Abstract
Microorganisms (MO) use many different mechanisms to survive in environments rich in silver. These mechanisms can either be endogenous involving spontaneous mutations or exogenous through the action of resistance genes that are either part of the chromosome or plasmids. In many bacteria, silver resistance has been associated with the presence of the sil operon which is inducible in the presence of the metal. Silver nanoparticles (AgNPs) are of great interest due to their use as alternatives to combat antimicrobial resistance (AMR). Previous studies have shown that there is a link between heavy metal resistance and the production of metal nanoparticles by bacteria, although not all bacteria exhibiting resistance are able to make nanoparticles. This study aimed to establish any differences in the nucleotide sequence of the sil operon that may promote the biosynthesis of AgNPs as a mechanism of silver resistance in bacteria. Seven bacterial isolates including Enterobacter xiangfangensis Pb204 were screened for resistance to silver and ability to synthesize AgNPs using growth assays. Enterobacter xiangfangensis Pb204 was resistant and synthesized AgNPs while other species were resistant but did not synthesize AgNPs. Whole genome sequencing revealed that Bacillus safensis, a resistant but non-nanofactory isolate did not house the sil operon and nitrate reductases in its sequence but had many other efflux pumps known to contribute to metal resistance. On the other hand, the sil operon, several heavy metal efflux pumps, and nitrate reductases essential for the reduction of silver ion were present in Enterobacter xiangfangensis Pb204. Although the findings indicate that bacteria do not necessarily need the sil operonto be resistant to silver due to the presence of other efflux mechanisms it is suggested that both the sil operonand reductases play an important role in the reduction of silver ions into nanoparticles. This study had some limitations in the sample size that was sequenced precluding a conclusive link between nanoparticle synthesis and the sil operon. Furthermore, isolates used in this study were selected based on the extracellular production of AgNPs. However, the mechanism of silver resistance used by a bacterial species may influence whether nanoparticles are made intracellularly or extracellularly. Hence, further studies can be conducted to track how much silver ions are pumped out of the cell by efflux mechanisms and how much silver ions were left in the cell that could have promoted intracellular synthesis of AgNPs.
Keywords: AgNPs, metal resistance, nanoparticle synthesis, sil operon, whole genome sequence