Mixed Anaerobic Infections

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

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

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:

Clostridial infections Overview of Clostridial Infections Clostridia are spore-forming, gram-positive, anaerobic bacilli present widely in dust, soil, and vegetation and as normal flora in mammalian gastrointestinal tracts. Pathogenic species produce... read more can lead to septic shock Sepsis and Septic Shock Sepsis is a clinical syndrome of life-threatening organ dysfunction caused by a dysregulated response to infection. In septic shock, there is critical reduction in tissue perfusion; acute failure... read more , 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 the elderly and in patients with cancer.

Patients usually have fever, rigors, and critical illness; shock is usually absent. DIC Disseminated Intravascular Coagulation (DIC) Disseminated intravascular coagulation (DIC) involves abnormal, excessive generation of thrombin and fibrin in the circulating blood. During the process, increased platelet aggregation and coagulation... read more may occur in Fusobacterium sepsis.

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

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

Clinical clues to the presence of anaerobic organisms include

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.

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

Most surgeons give both oral and parenteral antibiotics. For emergency colorectal surgery, parenteral antibiotics are used alone. Examples of oral regimens are neomycin (or kanamycin) plus erythromycin or metronidazole; these drugs are given no more than 18 to 24 hours before the procedure. Examples of parenteral preoperative regimens are cefotetan, cefoxitin, cefazolin plus metronidazole, and ertapenem; these drugs are given within 1 hour before the procedure. Preoperative parenteral antibiotics control bacteremia, reduce secondary or metastatic suppurative complications, and prevent local spread of infection around the surgical site.

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: