Combination of bacteriostatic and bactericidal drugs: lack of significant in vitro antagonism between penicillin, cephalothin, and rolitetracycline.

Although it is generally believed that bactericidal and bacteriostatic drugs should not be combined in vivo, in vitro experiments using the checkerboard dilution technique revealed no antagonism between penicillin/cephalothin and rolitetracycline but rather additive or synergistic activity of either drug combination in 40 to 50% of 20 Escherichia coli, and 14 Staphylococcus aureus strains. Slight antagonism occurred only between 3 and 8 h after combining penicillin/cephalothin and rolitetracycline in either bacteriostatic or bactericidal concentrations, but not after 24 h of incubation, nor was antagonism found with combinations of these drugs in bacteriostatic concentrations. Neither bacteriostatic nor bactericidal activity of penicillin/cephalothin and rolitetracycline was inhibited by pretreatment of one E. coli strain with bacteriostatic rolitetracycline or bacteriostatic penicillin/cephalothin concentrations. Penicillin and cephalothin could exert a bactericidal effect after 2-h exposure of the E. coli strain to bacteriostatic rolitetracycline concentrations. Combined action of subinhibitory penicillin and rolitetracycline concentrations resulted in more pronounced inhibition of growth than either drug alone. The higher activity of penicillin/cephalothin in combination with rolitetracycline on some E. coli and S. aureus strains might be due to a better access of rolitetracycline into bacterial cells whose cell walls have been weakened by cell wall-active, bactericidal drugs. Thus, growth of penicillin-induced spheroplasts of E. coli and stable staphylococcal L-forms was inhibited by much lower concentrations of rolitetracycline than were the corresponding parent cells with intact cell walls.