Is CepA from Klebsiella pneumoniae a biocide pump? Evidence suggests a metal efflux function

Amid the antibiotic resistance crisis, biocides, including antiseptics and disinfectants, are indispensable tools to limit the spread of nosocomial infections and extensively drug-resistant bacteria. Thus, it is crucial that they remain effective. Extruding biocides from the cell using efflux systems is one mechanism by which bacteria can survive in their presence. The CepA protein from Klebsiella pneumoniae belongs to a family of heavy metal exporters but has been reported to be associated with chlorhexidine resistance, a common biocide. This study aims to elucidate the function of CepA as a biocide transporter and clarify its substrate profile in Escherichia coli. The results support the role of CepA in the transport of Fe²⁺, Mn²⁺, and possibly Cu²⁺ and Zn²⁺ to a lesser extent. The transporter also contributes to cell tolerance to iron and manganese stress. However, in contrast to previous reports, no significant impact of the expression of this protein on bacterial resistance to quaternary ammonium biocides was detected. Therefore, we propose that the main role of K. pneumoniae CepA is as a heavy metal transporter.

Understanding the structure and function of efflux pumps is crucial for addressing efflux-mediated resistance. Transporters that pump out biocides or heavy metals are not uncommon. However, to date, no transport protein capable of extruding both types of substrates has been reported. Initially, it was proposed that Klebsiella pneumoniae CepA was associated with chlorhexidine resistance and subsequently considered a biocide efflux pump. That notion also hinted at a novel group of drug efflux pumps that can extrude both biocides and heavy metals, a direct mechanism of cross-resistance between the two. The findings of this study indicate that it is more plausible that K. pneumoniae CepA is involved in metal homeostasis rather than in chlorhexidine biocide resistance in the recombinant Escherichia coli.