The most clinically important anaerobes are the five genera of Gram-negative rods. Bacteroides, especially the B. fragilis group (made of up ten spec… - Sydney M. Finegold

The field of infectious diseases covers many entities that can be considered true medical emergencies. Included are meningitis, brain abscess, spinal epidural abscess, epiglottitis, pneumonia, bacteremia, endocarditis, certain intraabdominal infections, gas gangrene, and necrotizing fasciitis. Because emergencies related to infectious agents are potentially the most readily reversible of all medical emergencies, it behooves us to diagnose them as rapidly and specifically as possible so that appropriate life-saving therapy may be begun expeditiously.

Most gastrointestinal infections secondary to the use of antimicrobial agents that have been documented are related to overgrowth of Clostridium difficile which produces a spectrum from severe pseudomembranous colitis to mild diarrhea or asymptomatic carriage. The most common inducers of pseudomembranous colitis or antimicrobial agent-associated diarrhea are ampicillin, clindamycin, and various cephalosporins, but almost all antimicrobials may cause this problem. Symptoms vary from watery to bloody diarrhea; the extent and severity of the diarrhea, fever, and abdominal cramps and the incidence of complications (such as toxic megacolon and perforation of the bowel) and of fatality are variable. Normal carriage of C. difficile in infants and asymptomatic carriage in adults who have received antimicrobial therapy make it impossible to rely on culture for diagnosis. The presence of cytotoxin or enterotoxin produced by C. difficile is much more reliable diagnostically, but there may be false-positives with this as well, particularly in infants.

Anaerobic bacteria currently demonstrate increased resistance to antimicrobial agents, primarily by the production of beta-lactamase. A number of species of Bacteroides, most notably those in the Bacteroides fragilis group, produce these enzymes. A few species of Fusobacterium and Clostridium produce beta-lactamase as well. Fortunately, this mechanism of resistance is readily overcome by administering beta-lactamase inhibitors coupled with a beta-lactam antibiotic that would otherwise be inactivated. Other types of resistance encountered in anaerobic bacteria include inactivating enzymes such as chloramphenicol acetyltransferase, plasmid-mediated transferable multiple-drug resistance, changes in porin molecules in the outer membrane of the bacterial cell, decreased uptake of drug by other mechanisms, changes in the target organs such as penicillin-binding proteins, and decreased reduction of the antibiotic to an active intermediate product. In many institutions, certain drugs such as cefoxitin, clindamycin, and piperacillin, which were previously active against almost all strains of B. fragilis, are now effective against only 70 to 85% of this group of anaerobes.