ANTIBIOTIC KILLING OF BACTERIA - COMPARISON OF BACTERIA ON SURFACES AND IN LIQUID, GROWING AND NONGROWING

The minimum inhibitory concentrations of antibiotics for bacterial pathogens are derived from broth suspensions (broth dilution) and from nutrient surfaces (agar dilution). These concentrations may not apply when bacteria are on a nonnutrient surface such as in a foreign body infection. We compared bacteria (Staphylococcus epidermidis and Escherichia coli) broth suspension MBCs with MBCs of the same bacteria when on a nonnutrient surface in broth the growing and nongrowing phases. Bacteria growing on cotton surfaces were much less susceptible to antibiotic killing than when freely suspended in the liquid nutrient. These results alone, independent of host factors, would explain the failure of antibiotics to eradicate infections involving bacteria on foreign body surfaces. The resistance of bacteria to antibiotic killing is not caused by a lack of antibiotic penetration to the site of the bacteria, but by an altered state of the bacteria when they are associated with a surface.