Effect of different heterotrophic plate count methods on the composition of culturable microbial community

Heterotrophic plate counts (HPC) are routinely determined within the scope of water quality
assessment. However, variable HPC methods with different cultivation conditions are applied.
These variabilities can have a significant effect on the outcome of the analysis, particularly with
regard to the composition of culturable microbial community. This was demonstrated by testing
commonly used low and high incubation temperatures (22 and 37°C) as well as high and low
nutrient media (yeast extract agar and R2A). The culturable microbial community composition was
investigated by 16S rRNA gene sequencing and additional statistical analysis (ANOVA permutation
test on CCA and PERMANOVA). Lower temperature(22°C) showed the abundance of naturally
occurring Pseudomonadaceae and Aeromonadaceae, whereas at high temperature (37°C)
numerous Enterobacteriaceae and Bacilli were identified. The highest biodiversity was detected at
lower temperature, especially on R2A medium. The most significant effect of tested cultivation
conditions was assigned to temperature (p < 0.01), although media also affected the abundance of
certain detected bacteria (as seen in the number of operational taxonomic units). These results
endorse the supposition that different temperatures (low and high) should be included into HPC
measurement, whereas the selection of media should take into account the monitored water
source itself. Accordingly it can be inferred that the HPC method is more suitable for continuous
monitoring of water quality than for single assessments.