HOW MANY BACTERIA HAVE RIBOSOME GENESIS THAT IS SENSITIVE TO WASTEWATER?

Activated sludge (AS) systems require growth of bacteria in order to avoid washout due to endogenous decay and routine wasting of sludge from secondary clarification. Solids measurements and operational data are used to estimate the net growth rate of the total biomass, but not the net growth rate of specific microbial populations. Recent developments in molecular biology based methods for studying microbial communities provide methods to investigate the in situ growth rates of specific microbial populations (Oerther et al., 2000). For pure cultures grown in laboratory media, these methods show a direct correlation between the production of ribosomes (a.k.a. ribosome genesis) and growth. Previously, we observed that individual cells of Acinetobacter calcoaceticusT contained variable levels of precursor 16S rRNA (p16S rRNA) when they were grown in primary effluent collected from a municipal wastewater treatment plant (Oerther et al., 2000). We hypothesize that this pattern of erratic ribosome genesis is an indication of variable growth rates amongst individual cells of specific microbial populations in AS systems.Although growth of individual microbial cells was not measured explicitly in this study, we speculate that the pattern of erratic ribosome genesis observed in A. calcoaceticusT when exposed to wastewater is an indication of differences between the growth rates of individual cells of a single microbial population. Recently, we evaluated four pure cultures exposed to four local wastewaters (Cincinnati, OH) for erratic ribosome genesis throughout a 24-hour incubation. All four wastewaters caused erratic ribosome genesis in A. calcoaceticusT, indicating that this is not an isolated incident or the result of an aberrant experiment. In addition, a significant fraction of the individual cells had high p16S rRNA levels that are comparable to cells exposed to chloramphenicol, a known growth inhibitor. We have named this phenomena subpopulation growth inhibition (SGI). To date, SGI has only been observed with A. calcoaceticusT grown in primary effluent. Although only one of the four microbial populations tested showed SGI when incubated in the four wastewaters, the phylogenetic distribution of this observation is still in question. If SGI is prevalent in many microbial populations in AS systems, this may have implications for preventing washout of critical microbial populations.