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Role of bacterial antioxidant defense in their resistance to bactericidal antibiotics

A. C. Matin

Statement of the problem: Bacterial antibiotic resistance is a world-wide public health problem requiring and new approaches. Background: Sigma S (ss) controls the synthesis of proteins that contribute to the resistance of bacteria like uropathogenic Escherichia coli (UPEC) in the stationary phase of growth, where bacteria are most virulent; ss is encoded by the rpoS gene. Methodology: Colony forming unit formation was used to determine antibiotic sensitivity; a novel microfluidic device determined sensitivity at single-cell level. Results: Lack of rpoS increased UPEC sensitivity to bactericidal antibiotics: gentamicin (Gm), ampicillin and norfloxacin. Gm will be discussed to illustrate the findings with the three antibiotics. Global proteomic analysis implicated weakened antioxidant defense. Use of the psfiA genetic reporter, 3-(p-hydroxyphenyl) fluorescein (HPF) dye, and Amplex Red showed that Gm generated more oxidative stress in the mutant. Cell elongation can compromise the results of HPF, but the antibiotic treatment did not affect the dimensions of stationary phase bacteria. The antioxidant, N-acetyl cysteine (NAC), & anaerobiosis decreased drug lethality. Thus, greater oxidative stress caused by. insufficient quenching of endogenous ROS and/or respiration-linked electron leakage contributed to the increased sensitivity of the mutant; this was confirmed also in vivo. Eliminating of quencher proteins, SodA/SodB and KatE/SodA, or the pentose phosphate pathway proteins, Zwf/Gnd and TalA, (source of NADPH required by the quenchers), also generated greater oxidative stress and killing by Gm. The results were confirmed at single-cell level, as well as under microgravity during space flight where astronaut immune response is compromised. Conclusion and Significance: Besides their established mode of action, bactericidal antibiotics also kill bacteria by oxidative stress. Targeting the antioxidant defense will therefore enhance their efficacy. Bioinformatic approaches have identified small molecules that inhibit these proteins and are under study.