In Vitro Susceptibility of Shigellae to Sodium Sulfadiazine and to Eight Antibiotics

More than 300 recently isolatedShigellastrains were tested for susceptibility, to sodium sulfadiazine by tube and plate dilution techniques. With a low inoculum (100 organisms), 59% ofShigella flexneriand 87% ofS sonneiwere sulfadiazine resistant. All strains were resistant by high inoculum testing (2 × 10⁶organisms). Antibiotic susceptibility testing by the plate dilution method using high inocula demonstrated by the following percentages of resistance: potassium penicillin G, 93%; streptomycin, 17%; tetracycline hydrochloride, 12%; chloramphenicol, 11%; ampicillin, 6%; sodium colistimethate, 2%; kanamycin sulfate, 1%; and neomycin sulfate, 1%. These studies indicate that sulfadiazine is no longer an appropriate drug for initiating treatment of shigellosis. Ampicillin and three orally administered non-absorbable antibiotics (colistimethate, kanamycin and neomycin) are the most effective drugs against shigellae in vitro.