Post-antibiotic effects of ofloxacin on Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Streptococcus pyogenes.

A viable counting technique was used to determine the post-antibiotic effect (PAE) of ofloxacin against four bacterial species, treated with either once of four times the minimum inhibitory concentration for 1 or 3 h. Similar to the results obtained previously with ciprofloxacin, ofloxacin gave PAE values with Escherichia coli, Staphylococcus aureus, and Streptococcus pyogenes. Cell division of Klebsiella pneumoniae was inhibited on removal of ofloxacin, but no clear PAE was demonstrated with this species because once replication recommenced, the mean generation times of drug-treated cultures were much shorter than those of untreated controls. Therefore, although the results obtained with ciprofloxacin and ofloxacin imply a consistency of PAE for 4-quinolones within a species, the response to DNA damage induced by 4-quinolones is multifaceted and species dependent. 4-quinolones inhibit both DNA replication and cell division, whilst at the same time stimulating DNA repair pathways. Thus, in some cases PAEs result from an increased post-treatment lag phase which may be followed by nearly normal multiplication, whereas in other cases a long lag may be followed by abnormally rapid cell division, with the generation times of treated cultures being shorter than those of corresponding drug-free controls. The PAE of a drug-induced lag may thus be masked by rapid cell division once growth resumes.