Selection of antibiotic resistant bacteria: a role for mercury?

Background: Diverse environmental stressors (e.g. metals) can participate in the selection and maintenance of antibiotic resistance (AB R) strains and/or genetic elements. Mercury (Hg) is present in environment, in part associated with anthropogenic contamination. The occurrence of mer operon among Enterococcus was scarcely analyzed, although it has been linked to mobile genetic elements with ABR. Our goal was to assess the occurrence of known HgR genes among Enterococcus spp from several sources, and to evaluate their genetic context. Methods: We analyzed 231 E. faecalis-Efl, 252 E. faecium-Efm, and 205 Enterococcus spp-Ep from hospitalized (H, n=47), healthy humans (HV, n=68), poultry (P, n=155), piggeries environment/swine PE, n=220), aquacultures (A, n=167) and sewage (S, n=52) (Portugal; 1997-2011). Genes linked to ABR (vanA, tetM, tetL, ermB, aac6”-aph2, blaZ), HgR (4 merA sequences) and Tn6009 were searched (PCR). Mating assays, clonal relatedness (PFGE/MLST) and analysis of the plasmid carrying HgR genes (S1-PFGE, rep typing, hybridization) were done. Results: Genes encoding HgR (merA1-2%; merA2-3%; merA3-0,5%) were identified in 16 Efm, 2 Efl, and 2 Ep, and linked to 8 PFGE types (10 Efm, 1 Efl) from PE/H/HV (9%) and S (8%). Representative isolates corresponded to Efm CC17 (ST132, ST393 and ST431; PE and H), Efm CC5 (ST185; PE), STnew (S) and to Efl ST159 (H). The merA3 and tetM were located at chromosome (Efl-H) while other Hg/ABR genes were detected on plasmids: i) merA2 (185Kb, 190Kb; Efm-2PE); ii) merA2+tetM (250kb, 270Kb; Efm-1S, 1PE); iii) merA2+tetM+ermB (200kb; Efm-1HV); iv) merA2+merA3+ermB+vanA (75kb; Efl-1H). ABR (erythromycin, tetracycline, ampicillin, HLR- streptomycin, HLR-gentamicin) was co-transferred with HgR genes at different rates. The repA-pLG1 was only identified in Efm plasmids tested which carry only HgR (n=1) or HgR +ABR genes (n=2). The merA were not linked to Tn6009 or plasmids carrying blaZ as described. Conclusion: This work represents the first study addressing the diversity of merA genes among Enterococcus from different ecological niches. Their co-location with ABR genes on widespread large pLG-like conjugative plasmids of E. faecium may favoured their persistence under different selective pressures linked to antibiotic use or environmental Hg contamination. The diversity of merA genes also suggests frequent lateral gene transfer between bacteria sharing common ecological settings.