na thioglycollate broth containing 0.1% agar, according to a titration based on the method of Price, Nielson, and Welch, with examination after 48 hrs.'' incubation. The first tube showing growth inhibition was considered the minimal inhibiting dose. All strains grew in the presence of 100 U./ml. of penicillin. B.funduh-formis was slightly more sensitive to streptomycin than was B. fragilis, but for both, minimal inhibitive doses exceeded those obtainable in vivo. To detm. the effect of Na thioglycollate on the in vitro streptomycin resistance of Bacteroides, the strains used were tested for sensitivity in boiled tryptic digest broth with 0.1% dextrose. Each titration tube was sealed and incubated at 37[degree]C for 48 hrs. There was no significant difference between the results obtained by this method and those obtained in Na thioglycollate with the same strains. As streptomycin concs. needed to inhibit the Bacteroides growth are high, Bacteroides is categorized as highly streptomycin-resistant. Certain strains, with one exception, grew in the presence of sulfathiazole and sulfadi-azine.