Disarming the phenotypic expression involved in coselection of metal and antibiotic resistance in salmonella Enterica Serovar Typhi

Abstract

Although many reports highlight an indirect relationship between metal and antibiotic resistance, however studies in reference to clinical Salmonella strains are still lacking. In view of this, the present study was carried out to substantiate this association in serovar Ty2 and clinical Salmonella Typhi isolates. Interestingly, the increased cadmium levels were found in clinical isolates which co-related with their antibiotic sensitivity/resistance pattern. On further cadmium accumulation, antibiotic(s) sensitive isolates were rendered resistant and the resistant isolates were rendered more resistant as per their minimum inhibitory concentration(s). Further, cadmium accumulation led to changes in growth response, morphology, proteome, elevated antioxidants other than SOD, increased biofilm formation, decreased intracellular macrophage killing coupled with upregulation of genes encoding metallothionein, metal transporters and efflux pumps (acrB, tolC) along with its regulators newline(soxS, marA). The pathogen elevated the expression of efflux genes even in the presence of three efflux pump inhibitors i.e. PAand#946;N, NMP and Plumbagin, perhaps by sensing the pressure of the latter in addition to cadmium stress. Interaction of different EPIs with efflux pumps of CdA Ty2 cells was confirmed using ethidium bromide (EtBr) accumulation and efflux assay, wherein all the EPIs could cause retention of EtBr. Considering this potential of EPIs, retention of antibiotics was evaluated in CdA Ty2 cells wherein EPIs were used in combination with selected antibiotics. Also, decrease in the effective concentration of antibiotics was observed (both in efflux pump overexpressing CdA standard Ty2 strain and clinical isolates). Overall, these results indicated that cadmium, if acquired from the environment, being non-degradable can exert a long-lasting selective pressure on Salmonella in the host which may display antibiotic resistance later on, as a result of co-selection. Thus, a combination of EPI(s)-antibiotics may be exploited in future as o