Biofilms in the potable water distribution network
The roles of vegetative dormancy and attachment to surfaces in the survival and growth of bacteria in potable water systems were investigated. Species present in the water were identified following isolation or direct observation of static batch enrichment cultures. Using the latter approach, many prosthecate and other stalked bacteria were found. Prosthecate bacteria undergo bi- or poly-phasic life cycles involving asymmetric division to produce reproductive cells and dormant swarmer cells and their presence in tap water supports the theory that vegetative dormancy is an important survival mechanism in this environment. A continuous flow model was established to analyse the metabolic activity of planktonic and attached bacteria in potable water. A physiological dye, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), was shown to stain active cells specifically in batch cultures of a Sphingomonas sp. and Caulobacter crescentus. Unsuccessful attempts were made to identify cellular proteins of Sphingomonas sp. cells that were specific to the attached or planktonic phenotype. By comparing the total bacterial counts in potable water with the total viable counts it was shown that a large proportion of the microflora was not cultivable on heterotrophic media. However, a proportion of these cells became culturable following enrichment with peptone. After ceasing the exogenous nutrient addition cellular aggregation occurred, presumably reflecting physiological changes in response to nutrient depletion. No clear trend in the activity of attached cells during biofilm development was detected. However, firmly attached cells were buffered against changes in the chemistry of the water. Growth within biofilms and release into the water column elevated the concentration of bacteria in the water. Attached cells were resistant to 0.3 mg free chlorine 1-1 added for 3 hrs, although this did weaken the architecture of the biofilm. Long term biofilms (one year-old) were almost devoid of bacteria - an observation that could not be adequately explained.