Page 6: Guidance on Waterborne Bacterial Pathogens

Part B. Supporting information

B.3 Issues of emerging interest

B.3.1 Disinfection and antibiotic-resistant organisms

Disinfectants and antibiotics exert action on bacteria through very different mechanisms. Antibiotics characteristically act against specific target sites within the bacteria, interfering with a particular component of an essential process or pathway. In contrast, disinfectants act in a general manner against multiple targets that are fundamental components of the bacterial cell (e.g., proteins and DNA/ribonucleic acid [RNA]). Free chlorine, chloramine, chlorine dioxide and ozone are all very strong oxidizers ,which inactivate bacterial cells by destroying the activity of cell proteins that can be involved with cell structure or metabolism. UV light inactivates bacterial cells by altering the DNA in such a way that the cell can no longer multiply. Because of the fundamental differences in the way in which these two types of antibacterial strategies operate, antibiotic-resistant bacteria are not expected to show increased resistance to the action of drinking water disinfectants.

Antibiotic-resistant pathogens have the ability to change and become less susceptible to drugs. Bacterial resistance to antibiotics can be brought about in a variety of ways; for example, cells may not allow penetration of the antibiotic, they may lack the required target site or they may possess enzymes that can modify or destroy the antibiotic. Repeated exposure of bacteria to antibacterial agents and access of bacteria to increasingly large pools of antibiotic-resistant genes in mixed bacterial populations are the primary driving forces for emerging antibacterial resistance.

There are numerous types of antibiotics, which can be categorized into different classes based on their structure or mode of action. Bacteria having a particular resistance mechanism may be unaffected by antibiotics of a similar class or that target the same site. These same bacteria may be vulnerable to different antibiotics or may possess mechanisms that make them resistant to multiple classes of antibiotics. The growing problem with antibacterial resistance is diminution of the effectiveness of antibacterial agents, resulting in antibiotic-resistant pathogens that are more virulent than their susceptible counterparts, causing more prolonged or severe illnesses.

Very few data have been generated to date regarding the effects of disinfectants on antibiotic-resistant bacteria in drinking water. Some early work found that a greater proportion of HPC bacteria in treated water are antibiotic-resistant bacteria, compared with those in untreated water (Armstrong et al., 1981, 1982). Templeton et al. (2009) conducted an investigation on the susceptibility of ampicillin- and trimethoprim-resistant strains of E. coli to free chlorine and UV disinfection. The authors observed no differences in UV inactivation between antibiotic-resistant and antibiotic-sensitive E. coli under the doses and contact times tested. The trimethoprim-resistant E. coli strain did show slightly greater resistance to free chlorine compared with the antibiotic-sensitive E. coli; however, the authors concluded that the difference was likely to be negligible under chlorine doses and contact times typically observed in routine drinking water treatment. It was further concluded that these disinfectants did not likely select for ampicillin or trimethoprim resistance during drinking water treatment. No drinking water studies were found pertaining to the inactivation rates for other disinfectants, such as ozone or chlorine dioxide, against antibiotic-resistant bacteria.

At present, there is little evidence to indicate that the use of disinfectants in drinking water systems favours the selection of antibiotic-resistant bacteria in any way (Templeton et al., 2009). However, one study by Xi et al. (2009) suggested that water treatment could increase the antibiotic resistance of surviving bacteria or induce antibiotic resistance gene transfer. Additional study in this area is needed. The evidence at present, although limited, suggests that antibiotic resistance in bacteria is not an important factor in chlorine and UV treatment effectiveness at doses and contact times typically applied in drinking water treatment systems.

B.3.2 Showerheads

Shower use can provide a source of exposure to microorganisms through aerosolization, as the inside of a showerhead provides a moist, warm, dark environment that is frequently replenished with low-level nutrient sources.

Inhalation of aerosols from showerhead water has been implicated in respiratory disease (Falkinham et al., 2008; Feazel et al., 2009). Although opportunistic pathogens have been cultured from showerheads, little is known about either the prevalence or the nature of the microorganisms that can be aerosolized during showering. To determine the composition of showerhead biofilms and waters, Feazel et al. (2009) analysed ribosomal RNA gene sequences of biofilms from 45 showerheads from nine sites in the United States. The authors found that sequences representative of non-tuberculous mycobacteria and other opportunistic pathogens were highly enriched in many showerhead biofilms. They concluded that showerheads may present a significant potential exposure to aerosolized microorganisms and that the health risk associated with showerheads needs further investigation, particularly for individuals with compromised immune or respiratory systems.

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