Mixed Anaerobic Infections

The principal anaerobic gram-positive cocci involved in mixed anaerobic infections are

• Peptococci

• Peptostreptococci

These anaerobes are part of the normal flora of the mouth, upper respiratory tract, and large intestine.

The principal anaerobic gram-negative bacilli involved in mixed anaerobic infections include

• Bacteroides fragilis

• Prevotella melaninogenica

• Fusobacterium species

The B. fragilis group is part of the normal bowel flora and includes the anaerobic pathogens most frequently isolated from intra-abdominal and pelvic infections. Organisms in the Prevotella group and Fusobacterium species are part of the normal oral, vaginal, and large-bowel flora.

Mixed anaerobic infections can usually be characterized as follows:

• They tend to occur as localized collections of pus or abscesses.

• The reduced oxygen tension and low oxidation-reduction potential that prevail in avascular and necrotic tissues are critical for their survival.

• When bacteremia occurs, it usually does not lead to disseminated intravascular coagulation (DIC) and purpura.

Clostridial infections can lead to septic shock, but most other anaerobic infections do not.

Some anaerobic bacteria possess distinct virulence factors. The virulence factors of B. fragilis probably account for its frequent isolation from clinical specimens despite its relative rarity in normal flora compared with other Bacteroides species. This organism has a polysaccharide capsule that apparently stimulates abscess formation. An experimental model of intra-abdominal sepsis has shown that B. fragilis alone can cause abscesses, whereas other Bacteroides species require the synergistic effect of another organism. Another virulence factor, a potent endotoxin, is implicated in septic shock and DIC associated with severe Fusobacterium pharyngitis.

Morbidity and mortality rates for anaerobic and mixed bacterial sepsis are as high as those for sepsis caused by a single aerobic organism. Anaerobic infections are often complicated by deep-seated tissue necrosis. The overall mortality rate for severe intra-abdominal sepsis and mixed anaerobic pneumonias tends to be high. B. fragilis bacteremia has a high mortality rate, especially in older adults and in patients with cancer.

Patients usually have fever, rigors, and critical illness; shock is usually absent. DIC may occur in Fusobacterium sepsis.

For specific infections (and symptoms) caused by mixed anaerobic organisms, see elsewhere in THE MANUAL and see table Disorders Often Caused by Mixed Anaerobic Organisms.

Anaerobes are rare in urinary tract infection, septic arthritis, and infective endocarditis.

• Clinical suspicion

• Gram stain and culture

Clinical clues to the presence of anaerobic organisms include

• Infection adjacent to mucosal surfaces that bear anaerobic flora

• Ischemia, tumor, penetrating trauma, foreign body, or perforated viscus

• Spreading gangrene involving skin, subcutaneous tissue, fascia, and muscle

• Feculent odor in pus or infected tissues

• Abscess formation

• Gas in tissues

• Septic thrombophlebitis

• Failure to respond to antibiotics that do not have significant activity against anaerobic bacteria

Anaerobic infection should be suspected when any wound smells foul or when a Gram stain of pus from an infected site shows mixed pleomorphic bacteria but aerobic cultures show no growth. Only specimens from normally sterile sites should be cultured anaerobically because commensal anaerobic contaminants may easily be mistaken for pathogens.

Gram stains and aerobic cultures should be obtained for all specimens. Cultures for anaerobes, particularly if mishandled, may be falsely negative. Antibiotic susceptibility testing of anaerobes is exacting, and data may not be available for ≥ 1 week after initial culture. However, if the species is known, susceptibility patterns can usually be predicted. Therefore, many laboratories do not routinely test anaerobic organisms for susceptibility.

• Drainage and debridement

• Antibiotic choice varying by site of infection

In established infection, pus is drained, and devitalized tissue, foreign bodies, and necrotic tissue are removed. Organ perforations must be treated by closure or drainage. Whenever possible, blood supply should be reestablished. Septic thrombophlebitis may require vein ligation as well as antibiotics.

Because anaerobic culture results may not be available for 3 to 5 days, antibiotics are started empirically. Antibiotics sometimes work even when some of the bacterial species in a mixed infection are resistant to the antibiotic (eg, because of loss of support from other bacterial species or the necrotic anaerobic environment), especially if surgical debridement and drainage are adequate. Antibiotics are chosen based on infection site and thus likely organisms.

Before elective colorectal surgery, patients should have bowel preparation consisting of

• Cathartics

• Enemas

• Antibiotics

Most surgeons give both oral and parenteral antibiotics. For emergency colorectal surgery, parenteral antibiotics are used alone.

During lengthy procedures, intraoperative antibiotics may be given every 1 to 2 half-lives of the antibiotic. Typically, postoperative antibiotics are not continued beyond 24 hours after surgery.

For patients with confirmed allergy or adverse reaction to beta-lactams, one of the following regimens is recommended: