Effects of gentamicin on the intestinal bacterial flora of rats

1) Normal intestinal microbial flora (NF) in feces of rats were examined before and after administration of gentamicin (GM). Although GM caused no influence on NF after 6 hours, the number of NF decreased markedly 24 hours after GM, viz., the count of coliforms, enterococci and lactobacilli was reduced to 1/10-1/100 of the control value while proteus and fusobacteria declines in number down to 1/100-1/1,000 times as low as the control value. 2) The decreased count of NF tended to recover to some degree at the end of the 1st therapeutic week and the considerably recovered count continued thereafter indicating about 1/10 times as low as the control value until a 12-week experimental period, except that the count of lactobacilli returned to almost normal. 3) Susceptibility of coliforms isolated from GM-treated rats to GM was nearly equal to that of the bacteria isolated from the untreated control rats, which revealed no development of drug resistance to GM. 4) The increasing rate of body weight was higher in GM-treated rats than in the control, and the feed intake was consequently great in the former.