Effect of twelve antimicrobial drugs on the colonization resistance of the digestive tract of mice and on endogenous potentially pathogenic bacteria.

Twelve antimicrobial drugs were studied for their effect on the endogenous aerobic potentially pathogenic bacteria (Enterobacteriaceae, Streptococcus faecalis) in the intestines and on the colonization resistance (CR) of the digestive tract. Three subclasses of antimicrobial drugs could be recognized: (1) those which suppress the CR following low oral doses (rifamycin, penicillin V, cloxacillin, fenethicillin); (2) those in which the CR is suppressed only following relatively high oral doses (amoxycillin); and (3) those in which no obvious suppression of the CR was noticed even following substantial oral doses (nalidixic acid, cinoxacin, co-trimoxazole, oral cephalosporins, piv-mecillinam and doxycyclin). Some of the drugs in the third category were found to suppress endogenous Enterobacteriaceae (nalidixic acid, co-trimoxazole, piv-mecillinam and doxycyclin) and S. faecalis (doxycyclin) at dose levels at which they did not decrease CR.