(See also Overview of Anaerobic Bacteria Overview of Anaerobic Bacteria Bacteria can be classified by their need and tolerance for oxygen: Facultative: Grow aerobically or anaerobically in the presence or absence of oxygen Microaerophilic: Require a low oxygen concentration... read more .)
Hundreds of species of nonsporulating anaerobes are part of the normal flora of the skin, mouth, gastrointestinal tract, and vagina. If this commensal relationship is disrupted (eg, by surgery, other trauma, poor blood supply, or tissue necrosis), a few of these species together can cause infections with high morbidity and mortality. After becoming established in a primary site, infection can spread locally and hematogenously to distant sites.
Because aerobic and anaerobic bacteria are frequently present in the same infected site, appropriate procedures for isolation and culture are necessary to keep from overlooking the anaerobes.
Anaerobes can be the main cause of infection in the following:
The pleural spaces and lungs
Intra-abdominal, gynecologic, central nervous system, upper respiratory tract, and cutaneous diseases
Bacteremia
Etiology of 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.
Pathophysiology of Mixed Anaerobic Infections
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 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.
Symptoms and Signs of Mixed Anaerobic Infections
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.
Disorders Often Caused by Mixed* Anaerobic Organisms
Bartholin gland infections |
Decubitus or ischemic ulcer infections |
Dental abscesses |
Human bite infections |
Liver abscess |
Nongonococcal tubo-ovarian abscess |
Parametrial abscess |
Pelvic peritonitis |
Septic thrombophlebitis |
Skene gland infection |
* With aerobes or other anaerobes. |
Diagnosis of Mixed Anaerobic Infections
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 anaerobic activity
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.
Treatment of Mixed Anaerobic Infections
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.
Oropharyngeal anaerobic infections and lung abscesses
Oropharyngeal anaerobic infections may not respond to penicillin and thus require a drug effective against penicillin-resistant anaerobes (see below).
Oropharyngeal infections and lung abscesses should be treated with clindamycin or a beta-lactam/beta-lactamase inhibitor combination such as amoxicillin/clavulanate. In patients allergic to penicillin, clindamycin or metronidazole (plus a drug active against aerobes and microaerophiles) is useful.
Gastrointestinal (GI) or female pelvic anaerobic infections
GI or female pelvic anaerobic infections are likely to contain obligate anaerobic gram-negative bacilli such as B. fragilis plus facultative gram-negative bacilli such as Escherichia coli; antibiotic regimens must be active against both. Resistance of B. fragilis and other obligate anaerobic gram-negative bacilli to penicillins and 3rd- and 4th-generation cephalosporins occurs. However, the following drugs have excellent in vitro activity against B. fragilis and are effective:
Metronidazole
Carbapenems (eg, imipenem/cilastatin, meropenem, ertapenem, doripenem)
Beta-lactam/beta-lactamase combinations (eg, piperacillin/tazobactam, ampicillin/sulbactam, amoxicillin/clavulanate, ticarcillin/clavulanate)
Tigecycline
Moxifloxacin
No single regimen appears to be superior. Drugs that are less predictably active in vitro against B. fragilis include clindamycin, cefoxitin, and cefotetan. All except clindamycin and metronidazole can be used as monotherapy because these drugs also have good activity against facultative anaerobic gram-negative bacilli.
Metronidazole is active against clindamycin-resistant B. fragilis, has unique anaerobic bactericidal activity, and usually avoids the pseudomembranous colitis sometimes associated with clindamycin. Concerns about metronidazole's potential mutagenicity have not been of clinical consequence.
Because many regimens currently used to treat GI or female pelvic anaerobic infections (see also treatment of pelvic inflammatory disease Treatment Pelvic inflammatory disease (PID) is a polymicrobial infection of the upper female genital tract: the cervix, uterus, fallopian tubes, and ovaries; abscess may occur. PID may be sexually transmitted... read more ) are also effective against facultative gram-negative bacilli, use of a potentially nephrotoxic aminoglycoside (to cover enteric facultative gram-negative bacilli) plus an antibiotic active against B. fragilis is no longer warranted.
Prevention of Mixed Anaerobic Infections
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. 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:
Clindamycin plus gentamicin, aztreonam, or ciprofloxacin
Metronidazole plus gentamicin or ciprofloxacin
Key Points
Mixed anaerobic infections occur when the normal commensal relationship among the normal flora of mucosal surfaces (eg, skin, mouth, gastrointestinal tract, vagina) is disrupted (eg, by surgery, injury, ischemia, or tissue necrosis).
Infections tend to occur as localized collections of pus or abscesses.
Base clinical suspicion on the clinical setting and the presence of gangrene, pus, abscess, tissue gas, and/or feculent odor.
Drain and debride the infected area, and give antibiotics selected based on the infection location (and thus likely organisms).
