Bacteroides species are anaerobic, aero-intolerant, nonsporeforming, nonmotile, gram-negative bacilli that colonize the oropharynx, female genital tract, and large bowel, comprising 95% of the colonic bacterial population and 20% of the dry weight of stool. Bacteriologic identification of Bacteroides organisms is frequently difficult because they are fastidious, sometimes slow-growing, and often resemble Escherichia coli and other gram-negative bacilli morphologically. Specimens infected by Bacteroides organisms may contain many gram-negative rods on gram stain but show no growth on culture if not inoculated promptly into anaerobic media. Infections are often mixed and sometimes fail to yield Bacteroides organisms because of overgrowth by less fastidious bacteria.
Commonly, the clinician first recognizes Bacteroides infection in his patient when the laboratory reports blood cultures positive. Accumulating experience suggests that the apparent incidence of bacillemia due to Bacteroides organisms is rising. In 1971 in the Mayo Clinic, Bacteroides was the second most common gramnegative bacillus isolated from