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Mixed Anaerobic Infections


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University;

Maria T. Vazquez-Pertejo

, MD, FACP, Wellington Regional Medical Center

Reviewed/Revised Jun 2023
Topic Resources

Anaerobes can infect normal hosts and hosts with compromised resistance or damaged tissues. Anaerobic infections can include both single anaerobic species or multiple anaerobic species with any number of nonanaerobic isolates. Symptoms depend on site of infection. Diagnosis is clinical combined with Gram stain and anaerobic cultures. Treatment is with antibiotics and surgical drainage and debridement.

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 skin and soft-tissue 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.

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.

Symptoms and Signs of Mixed Anaerobic Infections

For specific infections (and symptoms) caused by mixed anaerobic organisms, see elsewhere in THE MANUAL and see table .

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


Disorders Often Caused by Mixed* Anaerobic Organisms

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 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.

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 an antibiotic 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 an antibiotic 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 antibiotics 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)

  • Tigecycline

  • Moxifloxacin

No single regimen appears to be superior. Antibiotics that are less predictably active in vitro against B. fragilis include clindamycin, cefoxitin, and cefotetan.

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 caused by sexually... 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.

Preoperative parenteral antibiotics control bacteremia, reduce secondary or metastatic suppurative complications, and prevent local spread of infection around the surgical site. Examples of oral antibiotics are neomycin plus either erythromycin or metronidazole; these antibiotics are given no more than 18 to 24 hours before the procedure. Examples of parenteral preoperative antibiotics include cefazolin plus metronidazole, or cefoxitin, cefotetan, or ertapenem each alone; these antibiotics are given within 1 hour before the procedure.

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).

Drugs Mentioned In This Article

Drug Name Select Trade
Cleocin, Cleocin Ovules, Cleocin Pediatric, Cleocin T, CLIN, Clindacin ETZ, Clindacin-P, Clinda-Derm , Clindagel, ClindaMax, ClindaReach, Clindesse, Clindets, Evoclin, PledgaClin, XACIATO
Amoxil, Dispermox, Moxatag, Moxilin , Sumox, Trimox
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
Primaxin, Primaxin IM
Zosyn, Zosyn Powder
Avelox, Avelox ABC Pack, Avelox I.V., MOXEZA, Vigamox
A/T/S, Akne-mycin, E.E.S., Emcin Clear , EMGEL, E-Mycin, ERYC, Erycette, Eryderm , Erygel, Erymax, EryPed, Ery-Tab, Erythra Derm , Erythrocin, Erythrocin Lactobionate, Erythrocin Stearate, Ilosone, Ilotycin, My-E, PCE, PCE Dispertab , Romycin, Staticin, T-Stat
Ancef, Kefzol
Garamycin, Genoptic, Genoptic SOP, Gentacidin, Gentafair, Gentak , Gentasol, Ocu-Mycin
Azactam, Cayston
Cetraxal , Ciloxan, Cipro, Cipro XR, OTIPRIO, Proquin XR
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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