Differential effects of antibiotics on adhesins of antibiotic resistant strains of Escherichia coli

Sublethal concentrations of antibiotics that inhibit protein synthesis were found to diminish the ability of strains of Escherichia coli to bind to human leukocytes and to oral epithelial cells, guinea pig erythrocytes, and mannan-containing yeast cells. In general depressed adherence correlated with diminished production of type 1 fimbriae by the drug-treated bacteria and was affected by the antibiotics in the following order: aminoglycoside (gentamicin, streptomycin, neomycin) > spectinomycin > tetracycline > chloramphenicol. With one notable exception, mutation to resistance to antibiotic-induced growth inhibition resulted in resistance to the sublethal, anti-adherence effects of the same antibiotic. The exceptional strain, VL-2, was the only one out of many streptomycin-resistant strains that was not also resistant to the anti-adherence effects of subinhibitory concentrations of the drug. Compared to control cultures of VL-2, those grown in antibiotic demonstrated decreased amounts of mannose-sensitive hemagglutination (by > 99%) and adherence to human epithelial cells (58%) and leukocytes (93%). When treated bacteria were examined by electron microscopy, they were found to be as heavily fimbriate as control bacteria, but their fimbriae were twice as long. Moreover, fimbriae isolated and purified from drug-treated bacteria had no lectin-like properties. Thus, although most antibiotics diminished the adhesive properties only of antibiotic-sensitive bacteria, streptomycin caused one streptomycin-resistant strain of bacteria to produce aberrant (non-adhesive) fimbrial protein.