THE MICROBIAL CONTRIBUTION TO HUMAN FAECAL MASS

Abstract

PLATE II THE NORMAL daily faecal output in Britain is 100-200 g per person, of which 25-50 g is solid matter (Wyman et al., 1978). It is commonly thought that a major component of human faeces is undigested plant material or fibre, and that when stool output increases on addition of fibre to the diet, the increase is due to water held in the colon by this material. On an average British diet however, only 3-5 g/day of dietary fibre remains in the stool (Cummings et al., 1979) and, on microscopic examination, what remains appears lignified and physically inert and is unlikely to account for the water content of normal faeces. The high nitrogen content of faeces, 6% of the dry weight (Cummings 1978), suggests that the bacterial component of faecal material may be larger than previously thought. Previous estimates of the bacterial component of the wet faecal mass are 30-40% (van Houte and Gibbons, 1966; Moore and Holdeman, 1975). These estimates were based on direct microscopic counts which were then converted to a weight, assuming an average size for the bacteria. A more accurate method of assessing bacterial mass would be to separate the microbial fraction from the other faecal material and weigh it. We have therefore developed a method that fractionates faeces into three main components: the bacteria, undigested fibre, and soluble substances. The procedure has been developed from techniques used to isolate microbial matter from the rumen (Hoogenraad and Hird, 1970; Smith and McAllan 1974), with several steps altered or omitted to improve the separation of bacteria from fibrous debris and to ensure the purity of the bacterial fraction. By this method, a direct estimate of the microbial contribution to the weight of the stool has been obtained.