The Effect of Short-Term Drying on Biofilm Formed in a Model Water Distribution System

Abstract

In this study the effect of short-term drying on biofilm-related bacteria was investigated. Biofilm formation was encouraged to develop for nine months in a model water-distribution system. Biofilms were analyzed monthly for enumeration of aerobic and anaerobic heterotrophic bacteria, and sulphate reducing bacteria (SRB) after 6, 24, 48, and 72 hours of exposure to drying. The numbers of live and dead bacteria were directly analyzed by epifluorescence microscopy. In addition, extracellular polysaccharide substances (EPS) extraction, carbohydrate analysis, and scanning electron microscope (SEM) observation were performed. The formation of a brown-colored, thin biofilm layer was observed on the inner surface of polypropylene pipes at the end of the experimental study. SEM micrographs showed that ruptures occurred in the biofilm layer due to effects of drying. The counts of aerobic heterotrophic bacteria and SRB in dried biofilm samples decreased significantly after 6 and 48 hours, respectively. According to 5-cyano-2,3-ditolyl-tetrazolium chloride (CTC) staining results, bacteria can remain viable for up to 72 hours after exposure to drying. The significant increase in the amount of carbohydrate after 48 hours of exposure to drying indicates that bacteria produce EPS as a protective mechanism against drying stress.