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Abstract
GRAM-NEGATIVE anaerobic bacilli are a major component of the normal flora of human faeces. In quantitative studies, Bacteroides spp. have been isolated in counts of 1011 organisms/g of faeces (Hill et al., 1971; Finegold, Attebury and Sutter, 1974; Finegold, 1977). Drasar (1974) isolated Bacteroides in greater numbers from subjects from developed countries whereas gram-positive anaerobes were predominant in the faeces of subjects from developing countries.
Investigators who have identified faecal Bacteroides have found that the B. fragilis group are the dominant species. A previous taxonomic scheme (Hol-deman and Moore, 1974a) assigned all the bile- and penicillin-resistant faecal strains to the single species B. fragilis, with five subspecies; when detailed identification was undertaken the commonest faecal subspecies were ss. vulgatus and ss. thetaiotaomicron (Werner, 1974; Moore and Holdeman, 1975; Finegold et al., 1975). However, DNA-homology studies have shown that each subspecies is unique and should be assigned species status (Cato and Johnson, 1976) and several new species have been described to accommodate strains with different properties within the B. fragilis group (Holdeman and Moore, 1974b; Werner, Rintelen and Kunstek-Santos, 1975). Many other Bacteroides and Fusobacterium spp. have been found inconsistently in small numbers in faeces. This may reflect the problems of isolation, identification and classification of these organisms. B. asaccharolyticus (B. melaninogenicus ss. asaccharolyticus; Finegold and Barnes, 1977) has been isolated regularly from faeces (Werner, Pulverer and Reichertz, 1971) but in much smaller numbers than the B. fragilis group.
The present study was undertaken to assess the contribution of the different species and subspecies of Bacteroides to the normal faecal flora as part of the evaluation of a scheme for the identification of gram-negative anaerobic bacilli by conventional bacteriological tests (Duerden et al., 1980).
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