Isolation of an Escherichia coli mutant susceptible to a quinolone in an anaerobic environment

Quinolones are bactericidal agents that interfere with the essential prokaryotic enzyme DNA gyrase. While their mechanism of killing appears to be elucidated, one interesting feature is represented by the fact that, under anaerobic conditions, the growth of bacteria is inhibited but their viability is not affected by the first generation of quinolones such as nalidixic acid. More information about the mode of action of these drugs in anaerobiosis might be gained through the availability of strains subjected to enhanced killing in oxygen-deprived media. It has been assumed that when a population of a AB1157(F’lac) strain is exposed to nalidixic acid, plasmid-free cells could be recovered from culture treated with sub-inhibitory concentrations of the drug (2 mg/L) in aerobiosis, and, at the same drug level, only from the rare spontaneous susceptible mutant(s) in anaerobiosis. Among plasmid free bacteria found, 1 isolate demonstrated the same MIC value to nalidixic acid in both aerobic and anaerobic conditions. The mutation was co-transferred with Tn10 inserted at 28.5 min of the Escherichia coli genetic map into a wildtype strain. These transductants revealed the same phenotypes of the original mutant: susceptibility to nalidixic acid under anaerobic conditions (assessed by time-kill tests) and elongated cells during the aerobic growth, generation time about 65 min in comparison to 25 min of the control. Time kill experiment under aerobic environment revealed that the transductant was also susceptible to ciprofloxacin but not nalidixic acid in the presence of chloramphenicol (50 mg/L). These results suggest a possible role of bacterial topoisomerase in the anaerobic susceptibility to nalidixic acid of the mutant.