Effect of Subinhibitory Concentrations of Antibiotics on Cell Surface Properties of Streptococcus gordonii and Staphylococcus aureus

ABSTRACT

Antibiotics were reported to be able to alter bacterial surface properties in subinhibitory concentrations (sub-MICs). The effects of sub-MICs of certain antibiotics on a bacterial surface property such as hemagglutination, as well as on the cell morphology were studied using Streptococcus gordonii and Staphylococcus aureus. The effect of sub-M1Cs of antibiotics on the binding of these bacteria to immobilized fibrinogen were also investigated. The MICs of antibiotics were determined by culturing S. gordonii and S. aureus in media supplemented with serially diluted drug solutions, and one-half the MIC was used as the sub-MIC of the drugs, unless stated otherwise. Sub-MICs of antibiotics did not affect bacterial agglutination of erythrocytes. Microscopic observation of S. gordonii grown at sub-MIC concentration of 0.02 ug/ml of amoxicillin revealed cell enlargement of 1.6 times those grown without the drug. When grown in the sub-MIC amount of 0.08 ug/ml of cefazolin, most S. gordonii cells were enlarged and elongated into rod-shape, resulting in 3 times the size of the cells grown without the antibiotic. The data from the fibrinogen-binding experiments showed that the binding of S. gordonii to immobilized fibrinogen was increased with all the B-lactam drugs tested; the binding of S. aureus to immobilized fibrinogen, on the other hand, was decreased with the same drugs. The results show that low concentrations of certain B-lactam antibiotics are able to cause alterations in cellular morphology of S. gordonii and affect the binding of S. gordonii and S. aureus to immobilized fibrinogen.