Siderophores in clinical isolates of Klebsiella pneumoniae promote ciprofloxacin resistance by inhibiting the oxidative stress

To explore the relevance of and understand the potential mechanisms behind the production of siderophores by clinical isolates of K. pneumoniae and ciprofloxacin (CIP) resistance, we divided the K. pneumoniae isolates into two groups based on bacterial siderophores production: high siderophore-yielding group (39 strains) and low siderophore-yielding group (38 strains). The rate of CIP resistance in K. pneumoniae (27/39 = 69.23%) from the high siderophore-yielding group was significantly higher than that (16/38 = 42.11%) in the low siderophore-yielding group (p < 0.05). Furthermore, we noted that bacterial siderophores production was positively correlating with CIP resistance as indicated by the minimum inhibitory concentration (MIC; p < 0.05). However, siderophore-related antibiotic resistance had no relationship with DNA gyrase GyrA mutation (p > 0.05). Siderophore-related antibiotic resistance was accompanied by efflux pump functions, but was not directly relevant to it. Furthermore, we found that the oxidative stress response was significantly lower in high siderophore-yielding strains compared to those isolates which had a low siderophores yield (12.17 vs. 30.91 of average fluorescence value; p < 0.01). There was a consistent inverse correlation between the production of bacterial siderophores and oxidative stress response (p < 0.05). Although CIP induced oxidative stress in both high and low siderophore-yielding strains (p < 0.01), oxidative stress in high siderophore-yielding strains was significantly lower than in low siderophore-yielding strains (p < 0.01). Our data suggest that siderophores of K. pneumoniae clinical isolates promote CIP resistance through inhibition of the bacterial oxidative stress response, indicating that reduction of bacterial oxidative stress could provide a new avenue for control of bacterial drug resistance.