Is TBX agar a suitable medium for monitoring Escherichia coli in bathing water using the membrane filtration method?

The use of a suitable method for the enumeration of indicator microorganisms is of crucial importance for reliable monitoring and assessment of the quality of bathing waters. Among other characteristics, the method should be selective enough and ensure acceptable relative recovery of target microorganisms. This study presents the basic parameters, relative recovery and categorical performance characteristics of Tryptone Bile X-glucuronide (TBX) agar for Escherichia coli (E. coli) enumeration in bathing water samples using the membrane filtration method.The results of the relative recovery study, in which TBX agar was compared against temperature-modified ISO 9308-1:2014, showed that in order to achieve a satisfactory relative recovery of E. coli with TBX agar at 44 ± 0.5 °C, the resuscitation period on a non-selective medium (Minerals Modified Glutamate Agar, MMGA) at 36 ± 2 °C is crucial. Incubation on a double-layer MMGA/TBX medium with a 6-h resuscitation period and alternating incubation on single-layer MMGA and TBX agar with a 4-h resuscitation period resulted in acceptable and very similar relative recovery. The achieved performance characteristics of the tested medium, double-layer MMGA/TBX agar, are acceptable. The selectivity was matrix-dependent and was 60.6% for inland and 69.9% for coastal waters. No significant effect of the resuscitation period on selectivity was recorded. Finally, the results showed that when the resuscitation period on a non-selective medium is included, TBX agar is a suitable medium for E. coli enumeration in bathing water samples using the membrane filtration method and that its use, theoretically, would not have negative effects on the assessment of bathing water quality.