Enumeration of anaerobic chytridiomycetes as thallus-forming units: novel method for quantification of fibrolytic fungal populations from the digestive tract ecosystem.

An endpoint dilution procedure, based on the technique of most probable numbers, was developed to enumerate anaerobic chytridiomycetes as thallus-forming units. The method does not distinguish between zoospores and thalli, but does permit enumeration of fungal populations with respect to their ability to digest plant cell walls. Fibrolytic populations in batch culture, ruminal contents, and feces were compared by relating viable counts to the dry matter content of enumerated samples (i.e., thallus-forming units per gram of dry matter). Batch cultures of Neocallimastix sp. strain R1 grown on wheat straw were used to assess the enumeration procedure and to demonstrate the potential of the technique for quantification of anaerobic fungi in vivo. Determination of total ruminal contents from steers enabled the quantification of the entire population of fiber-degrading anaerobic fungi in the reticulorumen. The enumeration procedure revealed substantial populations of fibrolytic anaerobic fungi in fresh and air-dried feces. Populations in fresh feces were equivalent to those in ruminal contents, but declined exponentially with time in dry feces. Minimum values were obtained from dry feces 90 days after drying, and anaerobic fungi were detectable for up to 210 days thereafter.