Aeromonas Species from Nonchlorinated Distribution Systems and Their Competitive Planktonic Growth in Drinking Water

ABSTRACT

Aeromonas is included in the Dutch Drinking Water Decree as an indicator for elevated microbial regrowth in nonchlorinated drinking water distribution systems (DWDS). The temporal and spatial diversity of Aeromonas species in 10 DWDS and their planktonic growth characteristics with regard to different carbon sources were investigated. Genotyping of the gyrB gene of isolates showed a nonsystematic temporal and spatial variable prevalence of seven different Aeromonas species in these DWDS and no correlation with the level of assimilable organic carbon determined with Pseudomonas fluorescens strain P17 and Spirillum sp. strain NOX (AOC-P17/NOX concentration) and Aeromonas concentrations. Pure cultures of these seven species showed a high affinity for low concentrations (micrograms per liter) of individual amino acids and fatty acids, compounds associated with biomass. Growth occurred at 0.5 µg C/liter of an amino acid mixture. Growth of a mixed community of Aeromonas rivuli, Aeromonas salmonicida, Aeromonas sobria, and Aeromonas veronii in drinking water occurred in pasteurized samples; however, either no growth or decay occurred in competition with the autochthonous bacteria (nonpasteurized samples). This community also failed to grow in nonpasteurized distribution samples from a location with a clear increase in planktonic Aeromonas concentrations in the transported drinking water. For competitive planktonic growth of Aeromonas, an amino acid concentration of ≥5 µg C/liter is required. AOC-P17/NOX concentrations showed that such concentrations are not expected in Dutch drinking water. Therefore, we suspect that competitive planktonic growth is not the major cause of the observed noncompliance with the Aeromonas standard in nonchlorinated DWDS.

IMPORTANCE The occurrence of the bacterial genus Aeromonas in nonchlorinated drinking water in the Netherlands is regarded as an indication of elevated microbial regrowth in the distribution system. Identification of the prevalent species in 10 distribution systems by genotyping yielded seven different species, with A. rivuli, A. veronii, and A. sobria being the most dominant ones. Planktonic growth experiments of pure cultures confirmed former published affinity of Aeromonas for certain biomass compounds (amino and fatty acids). In competition with the autochthonous microflora, however, planktonic growth was observed only after addition of a threshold amino acid concentration of 5 µg C/liter. Based on our results and further observations, we deduced that planktonic growth of Aeromonas in the DWDS is not very likely. Benthic growth in loose deposits and planktonic release are a more plausible explanation for the observed planktonic increase of Aeromonas.