Anaerobic Bacteria Culture Health Article

Definition

An anaerobic bacteria culture is a method used to grow anaerobes from a clinical specimen. Obligate anaerobes are bacteria that can live only in the absence of oxygen. Obligate anaerobes are destroyed when exposed to the atmosphere for as briefly as 10 minutes. Some anaerobes are tolerant to small amounts of oxygen. Facultative anaerobes are those organisms that will grow with or without oxygen. The methods of obtaining specimens for anaerobic culture and the culturing procedure are performed to ensure that the organisms are protected from oxygen.

Purpose

Anaerobic bacterial cultures are performed 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 serious consequences such as amputation, organ failure, sepsis, meningitis, and death. Culture is required to

correctly identify anaerobic pathogens and institute effective antibiotic treatment.

Precautions

It is crucial that the health care provider 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 necrotic tissues. The specimen must be protected from oxygen during collection and transport and must be transported to the laboratory immediately. The health care team member who performs the collection should follow universal precautions for the prevention of transmission of bloodborne pathogens.

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 mucus 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 and spores may be introduced via wounds especially punctures. Anaerobic infections are most likely to be found in persons who are immunosuppressed, those treated recently with broad-spectrum antibiotics, and persons who have a necrotic, discolored injury on or near a mucus membrane, especially if the site is foul- smelling.

Some specimens from which anaerobes are likely to be isolated are:

• blood
• bile
• bone marrow
• cerebrospinal fluid
• direct lung aspirate
• tissue biopsy from a normally sterile site
• fluid from a normally sterile site (like a joint)
• dental abscess
• abdominal or pelvic abscess
• knife, gunshot, or surgical wound
• severe burn

Some of the specimens that are not suitable for anaerobic cultures include:

• coughed throat discharge (sputum)
• rectal swab
• nasal or throat swab
• urethral swab
• voided urine

Specimen collection

The keys to effective anaerobic bacteria cultures include collecting a contamination-free specimen and protecting it from oxygen exposure. Anaerobic bacteria cultures should be obtained from an appropriate site without the health care professional contaminating the sample with bacteria from the adjacent skin, mucus membrane, or tissue. Swabs should be avoided when collecting specimens for anaerobic culture because cotton fibers may be detrimental to anaerobes. Abscesses or fluids can be aspirated using a sterile syringe that is then tightly capped to prevent entry of air. Tissue samples should be placed into a degassed bag and sealed, or into a gassed out screw top vial that may contain oxygen-free prereduced culture medium and tightly capped. The specimens should be plated as rapidly as possible onto culture media that has been prepared as prereduced anaerobically sterilized media (PRAS). Alternatively, media that contains a reducing agent such as dithiothreitol or palladium chloride and has been stored for one day in an anaerobic chamber can be used.