An anaerobic organism or anaerobe is any organism that does not require oxygen for ANAEROBESgrowth and may even die in its presence. Anaerobic bacteria are intolerant of O2 or replicating at low oxidation reduction potential sites, such as necrotic, de vascularized tissues.

Three types:

  • Obligate anaerobes: which cannot use oxygen for growth and are even harmed by it
  • Aerotolerant organisms: which cannot use oxygen for growth, but tolerate the presence of it
  • Facultative anaerobes: which can grow without oxygen, but if present can utilize it.

Distribution of Anaerobes flora in Human

In humans, anaerobic organisms are among the normal flora especially of the

  • GI tract
  • Mouth
  • Vagina
  • Human skin.
  • Urethra & Genito urinary tract
  • Respiratory tract (Specially in pleural cavity)
  • Soft tissues specially with covering of Mucous membranes.

Clinical clues to the presence of anaerobic organisms include

  • Infection adjacent to mucosal surfaces that bear anaerobic flora.
  • Ischemia, tumor, penetrating trauma, foreign body, or perforated viscus.
  • Spreading gangrene involving skin, subcutaneous tissue, fascia and muscle.
  • Feculent odor in pus or infected tissues.
  • Abscess formation
  • Gas in tissues.
  • Septic thrombophlebitis.
  • Failure to respond to antibiotics that do not have significant anaerobic activity.

Anaerobic infections are most likely to be found in persons who are immuno suppressed, those
Treated recently with broad-spectrum antibiotics

Prognosis of Anaerobic Infections:

  • As we know that hundreds of species of nonsporulating anaerobes are part of the normal flora of the skin, mouth, GI tract, and vagina
  • If this commensal relationship is disrupted (eg, by surgery, other
    trauma, poor blood supply, or tissue necrosis), a few of these species can cause infections with high morbidity and mortality.
  • After becoming established in a primary site, organisms can spread
    hematogenously to distant sites. Because aerobic and anaerobic bacteria are frequently present in the same infected site, appropriate procedures for isolation and culture are necessary to keep from overlooking the anaerobes.
  • Anaerobes can be the main cause of infection in the pleural spaces and lungs, in intra- abdominal, gynecologic, CNS, upper respiratory tract, and cutaneous diseases; and in bacteremia. May be serious and life-threatening.
  • Treatment is complicated by slow growth of these organisms and by the growing resistance of anaerobic bacteria to antimicrobial agents.
  • Anaerobic infections are often complicated by deep-seated tissue necrosis.
  • The overall mortality rate for severe intra-abdominal sepsis and mixed anaerobic pneumonias tends to be high.

The overall mortality rate for severe intra-abdominal sepsis and mixed anaerobic pneumonias tends to be high.

Mortality associated with Anaerobes

Mortality associated with anaerobes bacteremia is high and reported to be in the range of 24 to 31 in most of studies.

Anaerobic Species

Obligate anaerobic

Gram-positive: Clostridium spp. Clostridium botulinum ,Clostridium difficile ,Clostridium perfringens, Clostridium tetani, Peptostreptococcus spp. (P. anaerobius, P. lanceolatus, P. micros, P. parvulus, P. productus slow-growing bacteria ) Part of normal flora of the mouth,upper respiratory tract, large intestine; opportunistic pathogens causing soft tissue infections and bacteremias; causes a variety of infections (wound infections, gangrene, puerperal fever, appendicitis, pleurisy, sinusitis, osteomyelitis, diarrhea,dental infections); intra-abdominal mixed infections following surgery; intestinal perforation or cancer may lead to liver abscess, Actinomyces

Gram Negative: Bacteroides spp. (Bacteroides are normally mutualistic, making up the most substantial portion of the mammalian gastrointestinal flora, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine. As many as 1010-10 11 cells per gram of human feces have been reported. They can use simple sugars when available, but the main source of energy is polysaccharides from plant sources)”Bacteroides melaninogenicus” has recently been reclassified and split into Prevotella melaninogenica and Prevotella intermedia. Porphyromonas (A type of Bacteroides spp. aspiration pneumonia, periodontitis) Veillonella are gram-negative anaerobic cocci. This bacterium is well known for its lactate fermenting abilities. They are a normal bacterium in the intestines and oral mucosa of mammals. In humans they have been rarely implicated in cases of osteomyelitis and endocarditis, for example with the species Veillonella parvula. Fusobacterium nucleatum contribute to several human diseases, including periodontal diseases, Lemierre’s syndrome, and topical skin ulcers.

Facultative anaerobic

Enterobacteriaceae: E.coli (Gr -ve), Klebsiella pneumoniae (Gr -ve), Proteus mirabilis (Gr -ve),,Salmonella like S. enteriditis , S. typhi, (Gr -ve), Shigella (Gr -ve), & Yersinia spp. (Gr-ve), like Yersinia enterocolitica, Serratia spp. (Gr -ve).

Vibrionaceae: Vibrio spp (Gr -ve) like Vibrio Cholera (also called as a Asiatic cholera), Aeromonas (Gr -ve) (pathogens of fresh water fish), Photobacterium (Gr -ve) (Normal flora of some fish).

Pasteurellaceae: Pasteurella (Gr -ve) (Pathogen of domestic animals), Haemophilus (Gr -ve) like H. influenzae common inhabitants of upper respiratory tract mucous membrane, mouth, vagina, GI.

Risk factors for infection with Anaerobes :

Exposure of sterile body sites to large innoculum of indigenous mucous membrane flora

  • Poor blood supply and tissue necrosis
  • Trauma
  • Foreign body
  • Malignancy
  • Surgery (Any type of surgery)
  • Oedema
  • Shock
  • Colitis
  • Vascular disease
  • Diabetes mellitus
  • Splenectomy
  • Immunocompromise
  • Collagen vascular disease
  • Previous infection with aerobic or facultative organisms