Influence of cefaclor, phenethicillin, co-trimoxazole and doxycycline on colonization resistance in healthy volunteers.

The influence of oral administration of cefaclor, phenethicillin, co-trimoxazole and doxycycline on colonization resistance (CR) of the oropharynx and colon in healthy volunteers was studied. Antimicrobial agents were administered in a randomized cross-over design. No effect on CR of the oropharynx could be demonstrated. Phenethicillin decreased CR of the colon against Enterobacteriaceae (P = 0.001). Co-trimoxazole significantly decreased the concentration of Enterobacteriaceae in faeces (P = 0.03) but the decrease caused by cefaclor and doxycycline did not reach statistical significance. Administration of antimicrobial agents increased the appearance of secondary colonization by Enterobacteriaceae in faeces, especially when Escherichia coli was eliminated. During administration of phenethicillin, secondary colonization occurred at a concentration exceeding 10(7)/g in some volunteers. Following administration of cefaclor, co-trimoxazole and doxycycline, elimination of E. coli may result in the substitution by resistant Gram-negative bacilli in low concentrations.