Culturability as an indicator of succession in microbial communities

Successional theory predicts that opportunistic species with high investmen t of energy in reproduction and wide niche width will be replaced by equili brium species with relatively higher investment of energy in maintenance an d narrower niche width as communities develop. Since the ability to rapidly grow into a detectable colony on nonselective agar medium could be conside red as characteristic of opportunistic types of bacteria, the percentage of culturable cells may be an indicator of successional state in microbial co mmunities. The ratios of culturable cells (colony forming units on R2A agar ) to total cells (acridine orange direct microscopic counts) and culturable cells to active cells (reduction of 5-cyano-2,3-ditolyl tetrazolium chlori de) were measured over time in two types of laboratory microcosms (the rhiz osphere of hydroponically grown wheat and aerobic, continuously stirred tan k reactors containing plant biomass) to determine the effectiveness of cult urabilty as an index of successional state. The culturable cell:total cell ratio in the rhizosphere decreased from approximately 0.25 to less than 0.0 5 during the first 30-50 days of plant growth, and from 0.65 to 0.14 during the first 7 days of operation of the bioreactor. The culturable cell:activ e cell ratio followed similar trends, but the values were consistently grea ter than the culturable cell:total cell ratio, and even exceeded I in early samples. Follow-up studies used a cultivation-independent method, terminal restriction fragment length polymorphisms (TRFLP) from whole community DNA , to assess community structure. The number of TRFLP peaks increased with t ime, while the number of culturable types did not, indicating that the gene ral decrease in culturability is associated with a shift in community struc ture. The ratio of respired to assimilated C-14-labeled amino acids increas ed with the age of rhizosphere communities, supporting the hypothesis that a shift in resource allocation from growth to maintenance occurs with time. Results from this work indicate that the percentage of culturable cells ma y be a useful method for assessing the successional state of microbial comm unities.