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Three 2 μL of a 1-L sewage water sample harvested at the discharge of the grit chambers of the west wastewater treatment plant of Québec City were used for testing by MI agar, Chromocult® coliform agar, and Compass CC agar methods to verify their comparative ability to detect E. coli and total coliforms (Table 1). All these colonies were harvested and subjected to molecular identification using a 527 base-pairs fragment of the 16S rRNA gene. For the three culture-based methods tested, all blue colonies were identified as E. coli. Furthermore, none of the non-blue colonies was identified as E. coli, for a specificity and a sensitivity of 100%. However, MI agar and Compass CC agar detected, respectively, 12 and 13 E. coli colonies, whereas Chromocult® coliform agar detected 7 E. coli colonies. This result is in accordance with Maheux et al. (2015), demonstrating that for the detection of E. coli-positive water samples, all enzymatic culture-based methods tested were stastistically equivalent.

Table 1

Specificity, sensitivity, and predictive values of each method for the detection of total coliform as compared to 16S rRNA gene identification

Method and results16S rRNA identification
SpecificityaSensitivitybPositive predictive valuecNegative predictive valued
+
MI agar 
  + 45 69.2% 97.8% 91.8% 90.0% 
  − 
Chromocult® coliform agar 
  + 39 28 47.2% 97.5% 58.2% 96.2% 
  − 25 
Compass CC agar 
  + 24 25 40.5% 85.7% 49.0% 81.0% 
  − 17 
Method and results16S rRNA identification
SpecificityaSensitivitybPositive predictive valuecNegative predictive valued
+
MI agar 
  + 45 69.2% 97.8% 91.8% 90.0% 
  − 
Chromocult® coliform agar 
  + 39 28 47.2% 97.5% 58.2% 96.2% 
  − 25 
Compass CC agar 
  + 24 25 40.5% 85.7% 49.0% 81.0% 
  − 17 

aNo. of true negative results/(no. of true negative + false positive results).

bNo. of true positive results/(no. of true positive + false negative results).

cNo. of true positive results/(no. of true positive + false positive results).

dNo. of true negative results/(no. of true negative + false negative results).

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