|please click the image for large view in new window
Non-sporing anaerobes form the major part of the normal human
bacterial flora, outnumbering all other organisms in the gut by a
factor of 103. They are also found in the genital tract, oropharynx
Clinical syndromes due to non-sporing anaerobes
- Infection with non-sporing anaerobes is usually endogenous.
- Normal flora may escape into a sterile site following perforation
of a hollow viscus (e.g. large intestine).
- Ischaemia (e.g. strangulated hernia) permits anaerobic growth,
or the metabolism of facultative bacteria produce anaerobic
conditions (e.g. in deep skin ulcers or intraperitoneal
- Once established, anaerobic multiplication is promoted
by release of toxic metabolic products and proteolytic
- Toxic products of inflammatory cells, such as reactive oxygen
intermediates, exacerbate tissue damage creating a vicious cycle of
anaerobic sepsis that is rapidly progressive.
- Intra-abdominal sepsis may follow spontaneous bowel perforation
or postsurgical leakage and may lead to abscess formation
(e.g. abdominal or liver abscesses).
- Sepsis of the female genital tract is often secondary to septic
abortion, prolonged rupture of the membranes, complicated caesarean
section or retained products of conception.
- Non-sporing anaerobes are implicated in pelvic inflammatory
- Imbalance in the anaerobic flora of the vagina may lead to the
non-specific vaginosis syndrome (see Urinary and genital infections
- Non-sporing anaerobes play a part in polymicrobial liver
abscesses and biliary sepsis.
- Pneumonia following aspiration or associated with carcinoma
or foreign-body obstruction has a significant anaerobic component
and lung abscesses can develop.
- Brain abscesses often have an important anaerobic
- Chronic paranasal suppuration, such as in chronic otitis media
and chronic sinusitis, may contain non-sporing anaerobes.
- Anaerobes may colonize chronic skin ulcers.
- Rarely tropical ulcers may be caused by Fusobacterium
Non-sporing anaerobes are nutritionally fastidious, sensitive to
oxygen and difficult or slow to grow. Specimens should be plated
directly in theatre or at the bedside, or transported rapidly to the
laboratory in an anaerobic transport system. Pus rather than
swabs (which dry out quickly) should be sent. There is an increasing
role for nucleic acid amplification tests (NAATs) based on
amplification of the 16S rRNA gene.
Anaerobic species are identified by phenotypic laboratory tests,
by studying the end-products of metabolism using gas-liquid chromatography
and, increasingly, by molecular means.
Almost all anaerobes are susceptible to metronidazole, although
resistance has been reported. Other active agents include meropenem,
piperacillin-tazobactam, clindamycin, chloramphenicol,
penicillin and erythromycin. (Note
: Penicillin and erythromycin
are not active against Bacteroides fragilis
, the anaerobe most commonly
isolated from abdominal sepsis.)
Effective management depends on surgery and antimicrobial
therapy. Surgical procedures include:
- drainage of abscesses;
- closure of perforations;
- resection of gangrenous tissue;
- debridement of non-viable tissue from ulcers;
- treatment of coexisting infection.
Metronidazole is the most commonly used anti-anaerobic agent.
Prevention and control
The risk of anaerobic infection in elective surgery can be reduced
by good operative technique and perioperative antibiotics with
anti-anaerobic activity (see Antibacterial therapy
and Antibiotics in clinical use
Pathogens of anaerobic sepsis
is typically associated with postoperative
sepsis in abdominal and gynaecological surgery. It also contributes
to the polymicrobial flora found in cerebral, hepatic and lung
- The most common agent of serious anaerobic sepsis.
- Penicillin resistant through ß-lactamase production.
- Produces a protease (DNAse), heparinase and neuraminidase.
- Has an antiphagocytic capsule.
Prevotella melaninogenicus and Fusobacteria are found chiefly in
the oral cavity. They are associated with periodontal disease,
gingivitis, dental abscess, sinus infection, cerebral and lung
abscesses, and necrotizing pneumonia. They are also found in
association with Borrelia vincentii in Vincent's angina, and in
ulcerative diseases such as cancrum oris (Ludwig's angina), both
of which affect the head and neck. They may contribute to anaerobic
Peptococcus and Peptostreptococcus are the only anaerobic Grampositive
cocci that are regularly found in human specimens.
They are often found in mixed infections, such as dental sepsis,
cerebral or lung abscesses, and soft-tissue and wound infections.
They are also associated with necrotizing fasciitis, where a mixed
infection of anaerobic cocci, facultative streptococci and sometimes
Staphylococcus aureus usually progresses rapidly, with
destruction of the skin and deeper tissues that leads to septicaemia
Actinomyces spp. are associated with chronic abscesses following
dental sepsis, lung abscesses, gut perforation and infection of
intrauterine devices. Long-term penicillin is usually effective.