The objective was to study the susceptibility to ciprofloxacin (CIP) of Escherichia coli O157:H7 strains and relate to the bacterial oxidative stress.
Four clinical strains of E.coli O157:H7 provided by a pediatric hospital and the reference one were studied. The susceptibility to CIP was assayed by broth macrodilution method according to rules of Clinical and Laboratory Standards Institute (CLSI).
The generation of reactive oxygen species (ROS) was assayed in bacterial suspension
(OD600 1.0 ) grown overnight in tripticase soy broth. The suspensions were incubated without or with different concentrations of ciprofloxacin, then, nitro blue tetrazolium (NBT) was added for 30 min at 37ºC. The tubes were centrifuged and the extracellular ROS were measured at 575 nm. The pellets were treated with dimethyl sulfoxide to extract the reduced NBT (intracellular ROS).
The values of minimal inhibitory concentration (MIC) for CIP of the strains were between 0.0156 and 0.0312 µg/mL.
Reduction of NBT showed more intracellular than extracellular production of ROS in basal conditions, without antibiotic in all the studied strains. However, the clinical strains presented lower production of intra and extracellular ROS compared to the reference one.
There was a lower alteration of the oxidative metabolism in the clinical strains produced by ciprofloxacin compared to the reference strain. These results could suggest that these strains may have higher capacity of defense to the oxidative injury.
