Culture-based knowledge on biodiversity, development and stability of human gastrointestinal microflora

The human large intestine harbours an abundant and extremely diverse microflora with more than 400-500 microbial species estimated. The dominant microflora in faecal samples is obligately anaerobic: Especially Bacteroides spp., Eubacterium spp., Bifidobacterium spp., lactobacilli, anaerobic cocci and Clostridium spp. are detected in high numbers by bacterial culture. In addition, facultatively anaerobic organisms such as Escherichia coli, enterococci and streptococci are commonly encountered. Although several thorough studies on the diversity of the faecal flora has been performed during 1970's, investigation of intestinal flora by culture-based methods has several limitations: The lack of culture media supporting the growth of some components of dominant flora, difficulties in identification of bacteria by traditional phenotypic methods and continuous changes in taxonomy complicate studies on the intestinal flora by bacterial culture. Since invasive sampling techniques are required to obtain intestinal samples, most studies on human intestinal flora have been restricted mainly to the analysis of the faecal flora. A faecal sample seems to represent the microflora in the large intestine, whereas the microflora in the upper parts of the intestinal tract is sparse and less diverse. During the development of the intestinal flora in newborns, facultatively anaerobic species, especially E. coli, staphylococci and streptococci are colonising first and subsequently obligate anaerobes including Bacteroides spp., Bifidobacterium spp. and Clostridium spp. appear in faeces within few days. By the end of the second year of life the faecal flora resembles that of adults. Although a great variation in the flora composition exists between individuals, the faecal microflora in adults is relatively stable. However, several factors such as changes in age, diet, geographic location and disease may influence the composition of the normal intestinal flora.