Laboratory methods to detect pathogens in water

Contamination of surface waters by faecal material (which may contain many pathogens) represents a significant health risk to exposed humans and animals. The main pathways of exposure to contaminated water are related to the use of surface waters for drinking water production, recreation, and irrigation. The most frequently encountered pathogens in contaminated surface waters are the enteric pathogens.

However, the detection and isolation of enteric pathogens are expensive and time-consuming. That is why analysts use coliforms and enterococci as indicators instead. Coliforms are considered to be the backbone of faecal pollution monitoring. These are also the most used indicator organisms. Within this group, Escherichia coli is especially valuable. It is the most abundant among the coliforms which constitute the intestinal flora of warm-blooded animals. Besides the coliforms, enterococci are also commonly employed in water quality assessment. This group is represented mainly by Streptococcus faecali and S. faecium.

For both the detection of coliforms and enterococci, water sampling should be done in pre-sterilized bottles with 250mL minimum volume. For surface waters, it should be done 30 cm below the water’s surface and with the neck turned against the current. Samples should be kept refrigerated and protected from sunlight before analysis.

For the quantification of bacteria in water, two approaches are generally used. These are the membrane filtration (MF) and the most probable number (MPN). Both approaches are based on biochemical enzyme-substrate reactions which enable detection of the members of a target group.

Detection of the coliforms is based on the activity of the enzyme β –galactosidase which plays a significant role in the fermentation of lactose. Similarly, enterococci are detected through the activity of β –glucuronidase. Methods for the detection of faecal indicators are standardized (ISO 9308-1 and 9308-2 for E. coli; ISO 7899-1 and 7899-2 for enterococci). In the European Union, water quality monitoring is mandated by the Bathing Water Directive 2006/7/EC, the Drinking Water Directive 98/83/EC, and the Urban Wastewater Treatment Directive 91/271/EEC.

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