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The analysis of 100 ml filter sterilized drinking water artificially inoculated with E. coli revealed variable lower limits of detection (LLOD) depending on the bacterial viability. The LLOD from nutrient-rich cultures was ≥10 cell equivalents (CE) E. coli/100 ml. It rose to ≥1,000 CE E. coli/100 ml if nutrient-limited culture conditions were applied. For E. faecalis the LLOD was ≥100CE E. faecalis/100 ml from nutrient-rich cultures and ≥10 CE E. faecalis/100 ml from nutrient-limited cultures. This finding is partly in agreement with other studies (Bergeron et al. 2011). A tenfold increase in the sample volume to 1,000 ml resulted in a LLOD of ≥100 CE E. coli/100 ml from nutrient-rich cultures and ≥10 CE E. coli/100 ml from nutrient-limited cultures, respectively (Table 1, Figure S1, Supplementary material). The LLOD of E. faecalis was not altered (Table 1, Figure S2, Supplementary material). (Figures S1 and S2 are available with the online version of this paper.) In contrast, single contaminating bacteria, e.g. a minimum of 1 CFU/100 ml, could be reliably detected using the reference methods.

Table 1

Statistically estimated LLOD of fecal indicator bacteria in drinking water after application of different qPCR approaches

Sample volume
MethodOrganismCultivation100 ml1,000 ml
qPCR E. coli Nutrient rich 10 1–10 
Nutrient limited 10 1–10 
E. faecalis Nutrient rich 10 1–10 
Nutrient limited 10 1–10 
RT-qPCR E. coli Nutrient rich 10 100 
Nutrient limited 1,000 10 
E. faecalis Nutrient rich 100 100 
Nutrient limited 10 10 
v-qPCR E. coli Nutrient rich 100a 100a 
Nutrient limited 1,000a 10a 
E. faecalis Nutrient rich 100a 100a 
Nutrient limited 1,000a 100a 
Sample volume
MethodOrganismCultivation100 ml1,000 ml
qPCR E. coli Nutrient rich 10 1–10 
Nutrient limited 10 1–10 
E. faecalis Nutrient rich 10 1–10 
Nutrient limited 10 1–10 
RT-qPCR E. coli Nutrient rich 10 100 
Nutrient limited 1,000 10 
E. faecalis Nutrient rich 100 100 
Nutrient limited 10 10 
v-qPCR E. coli Nutrient rich 100a 100a 
Nutrient limited 1,000a 10a 
E. faecalis Nutrient rich 100a 100a 
Nutrient limited 1,000a 100a 

aViable cells after PMA treatment.

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