The influence of the SOS response on the activity of 4-quinolones and zidovudine against some strains of Enterobacteria.

The 4-Quinolones are known to induce the SOS response. This should also be the case with AZT (Zidovudine) which has the same bactericidal mechanism. SOS response might make the bacteria more sensitive or more resistant to subsequent doses of quinolones and AZT. NA (Nalidixic acid), the first quinolone of the early 1960s, sensitises a strain of E. coli isolated from the urine of patients with cystopyelitis and the E. coli AB1157 wild type strain which is a well-known SOS inducer. In this case, the SOS system is not involved but only the recombination repair mechanisms which make the bacteria more susceptible to further damage by NA. On the contrary, CPX (Ciprofloxacin) protects E. coli from further exposure to antibiotics. Therefore the SOS response induction assists the bacteria in recovering from the DNA damage caused by CPX. The SOS response induced by AZT in the tested E. coli strains does not seem to either contribute to the lethality of the drug or to be involved in protecting bacteria from the damage caused by AZT. In fact, the percentage of killing was the same for both pre-treated and non pre-treated bacteria (p = 0.5). On the contrary, it was found that in Salmonella typhimurium belonging to blood of a patient with recurrent bacteriaemia, the CPX added to pre-treated bacteria with AZT was less lethal than when it was added to non pre-treated bacteria. The SOS response, in this case, protects bacteria from the damage caused by AZT.