Diversity of the ars operon in arsenic resistant bacteria

The adverse health effects of arsenic (As) has led to an increase in the need for environmental research focusing on the fate of As in environmental waters including drinking water supplies. Because of the ubiquity of As in the environment, resistance to As can be observed across a diverse genera of bacteria. The ars genetic operon confers resistance to milimolar concentrations of arsenate (As(V)), arsenite (As(III)), and antimony (Sb) in bacteria. Although ars genes have been studied at the biochemical and genetic levels in certain clinically relevant bacteria, little is known about the existence of ars homologs in As-resistant bacteria originating from environmental waters containing As. This research concerns the use of the ars operon as a model to describe the genotype of bacteria phenotypically expressing As resistance. Specifically, homology to the Escherichia coli ars operon was investigated in bacteria isolated from As enriched creek waters. Total As concentration in these waters ranged from 200 μg/L in the hot springs of Hot Creek, to 740 μg/L in South Haiwee Drain, California. A positive relationship was observed between the abundance of As(III) resistant bacteria in Hot Creek and As(III) concentration (R2 = 0.71) in the water. In addition, DNA-DNA hybridization showed that 80% of bacterial isolates contained DNA sequences homologous to E. coli derived arsA, arsB , and arsC gene probes at low stringency (31–53% estimated mismatch). At higher stringency, ars genes were conserved in only 50% of the isolates (22–36% estimated mismatch). PCR was too stringent for detecting divergent sequences in non-enteric bacteria however, ars genes were detected in 50% of As resistant raw sewage E. coli spp. and enteric bacteria. Partial arsB fragments from six environmental enteric bacteria were sequenced. These genes exhibited greater similarity (84% homology) to the E. coli R773 arsB gene. Investigation with the Pseudomonas aeruginosa ars operon showed that 60% of isolates harbored homologous sequences with an estimated mismatch of 12–20%. In contrast, PCR was too stringent to detect ars genes in various Pseudomonas spp. However, an ars-like gene fragment was isolated in an Acinetobacter sp. This sequence was approximately 78% similar to the P. aeruginosa arsB gene. The overall divergence from the E. coli and P. aeruginosa ars models observed in As resistant environmental bacteria suggests that there may be additional divergent classes of ars genes or other strategies for arsenic resistance.