Potentially pathogenic biocide tolerant heterotrophic bacteria from sewage & river water
Abstract
A problem we are facing in South Africa is the release of antimicrobial compounds into our natural water bodies. There have been reports on the emergence of biocide resistance and the accompanying possibility of antibiotic cross-resistance. Bacteria that exhibit tolerance to an antimicrobial might be selected by means of recurring exposure to a low concentration or increasing concentration of an antimicrobial. The aim of the study was to identify and characterise potentially pathogenic Triclosan (TCS) - and/or Chloroxylenol (PCMX) tolerant, heterotrophic plate count bacteria isolates from the wastewater treatment plant (WWTP) in Potchefstroom and the Mooi River. Nutrient agar supplemented with the biocides was used to isolate and maintain tolerant bacteria. Isolates were identified by sequencing of the 16S rDNA region and where relevant clonal relationships between specific isolates were elucidated using ERIC-PCR. Selected isolates were characterised for their MICs against TCS and PCMX, as well as antibiotics resistance profiles. Synergistic and antagonistic interactions between the biocides and selected antibiotics were also evaluated. Isolates were also screened for the presence of extracellular enzymes associated with virulence. These results along with antibiotic resistance profiles were used to generate a pathogenic potential index to assess potential pathogenicity and the associated health risks. Of the isolates obtained Pseudomonas, Bacillus, Klebsiella and Aeromonas spp. are well described opportunistic pathogens. Based on current fingerprinting methods it is not yet clear if the WWTP is the source from which these organisms enter the environment and more study is required to obtain more conclusive results. Isolates exhibited various levels of resistance to antibiotics and the biocides in question as well as several occurrences of synergy and to antagonisms, but further study is required to determine the resistance mechanisms involved. HPLC revealed the presence of both biocides in the WWTP influent, bacteria are thus exposed to these biocides during their initial inflow into the WWTP and possibly during the wastewater treatment process, but both TCS and PCMX seem to be sufficiently removed by the WWTP as there were no traces found in the WWTP effluent. Extracellular enzyme production along with antibiotic resistance profiles was used to generate a pathogenic potential index. This revealed that several of the isolates had very high potential for virulence, but further study is required to identify the specific virulence genes associated with the isolates in question.