Basics
Description
- Anaerobic bacteria are organisms capable of growing in a reduced oxygen environment, either exclusively (obligate anaerobes) or in addition to growing in air (facultative anaerobes).
- Anaerobic bacteria can cause invasive and serious diseases.
- Anaerobic bacteria tend to participate in polymicrobial infections with other anaerobic and aerobic flora.
Epidemiology
- Although anaerobic bacteremia is less frequent in children than in adults, other anaerobic infections such as chronic sinusitis or chronic otitis media are common in children.
- Because of their fastidious nature, the ability of microbiology laboratories to identify anaerobic bacteria is highly dependent on proper collection and transport of culture specimens; hence, anaerobic bacteria are often missed and likely underreported.
Risk Factors
- Impaired host immunity
- Malignancy
- Splenic dysfunction
- Hypogammaglobulinemia
- Presence of devitalized tissue
- Surgery, trauma
- Vascular insufficiency
- Poorly controlled diabetes
- Presence of foreign bodies
- Colitis
Pathophysiology
- Anaerobic infections commonly derive from the normal flora of the oropharynx, skin, intestines, or the female genital tract; thus, anaerobic infections occur when there is a loss of integrity of anatomic or epithelial barriers at these sites.
- Virulence factors include production of exotoxins (e.g., Clostridia spp.), endotoxins (e.g., Fusobacterium spp.), and presence of phagocyte-inhibiting capsules (e.g., Bacteroides spp.).
Etiology
- The most common clinically-relevant anaerobes include the following:
- Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium)
- Gram-positive cocci (Peptostreptococcus, Peptococcus)
- Spore-forming gram-positive bacilli (Clostridia)
- Non-spore-forming gram-positive bacilli (Eubacterium, Bifidobacterium, Propionibacterium, Actinomyces, Lactobacillus)
- Gram-negative cocci (Veillonella, Acidaminococcus)
- Spirochetes (many of which are anaerobic)
Commonly Associated Conditions
- CNS infections:
- Brain abscess due to bacteremia
- Subdural empyema
- Epidural abscess (most commonly due to complications from sinusitis)
- Head and neck infections:
- Sinusitis (generally polymicrobial)
- Chronic otitis media
- Ludwig angina (infection of the submandibular space)
- Cervical adenitis
- Peritonsillar abscess
- Dental abscess
- Gingivitis
- Actinomycosis of jaw
- Lemierre disease (septic thrombophlebitis of the internal jugular vein owing to anaerobic bacteremia, most commonly with Fusobacterium spp., often resulting in pulmonary abscess formation)
- Pleuropulmonary infections:
- Aspiration of infected amniotic or vaginal secretions in neonates
- Aspiration of oral or gastrointestinal fluids in children (severe gingival or periodontal may be a risk factor)
- Pneumonia, abscess formation due to aspirated foreign bodies
- Actinomycosis
- Peritonitis/peritoneal abscess:
- Appendiceal abscess
- Perforated viscus
- Postoperative complication
- Trauma-related
- Actinomycosis
- Cholangitis
- Ascending infection may occur following biliary tract surgery (e.g., Kasai procedure)
- Soft tissue infections:
- Paronychia
- Pilonidal cyst
- Hidradenitis suppurativa
- Crepitant cellulitis
- Necrotizing fasciitis
- Gas gangrene (Clostridium spp.)
- Infected decubitus ulcers (may result in contiguous osteomyelitis)
- Penetrating wounds (may lead to tetanus)
- Infections of the female genital tract:
- Endometritis
- Salpingitis
- Tubo-ovarian or adnexal abscess
- Pelvic inflammatory diseases
- Pelvic abscess
- Bartholin gland, vulvar, or perineal abscess
- Bacterial vaginosis
- Infected bite wounds
- Anaerobes isolated from 50% of human or animal bites
- Bacteremia
- Often associated with focal primary site of involvement (gastrointestinal disease, abscess)
- Neonatal infections:
- Cellulitis at fetal monitoring sites
- Aspiration pneumonia
- Omphalitis
- Conjunctivitis
- Infant botulism
Diagnosis
Involvement of anaerobic bacteria should be suspected in infections with suppuration or foul smell, abscess formation, tissue necrosis, or in hosts with systemic disease or defective immunity.
History
- Impaired mental status
- Increased risk of aspiration
- History of thumb sucking
- Anaerobes frequently isolated from paronychia
- History of animal or human bites
- Recent surgery or trauma
- Poor drainage or devitalized tissue associated with anaerobic infection
- Underlying immunodeficiency or chronic illness
- Impaired phagocytic function
- History of pus that is "sterile"¯ (no growth on routine cultures).
Physical Exam
- Location of infection
- See "Commonly Associated Conditions."¯
- Poor dentition
- Increased colonization of oropharynx with anaerobic organisms
- Necrotic tissue
- Crepitus with gas gangrene
- "Dishwater"¯ pus or discharge with foul odor
- Characteristic of anaerobic infections
- Lateral neck pain in association with respiratory distress:
- Lemierre disease results in septic thrombophlebitis of the internal jugular vein and formation of lung abscesses; untreated or undiagnosed Lemierre disease carries a significant mortality rate.
Diagnostic Tests & Interpretation
Lab
- Certain anaerobic bacteria have unique morphology on Gram stains:
- Bacteroides spp.: small, pleomorphic gram-negative bacilli
- Clostridium spp.: large gram-positive organisms with "boxcar"¯ morphology
- Anaerobic cultures
- Should be performed on tissue or aspirated fluid obtained directly from the infected sites in a sterile fashion
- Anaerobically collected specimens should be transported to the laboratory promptly.
- Swabs should not be sent for anaerobic cultures.
Imaging
- Radiographs may show the following:
- Air-fluid levels
- Cavity formation
- Gas in tissue
- CT and/or MRI scans
- Often important to define anatomic location and extent of disease, and to determine surgical approaches to drainage or debridement
Treatment
Medication
Empiric antibiotics for anaerobic infections include the following:
- Metronidazole
- Carbapenems (e.g. meropenem or imipenem)
- Chloramphenicol (supplies in the United States may be limited)
- β-lactam/β-lactamase inhibitor combinations (e.g., amoxicillin-clavulanate, ampicillin-sulbactam, ticarcillin-clavulanate, or piperacillin-tazobactam)
- Clindamycin
- Cephamycins (e.g., cefoxitin or cefotetan)
- Vancomycin has activity against gram-positive but not gram-negative anaerobes.
- Penicillins, cephalosporins, tetracyclines, macrolides, aminoglycosides, trimethoprim-sulfamethoxazole, and monobactams have either variable or poor activity against anaerobes and should not be used empirically.
- Most fluoroquinolones also have variable activity, but exceptions exist (e.g., moxifloxacin).
- Antibiotic resistance is increasing in certain anaerobes, especially Bacteroides spp.; not all microbiology laboratories routinely test anaerobes for antibiotic susceptibility.
- If a patient with a documented anaerobic infection does not appear to be responding to empiric therapy, it is advisable to consult with a infectious diseases specialist for further recommendations.
Empiric drug therapy reflects the polymicrobial nature of infections in which anaerobic bacteria are predominantly found:
- CNS infections
- Vancomycin + 3rd or 4th generation cephalosporin (e.g., ceftriaxone) + metronidazole
- Avoid using β-lactam/β-lactamase inhibitor combinations for brain abscesses, as β-lactamase penetration across blood-brain barrier may be suboptimal.
- Head and neck infections
- Ampicillin-sulbactam, amoxicillin-clavulanate, or clindamycin alone if gram-negative aerobic bacteria are of little concern
- Pleuropulmonary infections due to aspiration
- Ampicillin-sulbactam, amoxicillin-clavulanate, or clindamycin
- Peritonitis/peritoneal abscess
- Ampicillin-sulbactam, ticarcillin-clavulanate, piperacillin-tazobactam, cefoxitin, meropenem, or imipenem
- Cholangitis
- Piperacillin-tazobactam, meropenem, or imipenem
- Soft tissue infection
- Clindamycin, ampicillin-clavulanate
- Infections of the female genital tract
- Ampicillin-sulbactam, ticarcillin-clavulanate, piperacillin-tazobactam, cefoxitin, meropenem, or imipenem
- Infected bite wounds
- Ampicillin-sulbactam, piperacillin-tazobactam, amoxicillin-clavulanate
- Bacteremia
- Isolate-dependent, but it may be reasonable to start with vancomycin + 3rd or 4th generation cephalosporin (e.g., ceftriaxone) + metronidazole until isolate speciated and bacteremia confirmed to be due to anaerobic bacteria
Complementary & Alternative Therapies
- Neutralization of toxins, especially in the case of botulism or tetanus, is critical.
- Hyperbaric oxygen, although still sometimes used (especially in clostridial infections), has not been shown to be of proven benefit, although it may help define and demarcate the borders of devitalized tissues.
Surgery/Other Procedures
- Effective drainage of abscesses and debridement of devitalized tissue is essential.
- Cultures should be obtained by aspirating fluids into sterile syringes, capping them, and transporting these to the laboratory promptly.
- Cultures can also be obtained from intact tissues that are transported to the laboratory promptly.
- Swabs should not be sent for anaerobic cultures.
Ongoing Care
Prognosis
- Determined by speed with which infection is appropriately treated with antibiotics and/or drainage
- High rates of morbidity and mortality associated with untreated anaerobic bacteremia
- Specific prognosis depends on the bacterial species involved and the status of the patient's immune system.
- Soft tissue infections, necrotizing fasciitis, or gas gangrene caused by Clostridium spp. may cause up to 20% mortality despite aggressive therapy.
Complications
- Vary with nature of infection, but can include extension of infection to adjacent structures
- Development of bacteremia
Additional Reading
- Brook I. Anaerobic infections in children. Adv Exp Med Biol. 2011;697:117-152. [View Abstract]
- Brook I. Clinical review: bacteremia caused by anaerobic bacteria in children. Crit Care. 2002;6(3):205-211. [View Abstract]
- Japanese Society of Chemotherapy Committee on Guidelines for Treatment of Anaerobic Infections, Japanese Association for Anaerobic Infections Research. Chapter 1-1. Anaerobic infections (General): epidemiology of anaerobic infections. J Infect Chemother. 2011;17(Suppl 1):4-12. [View Abstract]
- Nagy E. Anaerobic infections: update on treatment considerations. Drugs. 2010;70(7):841-858. [View Abstract]
Codes
ICD09
- 041.84 Other specified bacterial infections in conditions classified elsewhere and of unspecified site, other anaerobes
- 040.89 Other specified bacterial diseases
- 473.9 Unspecified sinusitis (chronic)
- 101 Vincent's angina
- 382.9 Unspecified otitis media
ICD10
- A49.9 Bacterial infection, unspecified
- A49.8 Other bacterial infections of unspecified site
- J32.9 Chronic sinusitis, unspecified
- A69.1 Other Vincent's infections
- H66.90 Otitis media, unspecified, unspecified ear
SNOMED
- 423451008 Infection due to anaerobic bacteria
- 56688005 Clostridial infection (disorder)
- 40055000 Chronic sinusitis (disorder)
- 409865002 Vincent's disease (disorder)
- 21186006 Chronic otitis media (disorder)