The SOS response is a conserved response to DNA damage that is found in Gram-negative and Gram-positive bacteria. When DNA damage is sustained and severe, activation of error-prone DNA polymerases can induce a higher mutation rate than is normally observed, which is called the SOS mutator phenotype or hypermutation. We previously showed that zinc blocked the hypermutation response induced by quinolone antibiotics and mitomycin C in Escherichia coli and Klebsiella pneumoniae. In this study, we demonstrate that zinc blocks the SOS-induced development of chloramphenicol resistance in Enterobacter cloacae. Zinc also blocked the transfer of an extended spectrum beta-lactamase (ESBL) gene from Enterobacter to a susceptible E. coli strain. A zinc ionophore, zinc pyrithione, was ~100-fold more potent than zinc salts in inhibition of ciprofloxacin-induced hypermutation in E. cloacae. Other divalent metals, such as iron and manganese, failed to inhibit these responses. Electrophoretic mobility shift assays (EMSAs) revealed that zinc, but not iron or manganese, blocked the ability of the E. coli RecA protein to bind to single-stranded DNA, an important early step in the recognition of DNA damage in enteric bacteria. This suggests a mechanism for zinc's inhibitory effects on bacterial SOS responses, including hypermutation.
BackgroundThe SOS response is a conserved response to DNA damage that is found in Gram negative and Gram-positive bacteria. When DNA damage is sustained and severe, activation of error-prone DNA polymerases can induce a higher mutation rate then normally observed, which is called the mutator phenotype or hypermutation. We previously showed that zinc blocked the hypermutation response induced by quinolone antibiotics and mitomycin C in E. coli and Klebsiella pneumoniae (Bunnell BE, Escobar JF, Bair KL, Sutton MD, Crane JK (2017). Zinc blocks SOS-induced antibiotic resistance via inhibition of RecA in Escherichia coli. PLoS ONE 12(5): e0178303. https://doi.org/10.1371/journal.pone.0178303.) In addition to causing copying errors in DNA replication, Beaber et al. showed that induction of the SOS response increased the frequency of horizontal gene transfer into Vibrio cholerae, an organism naturally competent at uptake of extracellular DNA. (Beaber JW, Hochhut B, Waldor MK. 2003. SOS response promotes horizontal dissemination of antibiotic resistance genes. Nature 427:72–74.) Methods. In this study, we tested whether induction of the SOS response could induce transfer of antibiotic resistance from Enterobacter cloacae into E. coli, and whether zinc could inhibit that inter-species transfer of antibiotic resistance. Results. Ciprofloxacin, an inducer of the SOS response, increased the rate of transfer of an extended spectrum β-lactamase (ESBL) gene from Enterobacter into a susceptible E. coli strain. Zinc blocked SOS-induced horizontal transfer of §-lactamase into E. coli. Other divalent metals, such as iron and manganese, failed to inhibit these responses. Conclusion. In vitro assays showed that zinc blocked the ability of RecA to bind to ssDNA, an early step in the SOS response, suggesting the mechanism by which zinc blocks the SOS response. Disclosures All authors: No reported disclosures.
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