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Results of the confirmation tests are reported in Table 1. False positive rates were around 11% (11/97) and 4% (4/92) for the TBX and the Colilert®-18 methods, respectively. The value related to the Colilert®-18 test is lower than those reported for wastewater samples and comparable to those observed for samples of natural water in previous studies (Yakub et al. 2002; Niemela et al. 2003; Bonadonna et al. 2007; McLain et al. 2011). This apparent discordance can be explained by considering the distribution of false positives among the samples analysed. Over a total of 15 false positives, 14 were obtained from just two samples (a raw wastewater and a secondary settled effluent) collected on the same day from the same municipal WWTP (WWTP1). Considering just the results related to these two samples, false positive rates were much higher: 28% (10/36) for TBX and 14% (4/29) for Colilert®-18. On the other hand, for the other eight samples submitted to confirmation, three of which were collected at the same WWTP1, the percentages of false positives were close to zero: 2% (1/61) for TBX and 0% (0/63) for Colilert®-18. These results highlight the strong variability of the microbial ecosystem of wastewater, both among different WWTPs and over time. Moreover, they suggest that possible interferences due to the presence of bacteria other than E. coli that are capable of β-D-glucuronidase activity cannot be predicted for wastewater samples.

Table 1

Results of confirmation tests performed on presumptive E. coli-positive and on presumptive E. coli-negative colonies/wells

 MF with TBX agar, incubation at 37 °C for 24 h
Colilert®-18 Quanti-Tray®/2000
Presumptive E. coli-positivePresumptive E. coli-negativePresumptive E. coli-positivePresumptive E. coli-negative
Number of colonies/well analysed 97 30 92 79 
Oxidase negative 97 30 92 79 
Lactose positive 97 24 92 59 
Indole positive 86 88 10 
False positivea 11   
False negativeb   
 MF with TBX agar, incubation at 37 °C for 24 h
Colilert®-18 Quanti-Tray®/2000
Presumptive E. coli-positivePresumptive E. coli-negativePresumptive E. coli-positivePresumptive E. coli-negative
Number of colonies/well analysed 97 30 92 79 
Oxidase negative 97 30 92 79 
Lactose positive 97 24 92 59 
Indole positive 86 88 10 
False positivea 11   
False negativeb   

aNumber of presumptive E. coli-positive colonies/wells for which at least one confirmation test was not satisfied.

bNumber of presumptive E. coli-negative colonies/wells for which all confirmation tests were satisfied.

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