Effect of Oral Neomycin on Normal Intestinal Flora of Dogs and Man

1. Ten random strains of each of the following species were sensitive in vitro (3 exceptions in 40 cultures) to 6.2 μg/cc or less of neomycin in a 48-hour nutrient broth culture: E. coli, A. aerogenes, Pseudomonas aeruginosa, B. proteus vulgaris, Hemolytic staphylococcus aureus and Hemolytic streptococcus. In 5 strains of enterococci the range of sensitivity was 25-3.2 μg neomycin/cc, while Clostridia were not sensitive at a concentration of 200 μg/cc or higher. When neomycin is effective, its antibacterial action is bactericidal and not bacteriostatic under the conditions of the test(8). 2. Oral administration of neomycin usually eliminates coliform bacilli from stool cultures in dogs and in man within 48 hours and frequently within 24 hours. Continuation of the drug for 5-6 days resulted in the appearance of resistant strains of E. coli in dogs. Such resistance was not observed in 25 humans. 3. The numbers of Enterococci, Ps. aeruginosa, and B. proteus vulgaris in the stool are reduced in most cases by oral neomycin therapy. 4. Clostridia are not noticeably affected by neomycin.