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Superinfections originating from a digestive tract colonized by abnormally high concentrations of aerobic microorganisms as a result of impaired resistance to colonization (CR) may complicate antibiotic therapy. In this study, patients with a moderate to severe systemic infection were randomized to receive either cefotaxime (CTX, n = 10) or cotrimoxazole (CTR, n = 10), 2 antibiotic regimens presumed to spare CR; or imipenem/cilastine (I/C, n = 19). The effect on CR was measured indirectly by comparing the aerobic faecal flora before antibiotic treatment with that on day 8 of treatment. An increase in aerobic faecal flora denotes a disturbed CR, whereas a decrease means that the organism is sensitive to the effective faecal concentration of the antibiotic. Imipenem/cilastine-treated patients showed a significant increase in enterococci and Candida spp., while the number of aerobic Gram-negative rods remained constant. Cefotaxime-treated patients had evidence of an increase in enterococci, but not of Candida spp., and Escherichia coli numbers decreased significantly. In these patients the concentration of other Gram-negative aerobic rods showed a slight increase in 6 patients with a resistant Pseudomonas strain. Cotrimoxazole-treated patients showed a significant decrease in aerobic Gram-negative rods, a significant increase in Candida spp. and no change in enterococci. It is concluded that all 3 antimicrobial agents impair colonization resistance. Whether or not this is followed by overgrowth with resistant micro-organisms depends on the active faecal concentration of the antimicrobial agent and the MIC of the aerobic micro-organisms. The risk of overgrowth of the bowel with resistant Gram-negative bacilli appears to be smaller following cotrimoxazole than following cefotaxime or imipenem/cilastine.

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