Aerobic and Anaerobic Bacteria

Anaerobic Bacteria Characteristics

• Anaerobes can thrive in environments containing less than 20% oxygen
• They exhibit poor growth in room air or may not grow at all
• They are a diverse group, including bacteria that tolerate 20% oxygen and those requiring less than 0.02% oxygen

Types of Aerobes

• Obligate aerobes, such as Pseudomonas aeruginosa, favor 20% oxygen in room air and cannot survive in anaerobic conditions
• Facultative anaerobes, such as E. coli, can adapt and grow under various conditions
• Microaerophilic organisms, such as Campylobacter jejuni, need reduced O2 concentrations for optimal growth

Obligate Anaerobes

• Obligate anaerobes like Bacteriodes fragilis and Clostridium perfringens need a near-total absence of oxygen to survive
• Many anaerobes utilize nitrogen as the terminal electron acceptor instead of oxygen

Oxygen Inhibition

• The reduced amount or complete absence of catalase and superoxide dismutase is the reason why oxygen inhibits the growth of anaerobes
• Catalase and SOD neutralize toxic hydrogen peroxide and superoxide compounds formed during energy production

Tissue Environment

• Beyond oxygen concentration, the oxidation-reduction potential (Eh) of tissue impacts the growth of anaerobes
• Low-Eh areas, such as the periodontal region, dental plaques, and colon, foster anaerobic growth
• Crushing injuries result in devitalized tissue due to impaired blood supply, leading to low Eh and enabling anaerobic growth and disease

Anaerobes of Medical Interest

• This group contains both cocci and bacilli, encompassing Gram-positive and Gram-negative bacteria
• Bacilli are categorized into spore-forming Clostridium and non-spore-forming Bacteroides
• Clostridium, Bacteroides, and Actinomyces represent the major anaerobes of medical significance

Clinical Infections

• Many medically relevant anaerobes form part of the normal body flora
• They typically do not cause disease in their normal habitat but can do so if they move to other sites
• Clostridium botulinum and Clostridium tetani are notable exceptions as soilborne organisms

Anaerobic Diseases

• Anaerobic diseases commonly manifest as abscesses
• These often occur in the brain, lung, female genital tract, biliary tract, and other intra-abdominal locations
• Abscesses contain multiple organisms, including multiple anaerobes, mixtures of anaerobes, and facultative anaerobes

Facultative Anaerobes

• Facultative anaerobes consume oxygen, creating a suitable environment for anaerobes to thrive
• Hallmarks of anaerobic infection include a foul-smelling discharge, gas in the tissue, and necrotic tissue

Laboratory Diagnosis

• Lab diagnosis of anaerobic infection is important, even before culturing the specimen
• You must obtain appropriate specimen
• You must rapidly transporting the specimen under anaerobic conditions to the laboratory

Laboratory Specimens

• Suitable specimen should exclude normal flora members to avoid interpretive confusion
• Appropriate specimen include blood, pleural fluid, pus, and transtracheal aspirates
• Sputum and feces are not appropriate as specimen

Laboratory Techniques

• Cultures are handled and incubated under the correct anaerobic conditions in the laboratory
• Diagnostic standards such as Gram staining, morphology, and biochemical tests exist
• Gas chromatography is an important specialized technique that measures organic acids

Treatment of Anaerobic Infections

• Treatment involves surgical drainage of the abscess combined with antibiotics
• Commonly used medications are Penicillin G, cefoxitin, clindamycin, chloramphenicol, and metronidazole
• Many B. fragilis isolates have B lactamases and resistance towards penicillins