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Know All About Anaerobic Culture

Definition

  • An anaerobic bacteria culture is a method used to grow anaerobes from a clinical specimen.
  • Obligate anaerobes grow only in the absence of oxygen. They are destroyed when exposed to the atmosphere for as briefly as 10 minutes.
  • There are microaerophilic anaerobes that can tolerate 5% oxygen and aerotolerant ones that can grow in air e.g. 20% oxygen.
  • Few are facultative anaerobes that can grow in all atmospheres.

Purpose

  • To identify bacteria that grow only in the absence of oxygen and which may cause human infection.
  • If overlooked or killed by exposure to oxygen, anaerobic infections result in such serious consequences as amputation,
  • Culture is required to identify anaerobic pathogens and institute effective antibiotic treatment.

Features suggestive of anaerobic infection include

  • Foul smelling discharge.
  • Necrotic gangrenous tissue and abscess formation.
  • Free gas in tissue.
  • Black discoloration of exudates (Bacteroides melaninogenicus).
  • Sulphur granules in discharge (Actinomyces spp.).
  • Bacteraemia or endocarditis with no growth on aerobic blood cultures.

Precautions

  • Obtain the sample for culture via aseptic technique. Anaerobes are commonly found on mucous membranes and other sites such as the vagina and oral cavity.
  • Therefore, specimens likely to be contaminated with these organisms should not be submitted for culture (e.g., a throat or vaginal swab).
  • Some types of specimens should always be cultured for anaerobes if an infection is suspected.
  • These include abscesses, bites, blood, cerebrospinal fluid and exudative body fluids, deep wounds, and dead tissues.

Risk factors

Poor blood supply and tissue necrosis:

  • Foreign body.

Diabetes mellitus.

Splenectomy.

Immunocompromise.

Description

  • Anaerobes are normally found within certain areas of the body but result in serious infection when they have access to a normally sterile body fluid or deep tissue that is poorly oxygenated.
  • Some anaerobes normally live in the crevices of the skin, in the nose, mouth, throat, intestine, and vagina. Injury to these tissues (i.e., cuts, puncture wounds, or trauma) especially at or adjacent to the mucous membranes allows anaerobes entry into otherwise sterile areas of the body and is the primary cause of anaerobic infection.
  • A second source of anaerobic infection occurs from the introduction of spores into a normally sterile site. Spore-producing anaerobes live in the soil and water.
  • Gram-negative anaerobes and some of the infections they produce include the following genera.
  • Bacteroides (the most commonly found anaerobes in cultures; intra-abdominal infections, rectal abscesses, soft tissue infections, liver infection)
  • Fusobacterium (abscesses, wound infections, pulmonary and intracranial infections)
  • Porphyromonas (aspiration pneumonia, periodontitis)
  • Prevotella (intra-abdominal infections, soft tissue infections)

Gram-positive anaerobes include the following:

  • Actinomyces (head, neck, pelvic infections; aspiration pneumonia)
  • Bifidobacterium (ear infections, abdominal infections)
  • Clostridium (gas, gangrene, food poisoning, tetanus, pseudomembranous colitis)
  • Peptostreptococcus (oral, respiratory, and intra-abdominal infections)

 

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