Application of the fluorescent-antibody technique to the study of a methanogenic bacterium in lake sediments

Fluorescent antibody (FA) was prepared for a methanogenic bacterium isolated from Wintergreen Lake pelagic sediment. The isolate resembles Methanobacterium formicicum. The FA did not cross-react with 9 other methanogens, including M. formicicum strains, or 24 heterotrophs, 18 of which had been isolated from Wintergreen Lake sediment. FA-reacting methanogens were detected in heat-fixed smears of several different lake sediments and anaerobic sewage sludge. Pretreatment of all samples with either rhodamine-conjugated geletin or bovine serum albumin adequately controlled nonspecific absorption of the FA. Autofluorescent particles were observed in the sediment samples but, with experience, they could easily be distinguished from FA-reacting bacteria. FA direct counts of the specific methanogen in Wintergreen Lake sediments were made on four different sampling dates and compared with five-tube most-probable-number estimates of the total methanogenic population that was present in the same samples. The FA counts ranged from 3.1 X 10(6) to 1.4 X 10(7)/g of dry sediment. The highest most-probable-number estimates were at least an order ofmagnitude lower.