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All atypical colonies, also called background heterotrophic bacteria, harvested on MI agar, Chromocult® coliform agar, and Compass CC agar, were identified using 16S rRNA sequencing. To our knowledge, this is the first report of the identity of background heterotrophic bacteria detected on chromogenic culture-based methods. The amount of atypical colonies detected on each culture-based method was different. While MI agar detected only 8 atypical colonies, Chromocult® coliform agar and Compass CC agar detected, respectively, 27 and 17 colonies (Table 3). Furthermore, while MI agar detected mostly a bacterium called Uruburuella suis as the atypical colony, the population detected was larger for Chromocult® coliform agar and Compass CC agar where the most prevalent atypical colony was an Aeromonas spp. (85.2% and 47.1%, respectively). Thus, according to Maheux et al. (2015), the MI agar method is composed of the more stringent medium since concentrations of atypical colonies growing on MI agar are less important than the two other methods.

Table 3

Population of background heterotrophic bacteria detected by MI agar, Chromocult® coliform agar, and Compass CC agar from 2 μL of sewage water

MI agar
Chromocult® coliform agar
Compass CC agar
SpeciesNo.%%aSpeciesNo.%%aSpeciesNo.%%a
Uruburuella spp. 87.5  Aeromonas spp. 23 85.2  Aeromonas spp. 47.1  
Morganella sp. 12.5  Uruburuella spp. 7.4  Pseudomonas spp. 23.5  
    Acinetobacter sp. 3.7  Acinetobacter spp. 11.8  
    Lelliottia sp. 3.7  Shewanella spp. 11.8  
        Uruburuella sp. 5.9  
Total 100.0 13.6  27 100.0 28.0  17 100.0 24.3 
MI agar
Chromocult® coliform agar
Compass CC agar
SpeciesNo.%%aSpeciesNo.%%aSpeciesNo.%%a
Uruburuella spp. 87.5  Aeromonas spp. 23 85.2  Aeromonas spp. 47.1  
Morganella sp. 12.5  Uruburuella spp. 7.4  Pseudomonas spp. 23.5  
    Acinetobacter sp. 3.7  Acinetobacter spp. 11.8  
    Lelliottia sp. 3.7  Shewanella spp. 11.8  
        Uruburuella sp. 5.9  
Total 100.0 13.6  27 100.0 28.0  17 100.0 24.3 

aPercentage of background heterotrophic bacteria on each plate.

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