High occurrence of heavy metal tolerance genes in bacteria isolated from wastewater: A new concern?

Some heavy metals have antimicrobial activity and are considered as potential alternatives to traditional antibiotic therapy. However, heavy metal tolerance genes (HMTG) have been already detected and coding different tolerance mechanisms. Considering that certain metals are promising for antimicrobial therapy, evaluation of HMTG dissemination in bacteria from sewage is essential to understand the evolution of these bacteria and to predict antimicrobial use and control. The present study aimed to evaluate the occurrence of bacteria carrying HMTG in samples of hospital wastewater and from urban wastewater treatment plant (WWTP). The acquired HMTG were investigated by PCR in bacterial collection previously characterized for antibiotic resistant genes (ARGs). HMTG searched include arsB (arsenic efflux pump), czcA (cadmium, zinc and cobalt efflux pump), merA (mercuric reductase), pcoD (copper efflux pump), silA (silver efflux pump) and terF (tellurite resistance protein). Among 45 isolates, 82% of them carried at last one HMTG, in which the silA and pcoD tolerance genes were the most prevalent. A very strong positive correlation was found between these genes (r = 0.91, p < 0.0001). Tolerance genes merA, arsB, czcA and terF were detected in 47%, 13%, 13% and 7% of the isolates, respectively. It was found that 15 isolates co-harbored ARGs (β-lactamase encoding genes). HMTG are probably more dispersed than ARGs in bacteria, representing a new concern for heavy metals use as effective antimicrobials. To the best of our knowledge, this is the first study on the HMTG searched in Hafnia alvei, Serratia fonticola and Serratia liquefaciens. Hospital wastewater treatment implementation and additional technologies for treatment in WWTP can reduce the impacts on water resources and HMTG spread, ensureing the environmental and human health safety.