Response of Escherichia coli to Beta-lactam antibiotics: effect of the concomitant presence of staphylococci exhibiting inducible Beta-lactamase activity.

The response to Beta-lactam antibiotics of mixed cultures of Escherichia coli and Staphylococcus epidermidis was studied in static culture and in an in vitro model which simulates the dynamic conditions in which bacteria are exposed to antibiotics in the treatment of urinary infection. In static cultures, the concomitant presence of staphylococci exhibiting inducible Beta-lactamase activity substantially reduced the efficacy of benzylpenicillin and ampicillin (but not cefuroxime) against E. coli. In the conditions of the bladder model some interference with the activity of Beta-lactam antibiotics by Beta-lactamase producing staphylococci was also demonstrated. Nevertheless, relatively modest doses of ampicillin were still able to suppress growth of susceptible E. coli for periods exceeding the normal interdose interval, even in the presence of enzyme-producing staphylococci.