Background: Living organisms are exposed to oxidative stress due to internal or external stimuli. It results from the imbalance between the production and elimination of reactive oxygen species. This leads to loss of homeostasis. Objective: To test the effect of oxidative stress on the level of the production of reduced glutathione (GSH) as an antioxidant, malondialdehyde (MDA) as a measure of lipid peroxidation, and of the siderophore enterobactin as an oxidative stress response, in different bacterial species.
Materials and Methods: H₂O₂ minimum inhibitory concentration (MIC) was determined in Escherichia coli ATCC 25922 and Klebsiella pneumoniae ATCC 700603, using broth-macrodilution method. The levels of GSH and MDA were measured in E. coli ATCC 25922 and K. pneumoniae ATCC 700603 and in clinical isolates of E. coli, K. pneumoniae and Staphylococcus aureus after exposure to lethal H₂O₂ concentration, using Glutathione Reduced Kit and Lipid Peroxide-Malondialdehyde Kit, respectively. The level of expression of entC gene, involved in enterobactin biosynthesis, in presence of 0.25 and 0.5 MIC of H₂O₂ was determined using quantitative reverse transcription-polymerase chain reaction.
Results: H₂O₂ MIC for both E. coli ATCC 25922 and K. pneumoniae ATCC 700603 was 1.5 mM. Exposure of E. coli to H₂O₂ resulted in a significant increase in GSH (p=0.0001) and MDA (p=0.0001) levels. However, in K. pneumoniae, a significant decrease in the GSH (p=0.0001) and MDA levels (p=0.0001) was recorded upon H₂O₂ exposure. No change in MDA and GSH levels was detected in S. aureus isolates exposed to H₂O₂. The expression of entC gene in both E. coli ATCC 25922 and K. pneumoniae ATCC 700603 was reduced in presence of 0.25 and 0.5 H₂O₂ MIC.
Conclusion: Bacteria responded differently to oxidative stress, with S. aureus bacteria as the least affected by oxidative stress. Enterobactin role in oxidative stress needs reevaluation.