Sublethal concentrations of heavy metals Cu2+ and Zn2+ can induce the emergence of bacterial multidrug resistance

Abstract

The emergence of multidrug resistance has become an environmental safety issue of worldwide concern. Numerous studies have confirmed that heavy metals can promote the spread of bacterial resistance. However, in addition to co-resistance plasmids, the role and mechanism of metals at sublethal levels (< minimum inhibitory concentration [MIC]) on antibiotic resistance remain poorly defined. Here we investigated the alteration of resistance phenotype under Cu 2 + and Zn 2 + exposure and explored the molecular mechanism by which heavy metals affect antibiotic resistance. The results showed that the MIC of Escherichia coli and Staphylococcus aureus on antibiotics, especially norfloxacin and tetracyclines, was significantly increased after seven consecutive days of exposure to Cu 2 + and Zn 2 + and exhibited a higher cross-resistance rate, which meant that heavy metals can induce bacteria to develop antibiotic resistances that were not previously available. It was observed the downregulation of outer membrane protein gene omp C after heavy metal exposure, while the expression of other efflux pump and resistance genes in E. coli was upregulated, especially tet B, tol C and arc AB genes, suggesting that heavy metals might enhance antibiotic resistance by altering the expression of efflux pump genes. Moreover, many point mutations were found in the efflux system of resistant mutant strains. In particular, quinolone-resistant mutant strains showed a point mutation in the conversion of serine (Ser) to phenylalanine (Phe) at residue 464 of the gyr B gene, which may also be an important cause of increased quinolone resistance. This study provides an explanation for the effect of heavy metals on bacterial resistance. • The MIC of E. coli and S. aureus on antibiotics increased after Cu 2+ and Zn 2+ exposure. • After exposure to heavy metals, the cross resistance rates also showed an increase. • Heavy metals may enhance antibiotic resistance by altering efflux pump expression. • Many point mutations were found in the efflux system of resistant mutant strains.