4-QUINOLONE BACTERICIDAL MECHANISMS

The bactericidal activity of nalidixic acid against Escherichia coli s train KL16 in nutrient broth was abolished by the addition of rifampic in. Cells suspended in phosphate-buffered normal saline (PBS) were als o not killed by nalidixic acid. Experiments with modern 4-quinolones s howed their activities varied according to the conditions under which they were tested. Rifampicin did not affect the concentration at which ofloxacin became bactericidal in nutrient broth, but did limit the ex tent of ofloxacin-induced death. However, rifampicin produced a 10-fol d increase in the concentration at which ciprofloxacin became bacteric idal in nutrient broth, and completely abolished the bactericidal acti vity of norfloxacin. Unlike nalidixic acid all of the modern 4-quinolo nes killed cells suspended in PBS. Based on these results it was possi ble to differentiate 3 processes by which 4-quinolones induced death. Mechanism A was only active against dividing bacteria and required RNA and protein synthesis; it was therefore not active against bacteria s uspended in PBS and was inhibited in nutrient broth by the addition of rifampicin. Mechanism B required neither RNA nor protein synthesis an d was also active against non-dividing bacteria; it was therefore not inhibited by rifampicin nor by suspending bacteria in PBS. Mechanism C killed non-dividing bacteria, but required protein and RNA synthesis; it therefore functioned in PBS, but was inhibited by rifampicin. Dete rmination of the concentrations at which these mechanisms became opera tive showed that four times as much ciprofloxacin and five times as mu ch ofloxacin were required for initiation of mechanism B as for mechan ism C and that the concentration at which mechanism C of norfloxacin b ecome effective (0.2 mug/ml) correlated extremely well with the concen tration previously shown to inhibit DNA supercoiling.