The controversy regarding routine anaerobic blood cultures.

Anaerobic bacteria and the infections they cause have had their ups and downs through medical history. Although some of the best studies were done at the turn of the century, anaerobic infections were largely neglected in the early decades of the antibiotic era. The period from 1940 to 1960 was a time when putrid lung abscesses were euphemistically referred to as “nonspecific lung abscess,” intra-abdominal sepsis was commonly treated with penicillin plus streptomycin and subsequently with cephalothin, and the medical education about anaerobes was largely restricted to the classic histotoxic clostridia syndromes of tetanus, botulism, and gas gangrene. Nevertheless, the anaerobes themselves flourished. A review of bacteremia at the Mayo Clinic from 1968 to 1970 showed that Bacteroidaceae were the second most common organism that was identified, after Escherichia coli, in patients with gram-negative bacillary bacteremia (1). The period from 1965 to 1980 is sometimes referred to as the “anaerobic renaissance,” as it was characterized by the rediscovery of this group of organisms and their relevance in clinical medicine. This was facilitated by the development of a good taxonomic order for anaerobes, the development of the GasPak jar that made it possible for laboratories to cultivate oxygen-sensitive forms, several clinical studies demonstrating high yield and relevance of anaerobes, the development of clindamycin and metronidazole as favored drugs for their treatment, and the characterization of virulence factors for anaerobes, such as the capsular polysaccharide of B. fragilis. This is one of the most intensive campaigns in the history of medicine; it is also one of the most successful. Clindamycin has become a favored drug for “nonspecific lung abscess,” virtually all intra-abdominal infections are treated with antibiotics directed against anaerobes, and Bacteroides bacteremia has nearly disappeared (2). In this issue of the The American Journal of Medicine, Ortiz and Sande (3) have provided a review of blood culture results from 1994 to 1996 at the Salt Lake City Veterans Administration Medical Center. Anaerobic bacteria that was considered to be clinically significant accounted for only 7 (0.4%) of 1,597 positive cultures. The authors consequently asked whether the routine use of anaerobic blood cultures was still indicated. Standard blood culture practice in most laboratories is a two-bottle system, with one vented bottle for aerobes and a second unvented bottle for anaerobes. Many investigators have noted the low yield of anaerobic bacteria with this system. Most reports from the 1980s and 1990s show that obligate anaerobes accounted for only 2% to 4% of the bacteria recovered in blood cultures (2, 4 ‐7). The largest review was the 8-year experience from the Mayo Clinic, with 281,797 blood cultures from 1984 to 1992 (2). Obligate anaerobes accounted for 920 (3%) of the 28,205 microbes that were recovered. A review of the blood culture experience at Johns Hopkins is similar: of 1,954 positive blood cultures that yielded clinically significant bacteria during 1998, 44 (2.3%) were obligate anaerobes. These data are similar to the findings from Salt Lake City VA Hospital and provide further testimony to the remarkable achievement of the “anaerobic renaissance” with the use of antimicrobials that have dealt a nearly lethal blow to anaerobic bacteremia.