The effect of penicillins on the gastrointestinal microbiota is summarized in Table 1. Phenoxymethylpenicillin

Phenoxymethylpenicillin has been shown to induce minor variations in numbers of aerobic and anaerobic gastrointestinal microorganisms in healthy adults (5,6) and in infants treated for upper or lower respiratory tract infections or otitis media (7). Penicillin that reaches the gastrointestinal tract is destroyed by beta-lactamase produced by the microorganisms. Despite the low concentration of the agent in feces, generally under the detection level, occasional new colonization with Gram-negative aerobic rods has been observed during administration.


Ampicillin has a broader antimicrobial spectrum than phenoxymethylpenicillin and is active also against species of Gram-negative microorganisms. The effect on the normal gastrointestinal microbiota is moderate with suppressed numbers of enterococci, streptococci, corynebacteria and enterobacteria. Minor effects on anaerobic species have also been observed in one study. Overgrowth of resistant aerobic Gram-negative rods is common and occasionally also of Candida species (8-10). The disturbances are increasing with increased doses.


The impact of ampicillin/sulbactam on the intestinal microbiota has been studied in patients undergoing colorectal surgery (11,12). From an ecological point of view it should be expected that it would be less favorable to combine ampicillin with a beta-lactamase inhibitor like sulbactam since the antimicrobial spectrum increases. The effect in particular on the aerobic microbiota has been shown to be mild while the number of anaerobic microorganisms was suppressed. With higher doses, overgrowth of yeasts has been observed and occasionally also overgrowth of Pseudomonas fluorescens.


Amoxicillin is an agent closely related to ampicillin and with a similar spectrum. Amoxicillin is acid-stable and is therefore better adsorbed. Overgrowth and emergence of amoxicillin-resistant enterobacteria have been the main outcome in studies on the effects on the normal gastrointestinal microbiota both in patients (13,14,16,19), in healthy

Table 1 Impact of Penicillins on the Intestinal Microbiota


Dose mg/day

Days of administration

Number of subjects

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