The effect of antimicrobial administration on the percentage of tetracycline‐resistant facultative and anaerobic bacteria in the intestines of humans and animals was studied. Tetracycline was chosen because it is an antibiotic commonly used both as a feed supplement for animals, and therapeutically for humans and animals. More than 200 faecal specimens of bovine, porcine, equine, canine, feline and human origin were collected into pre‐reduced anaerobic media. Ninety‐one subjects from groups receiving antimicrobials as feed supplements, therapeutically or with no recent antibiotic exposure were included in the study. Counts of both facultative and anaerobic bacteria were determined from media with and without tetracycline. The percentage of normal faecal specimens harbouring tetracycline‐resistant coliforms was 56 for humans. 100 for calves. 100 for pigs. 36 for horses, 90 for dogs and 67 for cats. All samples contained Gram negative anaerobes resistant to tetracycline. Subjects from all species receiving antimicrobials showed higher percentages of tetracycline‐resistant coliforms and anaerobes than subjects not receiving antibiotics. Greater increases in the percentage of tetracycline‐resistant facultative organisms were noted in subjects which received antibiotics as feed supplements compared with subjects which were given therapeutic doses of antibiotics. Percentages of tetracycline‐resistant anaerobes showed the opposite trend with more increase among those subjects receiving therapeutic levels of antimicrobial agents.
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AHART et al. (1978) studied this question.
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