The Mucosal Anaerobic Gram‐Negative Bacteria of the Human Colon.

Our recent work on gram-negative anaerobes has centered on inhibition of phagocytosis and intracellular killing are likely to be important. the host response to Bacteroides species, especially to the lipopolysaccharide component of the cell envelope. The main topics of Other studies suggest that although B. fragilis is a minor compointerest have been (1) to examine the antibody response to Bactenent of feces, it is in fact found as a major component of the roides lipopolysaccharide in health and disease [1, 2], (2) to invesmucosa-associated flora. Its proximity to the extraintestinal comtigate the mechanisms of resistance to serum complement [3], and partment of the body and its apparent superior adherence properties (3) to assess the inflammatory potential of the supposedly weakly are likely to contribute significantly to its role as the most important endotoxic lipopolysaccharide molecule of Bacteroides species. pathogen of the genus. One such study, by Namavar et al. [8], This last topic was the subject of a presentation at a previous tends to support this theory. B. fragilis was shown to be the major meeting of the Anaerobe Society of the Americas, in 1994 [4]. component of the mucosal flora, and it was suggested that this A sound knowledge of the basic bacteriology of the species was the main reason why B. fragilis is the Bacteroides species most belonging to the genus Bacteroides and of their natural history commonly isolated from patients with gut-associated abdominal within the colon is essential background to these immunologic sepsis. studies. Much information is available from the established literaBecause of these apparent contradictions and ambiguities and ture. There is a wealth of quantitative studies on the different the importance of such basic information for our current studies, Bacteroides species found in the gastrointestinal tract during health we decided to investigate for ourselves the prevalence of B. fragand disease [5]. It must be stressed that much of this information ilis, as well as other species of Bacteroides, in the adherent compois based on the bacteriology of feces. There has been little quantitanent of the colonic microflora. tive work on the bacteria that adhere to the colonic mucosa, and Biopsy specimens of mucosa were taken during routine colonosthis has resulted in some possibly erroneous statements. For incopy from the proximal colon (10 from the ascending colon and stance, there are many examples in the literature that state that 2 from the proximal transverse colon) and the rectum of 12 paBacteroides fragilis is a minor component of the colonic mitients, 6 with ulcerative colitis and 6 with noninflammatory condicroflora, but what is strictly meant is that B. fragilis is a minor tions. Informed consent was given by all patients. The areas of component of feces. colonic mucosa selected for sampling were free of any visibly The position of B. fragilis as the major pathogenic species of adherent fecal material. Biopsy specimens were not washed or the genus is well established. The reasons for its greater pathogenic processed in any way to preserve the mucous layer and associated potential, compared with that of the other species, have been the bacteria. subject of much research. This has caused some controversy. Its After anaerobic transport to the laboratory in Stuart’s medium mechanisms of pathogenicity are slowly being resolved, and now (Unipath, Basingstoke, U.K.) in an anaerobic environment, biopsy much is understood [6], especially the role of the capsule in the specimens were homogenized, diluted in prereduced anaerobe formation of abscesses [7]. identification medium, and examined by quantitative bacteriologic Factors that have not yet been satisfactorily resolved are whether culture on selective and nonselective media. Total aerobic counts the prevalence and/or location of B. fragilis and whether it is were made on Columbia agar (Unipath) containing 5% horse adherent (mucosa-associated) vs. planktonic/luminal (fecal) within blood. Total anaerobic counts were made on Fastidious Anaerobe the gastrointestinal tract have any relationship to virulence. The Agar (Lab M, Bury, U.K.), containing 4% whole horse blood and published data are ambiguous. Most authors stress that as B. frag1% freeze-thawed lysed horse blood (FABA). ilis is a minor component of feces (Ç4% of total Bacteroides Bacteroides species were selectively cultured on FABA conspecies), its main virulence factors, compared to those of the more taining kanamycin (75 mg/L) and vancomycin (2.5 mg/L). Lactocommonly found fecal Bacteroides species, must be especially bacillus species and asaccharolytic lactic acid producers (not idendominant once outside the gut. These will permit its survival, tified by species) were isolated on Rogosa agar (Unipath), and multiplication, and subsequent ability to cause harm. Here the roles Bifidobacterium species on reinforced clostridial cotton blue agar of capsular polysaccharide, resistance to complement action, and [9]. The results are summarized in table 1. Counts from the proximal colonic and rectal biopsy samples in the same patient were not significantly different, and the means are given in table 1. Counts of viable aerobes and facultative organisms ranged from 2.6 1 10 to 7.2 1 10 cfu/biopsy specimen (5.6 mg or 20 mm), Grant support: SmithKline Beecham and the Edinburgh Intestinal Immunology Fund. and total anaerobic counts were 10to 100-fold higher, at 6.6 Reprints or correspondence: Dr I. R. Poxton, Department of Medical Micro1 10 to 2 1 10 cfu/biopsy specimen. biology, University of Edinburgh Medical School, Teviot Place, Edinburgh Bacteroides species predominated at both sites (range, 3.9 EH8 9AG, Scotland, United Kingdom. 1 10 to 1.1 1 10 cfu/biopsy specimen), comprising Ç66% of Clinical Infectious Diseases 1997;25(Suppl 2):S111–3 total counts for the proximal colon (range in individual patients, q 1997 by The University of Chicago. All rights reserved. 1058–4838/97/2503–0007$03.00 31%–80%) and 68.5% for the rectum (range, 38%–91%). Lacto-