By definition, anaerobic bacteria are bacteria unable to grow in a medium containing more than 20 % of oxygen7. They produce energy by fermentation and most have none of the usual aerobic enzymes: cytochrome, catalase, peroxydase.

Even though many bacteria do not grow in 20 % oxygen - i.e. in ambient air - their level of susceptibility to oxygen varies widely. Some are extremely susceptible to oxygen and will not tolerate more than 0.1% of oxygen. These do not seem to have any pathogenic effects on man and are to be only saprophytic. Others only survive oxygen concentrations between 0.1 and 5% and are called strictly anaerobic. Other groups are optionally anaerobic or microaerophilic, tolerating higher concentrations in oxygen, even though their growth is increased in anaerobiosis. This concept of aerotolerance is important as it explains why some anaerobic bacteria are capable of surviving oxygen pressures equal to those found in human tissues8-10.

Anaerobic bacteria classification is complex and there are still frequent changes in taxonomy. Briefly, and considering only those relevant to medicine, anaerobic bacteria can be sorted into the following groups : (Table

- Spore-forming anaerobic bacteria belonging to the Clostridium genus. Their natural habitats are the soil, and the digestive systems of man and animals12. Most of them produce toxins which are partly responsible for their harmful clinical effects13.

- Non spore-forming anaerobic bacteria, belonging to many other genera. These make up large proportions of the saprophytic flora of the mucosa11,14.

Table 2.2.4-2. Classification of anaerobic bacteria and frequency of isolation in products of human origin11_

SPORE-FORMING BACTERIA Gram-positive spore-forming bacilli :


C. perfringens C. tertium

C. ramosum C. sporogenes

C. difficile C. histolyticum

C. septicum C. novyi, C. tetani

C. paraputrificum C. botulinum NON SPORE-FORMING BACTERIA Gram-negative non spore-forming bacilli

Bacteroides : B. fragilis, B. thetaiotaomicron

B. vulgatus, B. distanosis, B. uniformis, B. ovatus. Prevotella : P. melaninogenica, P. intermedia,

P. oralis, P. disiens, P. oris, P. buccae Porphyromonas : P. asaccharolytica, P. endodontalis, P. gingivalis. Fusobacteria F. nucleatum F. necrophorum F. mortiferum F. varium Gram-positive Cocci

Peptostreptococci F. magna

P. asaccharolyticus P. prevotii P. anaerobius Peptococcus P. niger Gram-negative Cocci

Veillonella parvula Gram-positive non spore-forming bacilli Actinomyces

(A. israelii, A. naeslundii, A. odontolyticus, A. viscosus) Bifidobacteria B. dentium B. adolescentis

Propionibacterium sp.

P. acnes

Eubacterium sp.

E. lentum


2.2.4. Necrotizing Soft Tissue Infections 1.2 Pathophysiology

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