Index >> Fundamentals of Microbial Ecology >> Trends in Succession

Microbial communities associated with detritus are complex, but predictable population changes occur during succession. If sterilised natural detrital particles are placed in sea water or fresh water inoculated with a small amount of natural detritus, a characteristic succession of organisms occurs. This succession leads to a microbial community closely resembling that of natural detritus. Bacteria occur in small numbers-on the particles after 6-8 hours and reach their maximal numbers after 15-150 hours. The bacterial populations then decrease and become relatively stable after about 200 hours. Small zooflagellates appear about 20 hours after inoculation and reach maximal population sizes  after 100-200 hours. Ciliates appear after abouf­100 hours and reach maximal numbers at 200-300 hours. Other groups of microorganisms including rhizopods and diatoms usually appear late in succession.

The succession of bacterial populations in humans and other non-ruminant mammals normally begins with colonisation of the gastrointestinal tract by Bifidobacterium and Lactobacillus species. This is followed by a succession of facultative anaerobes, such as E.coli and Streptococcus faecalis. Populations of strictly anaerobic bacteria such as Bacteroides, appear late in the succession, after the beginning of solid food ingestion. These populations of obligate anaerobes become dominant.

In ruminants, succession leads to the development of a complex, obligately anaerobic microbial community. Included in the climax community of the rumen are populations of cellulose-degrading bacteria such asBacteroides and Ruminococcus, starch-degrading bacteria such as Selenomonas, methanogenic bacteria such as Methanobacterium, cellulose and pectin degrading protozoa such as Polyplastrom, and other populations.