Comparing the bacterial growth potential of ultra-low nutrient drinking water assessed by growth tests based on flow cytometric intact cell count versus adenosine triphosphate

The bacterial growth potential (BGP) of drinking water is widely assessed either by flow cytometric intact cell count (BGPICC) or adenosine triphosphate (BGPATP) based methods. Combining BGPICC and BGPATP measurements has been previously applied for various types of drinking water having high to low growth potential. However, this has not been applied for water with ultra-low nutrient content, such as remineralised RO permeate. To conduct a sound comparison, conventionally treated drinking water was included in this study, which was also used as an inoculum source. BGPICC, BGPATP, intact cell-yield (YICC), and ATP-yield (YATP) were determined for conventionally treated drinking water (Tap-water) and remineralised RO permeate (RO-water). In addition, both BGPICC and BGPATP methods were used to identify the growth-limiting nutrient in each water type. The results showed that the BGPICC ratio between Tap-water/RO-water was ∼7.5, whereas the BGPATP ratio was only ∼4.5. Moreover, the YICC ratio between Tap-water/RO-water was ∼2 (9.8 ± 0.6 × 106 vs. 4.6 ± 0.8 × 106 cells/µg-C), whereas the YATP ratio was ∼1 (0.39 ± 0.12 vs. 0.42 ± 0.06 ng ATP/µg-C), resulting in a consistently higher ATP per cell in RO-water than that of Tap-water. Both BGPICC and BGPATP methods revealed that carbon was the growth-limiting nutrient in the two types of water. However, with the addition of extra carbon, phosphate limitation was detected only with the BGPICC method, whereas BGPATP was not affected, suggesting that a combination of carbon and phosphate is essential for biomass synthesis, whereas carbon is probably utilised for cellular activities other than cell synthesis when phosphate is limited. It was estimated that the intact cell-yield growing on phosphate would be 0.70 ± 0.05 × 109 cells/µg PO4-P