Experimental study using the dilution incubation method to assess water biostability

Water biostability is of particular concern to water supply as a major limiting factor for heterotrophic bacterial growth in water distribution systems. This study focused on bacterial growth dynamics in the series dilution of water samples with TOC（total organic carbon） values determined beforehand. The results showed that the specific growth rate of Pseudomonas fluorescens P17 varied dramatically and irregularly with TOC value when TOC concentrations were low enough during the initial periods of incubation under given conditions. According to this relationship between bacterial growth rate and TOC, a dilution incubation method was designed for the study of water biostability. With the method under a given condition, a turning-point TOC value was found at a relatively fixed point in the curve between bacterial growth rate and TOC of water sample, and the variation of growth rate had different characteristics below the turning-point TOC value relative to that over this value. A turning-point TOC value similarly existed in all experiments not only with tap water, but also with acetate and mixed solutions. And in the dilution incubation method study, the affections were analyzed by condition factors such as inoculum amount,incubation time and nature of the organic carbon source. In very low organic carbon water environments, the variation characteristics of bacterial growth rate will be useful to further understand the meaning of water biostability.

Microbial community structure and dynamics of starch-fed and glucose-fed chemostats during two years of continuous operation

The microbial community structures of two mesophilic anaerobic chemostats, one fed with glucose, the other with starch as sole carbon sources, were studied at various dilution rates （0.05-0.25 d-1 for glucose and 0.025-0.1 d-1 for starch） during two years continuous operation. In the glucose-fed chemostat, the aceticlastic methanogen Methanosaeta spp. and hydrogenotrophic methanogen Methanoculleus spp. predominated at low dilution rates, whereas Methanosaeta spp. and the hydrogenotrophic Methanobacterium spp. predominated together when dilution rates were greater than 0.1 d 1. Bacteria affiliated with the phyla Bacteroidetes, Spiro- chaetes, and Actinobacteria predominated at dilution rates of 0.05, 0.1, and 0.15 d-l, respectively, while Firmicutes predominated at higher dilution rates （0.2 and 0.25 d-l）. In the starch-fed chemostat, the aceticlastic and hydrogeno- trophic methanogens coexisted at all dilution rates. Although bacteria belonging to only two phyla were mainly responsible for starch degradation （Spirochaetes at the dilution rate of 0.08 d-1 and Firmicutes at other dilution rates）, different bacterial genera were identified at different dilution rates. With the exception of Archaea in the glucose-fed chemostat, the band patterns revealed by denaturing gradient gel electrophoresis （DGGE） of the microbial communities in the two chemostats displayed marked changes during long-term operation at a constant dilution rate. The bacterial community changed with changes in the dilution rate, and was erratic during long- term operation in both glucose-fed and starch-fed chemo- stats.