engleski

Enumeration of Escherichia coli in coastal bathing water by temperature modified ISO EN 9308-1:2014 method – does it work?

The new Bathing Water Directive (2006/7/EC) specifies two reference methods for the enumeration of E. coli in bathing water samples: ISO 9308-3 (miniaturized most probable number method), and ISO 9308-1 (membrane filtration method ) which was applied on the Croatian Adriatic coast from 2009 to 2014. Due to the low selectivity of Chromogenic coliform agar, a revised edition of ISO 9308-1 (2014) does not include the matrix of surface water in its scope, thus questioning the use of this method in the monitoring of bathing water quality. To reduce the growth of background bacterial flora and extend the scope to coastal bathing water, we modified reference method by changing the incubation temperature to 44 °C after the 4-hour resuscitation period at 37 °C. Reference method, ISO EN 9308-1:2014 against alternative, temperature modified ISO EN 9308-1:2014 method was compared according to ISO 17994:2014. The study area covered the entire Croatian Adriatic coast. Eight laboratories participated in the study, with more than 430 of paired data collected from more than 100 sampling sites. Increased incubation temperature significantly suppressed the growth of thermo-intolerant bacteria, including thermo-intolerant coliform bacteria. The reduction of non-E. coli coliforms was 47 %, while the reduction of all non-E. coli bacteria was 90 %. This made the reading of the plates significantly easier and faster. Based on the results of equivalence testing and criteria set by ISO 17994:2014, the methods were evaluated as “not different”, allowing the use of the alternative method for enumerating E. coli in coastal waters.

Escherichia coli, ISO method, method comparison, coastal water quality

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