Investigation of the levels and diversity of heterotrophic bacteria in drinking water biofilms of Potchefstroom, North-West Province, RSA
Vos, Elsie Petronella
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Heterotrophic bacteria in drinking water and implications thereof is a controversial subject. Though this group of organisms is generally considered not harmful, the occurrence and harmlessness of this group of bacteria in drinking water was recently reconsidered by several leading scientists. High levels of these bacteria in drinking water could be an indication of either inefficient purification, regrowth or that contamination from external sources occurred. The potential implications and effects of such bacteria in biofilms within distribution systems remain undetermined. The aim of the present study was firstly to determine the diversity and levels of antibiotic resistant bacteria in drinking water biofilms in Potchefstroom and secondly to determine whether these isolates are potential pathogens. There were two main objectives. The first objective was to determine the diversity, levels and characteristics of heterotrophic bacteria in biofilms from home water filtering systems. Here, 144 and 381 bacterial colonies were respectively isolated from 2 home water filtering devices and a biofilm device, using standard microbiological culturing and sub-culturing procedures. Using biochemical methods the following Gram-negative species were identified: Enterobacter spp, Citrobacter spp, Aeromonas hydrophilia, Providentia spp. No Gram-positive species were identified. Among the isolates from the home water filtering systems 88.2% were resistant to tetracycline, 6.6% to vancomycin (Gram-positive only), 56.0% to erythromycin, 69.0% to ampicillin, 34.0% to neomycin, 9.0% to chloramphenicol, and 4.1% to gentamycin. Isolates were also tested for haemolytic activity (potentially pathogenic features) and a considerable number showed a-or P-hemolysis. Scanning electron microscopy results indicated that relatively large particulate matter was present in the water and that typical multi-species biofilms formed within the water filtering devices. The second objective of the study was to determine the diversity, levels and characteristics of heterotrophic bacteria in biofilms isolated from an in situ biofilm device. The 381 isolates obtained from the biofilm device included Pseudomonas spp, Enterobacter spp. and Exiguobacterium spp. as well as some unidentified Gram-positive species. Among these isolates, 34.4% were resistant to ampicillin, 5.6% to penicillin (Gram-positive only), 36.7%) to tetracycline and 5.6% to vancomycin (Gram-postive only). Most of the isolates obtained over the 12 week period showed a- or p- hemolysis. The majority of isolates from the biofilm device were tolerant to high levels of copper (minimum inhibitory concentrations > 5mM). The results also demonstrated that some of the isolates were simultaneously tolerant to high concentrations of heavy metals and resistant to multiple antibiotics. It was not determined whether a significant correlation existed between these two parameters. The results presented in this study may not indicate risk to consumers of Potchefstroom water. However, the presence of some of the species in the drinking water biofilms is cause for concern and this aspect should be further investigated. Other aspects that require attention include the source of the bacteria, the population dynamics thereof in the water distribution system and the dynamics of genetic elements that could be responsible for the heavy metal tolerance and antibiotic resistance.