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

More than 300 recently isolated Shigella strains were tested for susceptibility, to sodium sulfadiazine by tube and plate dilution techniques. With a low inoculum (100 organisms), 59% of Shigella flexneri and 87% of S sonnei were sulfadiazine resistant. All strains were resistant by high inoculum testing (2 × 10⁶ organisms). 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.