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 (typically 2 to 10%) and, for many, a high carbon dioxide concentration (eg, 10%); grow very poorly anaerobically
Obligate anaerobic: Are incapable of aerobic metabolism but are variably tolerant of oxygen
Obligate anaerobes replicate at sites with low oxidation-reduction potential (eg, necrotic, devascularized tissue). Oxygen is toxic to them. Obligate anaerobes have been categorized based on their oxygen tolerance:
Strict: Tolerate only ≤ 0.5% oxygen
Moderate: Tolerate 2 to 8% oxygen
Aerotolerant anaerobes: Tolerate atmospheric oxygen for a limited time
The obligate anaerobes that commonly cause infection can tolerate atmospheric oxygen for at least 8 hours and frequently for up to 72 hours.
Obligate anaerobes are major components of the normal microflora on mucous membranes, especially of the mouth, lower gastrointestinal (GI) tract, and vagina; these anaerobes cause disease when normal mucosal barriers break down.
Gram-negative anaerobes and some of the infections they cause include
Bacteroides (most common): Intra-abdominal infections
Fusobacterium: Abscesses, wound infections, and pulmonary and intracranial infections
Porphyromonas: Aspiration pneumonia and periodontitis
Prevotella: Intra-abdominal and soft-tissue infections
Gram-positive anaerobes and some of the infections they cause include
Actinomyces: Head, neck, abdominal, bone, and pelvic infections and aspiration pneumonia (actinomycosis Actinomycosis Actinomycosis is a chronic localized or hematogenous anaerobic infection caused by Actinomyces israelii and other species of Actinomyces. Findings are a local abscess with multiple... read more
)Clostridia 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 : Intra-abdominal infections Clostridial Intra-Abdominal and Pelvic Infections Clostridia, primarily Clostridium perfringens, are common in mixed intra-abdominal infections due to a ruptured viscus or pelvic inflammatory disease. Symptoms of abdominal infection... read more (eg, clostridial necrotizing enteritis Clostridial Necrotizing Enteritis Clostridial necrotizing enteritis is necrosis of the jejunum and ileum caused by Clostridium perfringens. Symptoms can range from mild diarrhea to septic shock and sometimes death. Diagnosis... read more ), soft-tissue infections Clostridial Soft-Tissue Infections Clostridial soft-tissue infections include cellulitis, myositis, and clostridial myonecrosis. They usually occur after trauma. Symptoms may include edema, pain, gas with crepitation, foul-smelling... read more
, and gas gangrene due to C. perfringens; food poisoning Clostridium perfringens Food Poisoning Clostridium perfringens food poisoning is acute gastroenteritis caused by ingestion of contaminated food. Symptoms are watery diarrhea and abdominal cramps. Diagnosis is by identifying... read more due to C. perfringens type A; botulism Botulism Botulism is poisoning that is due to Clostridium botulinum toxin and that affects the peripheral nerves. Botulism may occur without infection if toxin is ingested, injected, or inhaled... read more and infant botulism Infant Botulism Infant botulism results from ingestion of Clostridium botulinum spores, their colonization of the large intestine, and toxin production in vivo. Symptoms are initial constipation followed... read more due to C. botulinum; tetanus Tetanus Tetanus is acute poisoning from a neurotoxin produced by Clostridium tetani. Symptoms are intermittent tonic spasms of voluntary muscles. Spasm of the masseters accounts for the name... read more 
due to C. tetani; and Clostridioides (formerly Clostridium) difficile–induced colitis and diarrhea Clostridioides (formerly Clostridium) difficile–Induced Diarrhea Toxins produced by Clostridioides difficile strains in the gastrointestinal tract cause pseudomembranous colitis, typically after antibiotic use. Symptoms are diarrhea, sometimes bloody... read more (pseudomembranous colitis)Peptostreptococcus and Finegoldia: Oral, respiratory, bone and joint, soft-tissue, and intra-abdominal infections
Cutibacterium (formerly Propionibacterium): Foreign body infections (eg, in a cerebrospinal fluid shunt, prosthetic joint, or cardiac device)
Anaerobic infections are typically suppurative, causing abscess formation and tissue necrosis and sometimes septic thrombophlebitis, gas formation, or both. Many anaerobes produce tissue-destructive enzymes, as well as some of the most potent paralytic toxins known.
Usually, multiple species of anaerobes are present in infected tissues; aerobes are frequently also present (mixed anaerobic infections Mixed Anaerobic Infections Anaerobes can infect normal hosts and hosts with compromised resistance or damaged tissues. Mixed anaerobic infections can include both single anaerobic species or multiple anaerobic species... read more ).
Clues to anaerobic infection include
Polymicrobial results on Gram stain or culture
Bacteria are seen on Gram stain, but aerobic cultures are sterile
Gas in pus or infected tissues
Foul odor of pus or infected tissues
Necrotic infected tissues
Site of infection near mucosa where anaerobic microflora normally reside
Testing
Specimens for anaerobic culture should be obtained by aspiration or biopsy from normally sterile sites. Delivery to the laboratory should be prompt, and transport devices should provide an oxygen-free atmosphere of carbon dioxide, hydrogen, and nitrogen. Swabs are best transported in an anaerobically sterilized, semisolid medium such as Cary-Blair transport medium.
