species from non-chlorinated distribution systems and their competitive planktonic growth in drinking water.

is included in the Dutch Drinking Water Decree as an indicator for elevated microbial regrowth in non-chlorinated drinking water distribution systems (DWDS). The temporal and spatial diversity of species in ten DWDS and their planktonic growth characteristics for different carbon sources was investigated. Genotyping of the B gene of isolates showed a non-systematic temporal and spatial variable prevalence of seven different species in these DWDS and no correlation with AOC-P17/NOX and concentrations. Pure cultures of these seven species showed a high affinity to low concentrations (μg/L) of individual amino acids and fatty acids, compounds associated with biomass. Growth occurred at 0.5 μg-C/L of an amino acid mixture. Growth of a mixed community of , , and in drinking water occurred in pasteurized samples, however, no growth and decay occurred in competition with the autochthonous bacteria (non-pasteurized samples). This community also failed to grow in non-pasteurized distribution samples from a location with clear increase in planktonic concentrations in the transported drinking water. For competitive planktonic growth of an amino acid concentration of ≥5 μg-C/L 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 non-compliance with the standard in non-chlorinated DWSD. The occurrence of the bacterial genus in non-chlorinated drinking water in the Netherlands is regarded as an indication for elevated microbial regrowth in the distribution system. Identification of the prevalent species in ten distribution systems by genotyping yielded seven different species, with , and as the most dominant ones. Planktonic growth experiments of pure cultures confirmed former published affinity of for certain biomass compounds (amino and fatty acids). In competition with the autochthonous microflora, however, planktonic growth was not observed, only after addition of a threshold amino acid concentration of 5 μg-C/L. Based on our results and further observations we deduced that planktonic growth of in the DWDS is not very likely. Benthic growth in loose deposits and planktonic release is a more plausible explanation for the observed planktonic increase of .