Overview
Anaerobic bacteria are widespread and medically important organisms that do not require oxygen for growth — in fact, oxygen is often toxic to them because they lack key enzymes such as superoxide dismutase, peroxidase and catalase, making them susceptible to oxygen-derived free radicals.
Non-sporing anaerobes form part of the normal commensal flora of the mouth, oropharynx, gastrointestinal tract and genitourinary tract, but under the right conditions they become important pathogens, often producing polymicrobial, foul-smelling infections that are notoriously difficult to diagnose in the routine laboratory.
Learning Objectives
After this lesson you will be able to…- Classify non-sporing anaerobes based on morphology and Gram-staining characters
- Differentiate facultative, obligate, microaerophilic and aerotolerant anaerobes
- Describe the pathogenic potential of anaerobic cocci and Gram-negative bacilli
- Explain proper specimen collection and transport for anaerobic culture
- Outline the laboratory diagnosis of important anaerobic pathogens
Clinical Story
Why This MattersA 45-year-old diabetic man presents with a deep, foul-smelling abscess on his foot. The pus is aspirated under strict anaerobic technique and sent to the lab in an airtight syringe. Gram stain reveals a mixed bacterial flora with abundant pus cells — a classic clue pointing the technologist toward polymicrobial anaerobic infection, most likely involving Bacteroides fragilis.
Core Concepts
Anaerobes are grouped by their tolerance to oxygen. Facultative anaerobes (e.g. Peptostreptococcus) can grow with or without oxygen. Obligate anaerobes (e.g. Bacteroides) find oxygen toxic and never use it as a terminal electron acceptor. Microaerophilic organisms (e.g. spirochetes) need only low oxygen levels. Aerotolerant anaerobes (e.g. Propionibacterium) metabolise anaerobically but are unaffected by atmospheric oxygen.
Peptostreptococcus species are Gram-positive cocci implicated in puerperal sepsis, wound infections, gangrenous appendicitis, UTIs, osteomyelitis and abscesses. Veillonella parvula is the key Gram-negative anaerobic coccus, usually non-pathogenic. Bacteroides fragilis is the most commonly isolated clinical anaerobe, causing meningitis, brain abscess, and pleural/peritoneal infections; Prevotella melaninogenica produces brick-red UV fluorescence.
Contact with air must be minimised. Pus and fluids are aspirated into an airtight syringe with the needle plunged into a rubber cork to seal it. Swabs should never be used. PRAS media, gassed-out vials, Hungate's tubes, Robertson's Cooked Meat medium (RCM) or Thioglycollate broth (TGB) are used for transport.
Laboratory Principle
Anaerobic culture relies on excluding atmospheric oxygen using anaerobic jars, the GasPak system, or anaerobic chambers, combined with enriched media such as blood agar and Bacteroides bile aesculin agar. Since most anaerobes are slow-growing, cultures require a minimum of 72 hours of incubation. Foul smell, gas production, and mixed flora on Gram stain are strong presumptive indicators of anaerobic infection.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Blood Agar | 5% sheep blood | Primary anaerobic culture medium | 2–8°C |
| Kanamycin-Vancomycin Blood Agar | Selective | Selective isolation of Gram-negative anaerobes | 2–8°C |
| Thioglycollate Broth | Standard | Enrichment broth for anaerobes | Room temperature |
Step-by-Step Procedure
Aspirate pus/fluid in an airtight syringe minimising air contact; avoid swabs.
Place specimen in PRAS media, gassed-out vials, or Robertson's Cooked Meat broth immediately.
Note foul odour; examine under UV light for brick-red fluorescence; Gram stain for mixed flora and pus cells.
Inoculate blood agar, BHI agar, PEA and selective anaerobic media; incubate anaerobically for minimum 72 hours.
Identify colonies by Gram stain, biochemical reactions and antibiotic susceptibility pattern.
Flow Diagram
Quality Control
Internal QC uses known anaerobic (e.g. B. fragilis ATCC 25285) and facultative control strains to verify that anaerobic jars/chambers achieve and maintain a truly anoxic environment, confirmed with a methylene blue or resazurin indicator strip.
Participation in External Quality Assessment Schemes (EQAS) for anaerobic bacteriology ensures inter-laboratory consistency in anaerobe identification and susceptibility testing.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Foul-smelling pus with mixed flora on Gram stain | Suggestive of polymicrobial anaerobic infection | Culture anaerobically for 72h; consider metronidazole |
| Brick-red UV fluorescence | Indicates Prevotella melaninogenica | Correlate with lung/liver abscess or oral infection |
| Clue cells + fishy odour on KOH + pH >4.5 | Bacterial vaginosis (Mobiluncus, Gardnerella, anaerobes) | Confirm with wet mount and treat accordingly |
Common Errors & How to Avoid Them
Cause: Swabs expose the specimen to air, killing strict anaerobes.
Prevention: Always use airtight syringe aspiration or tissue biopsy.
Cause: Prolonged air exposure reduces anaerobe viability.
Prevention: Transport immediately in PRAS or gassed-out transport media.
Cause: Slow-growing anaerobes may be missed if cultures are read too early.
Prevention: Incubate for a minimum of 72 hours before discarding as negative.
Laboratory Tips from the Bench
A putrid, foul smell from a specimen is one of the strongest bedside clues to anaerobic infection — always note it on the request form.
Examine anaerobic specimens under UV light early — Prevotella melaninogenica's brick-red fluorescence can be lost after prolonged exposure to air.
Remember 'BLAST' for common anaerobic pathogens: Bacteroides, Leptotrichia, Actinomyces, Spirochetes, Treponema/Peptostreptococcus.
Important Notes
Most anaerobic infections are polymicrobial — a pure anaerobic culture should always be interpreted alongside clinical findings.
Anaerobic cocci and Gram-negative bacilli are typically resistant to aminoglycosides like streptomycin and gentamicin; metronidazole remains the mainstay of therapy.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePresents with a deep, foul-smelling abscess on the sole of his right foot of 5 days duration, associated with fever and local swelling. History of poorly controlled type 2 diabetes.
The foul odour, mixed Gram stain flora and growth of Bacteroides fragilis with Peptostreptococcus on anaerobic culture confirm a polymicrobial anaerobic diabetic foot infection requiring surgical debridement and metronidazole-based antibiotic therapy.
- →Foul smell and mixed flora on Gram stain are strong clues to anaerobic infection
- →Diabetic patients are at high risk for polymicrobial anaerobic soft tissue infections
- →Metronidazole remains the drug of choice for anaerobic Gram-negative bacilli
Frequently Asked Questions
Swabs expose the specimen to atmospheric oxygen during collection and transport, which can kill strict anaerobes before they reach the laboratory. Aspiration into an airtight syringe better preserves anaerobe viability.
Mobiluncus species, along with Gardnerella vaginalis and other anaerobes, cause bacterial vaginosis, characterised by a thin foul-smelling discharge, vaginal pH >4.5, a positive KOH 'whiff test' and clue cells on microscopy.
Most anaerobes are slow-growing organisms; a minimum of 72 hours' incubation is required to reliably detect their growth, compared to 24–48 hours for typical aerobes.
Quick Revision
10-Minute ReviewKey Takeaways
- Non-sporing anaerobes are classified by oxygen tolerance and by Gram stain/morphology.
- Bacteroides species are the most frequently isolated non-sporing anaerobic pathogens.
- Proper anaerobic specimen collection (airtight syringe, no swabs) is essential for accurate diagnosis.
- Diagnosis relies on foul odour, mixed flora on Gram stain, UV fluorescence and prolonged anaerobic culture.
- Most anaerobes remain sensitive to metronidazole and penicillin but resistant to aminoglycosides.
- Anaerobic infections are typically polymicrobial and involve normal commensal sites.
Competency Checklist
Track Your MasteryReferences
- Ananthanarayan R, Paniker CKJ. Textbook of Microbiology. 10th ed. Universities Press.
- Chatterjee KD. Parasitology (Protozoology and Helminthology). 13th ed. CBS Publishers.
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 13th ed. Mosby.