Notes by dr Claudio Italiano
Anaerobic bacteria are microorganisms that do not require oxygen for growth and
develop in CO2-rich soils. Microaerophilic bacteria can grow in an atmosphere
containing 10% CO2. Non-spore-forming anaerobes are part of the normal bacterial
flora of the mucous surfaces of humans and animals, being present mainly in the
oral cavity, the gastrointestinal tract, the skin and the female genital tract.
In oral cavity, the gram-negative bacteria species associated with bad breath
are:
Treponema denticola
Porphyromonas gingivalis
Tannerella forsythensis
Porphyromonas endodontalis
Prevotella intermedia
Eubacterium
In the oral flora the anaerobes are predominant
commensal microorganisms,
varying in their concentration between 109 germs / ml in saliva and 1012 in the
gingival plaque. These microorganisms grow better in the presence of only a
small amount of atmospheric oxygen. Optional bacteria can grow in the presence
and absence of air. This chapter describes infections caused by
non-spore-forming anaerobic bacteria. In general, the pathogenic anaerobes for
humans tolerate air relatively. In fact, although they can not multiply, they
can survive for 72 hours in the presence of oxygen. A very small number of
pathogenic anaerobes (which are also part of the normal endogenous flora),
instead, die after brief contact with oxygen, even if in low concentration. In
the oral cavity the ratio between the concentrations of the anaerobic bacteria
and those of the aerobic bacteria ranges from 1: 1 on the surface of the teeth
to 1000: 1 in the gingival pockets. The anaerobic bacteria are not detectable in
appreciable number inside the intestine under normal conditions, if not starting
from the distal ileum; in the colon the share of anaerobes increases
significantly, as is the case with the total amount of bacteria. For example, in
the colon there are 1011 to 1012 microorganisms / g of faeces, with an anaerobic
/ aerobic ratio of about 1000: 1.
At the level of the female genital tract there are about 109 germs / ml of
secretions, with the ratio already cited equal to about 10: 1. Hundreds of
species of anaerobes have been identified as belonging to the normal bacterial
flora of man. In faeces, the identification of at least 500 different species of
anaerobes reflects the remarkable polymorphism of normal anaerobic flora and,
despite this huge variety, relatively few species isolate themselves during
human infections. Anaerobic infections occur when the balance between host and
bacteria is altered. Each district of the organism can be the site of infection
by these commensal germs, when the mucosal barriers or the skin are damaged by
surgery, trauma, neoplasia, ischemia or necrosis, which locally reduce the redox
potential. Because the districts colonized by the anaerobes contain numerous
other bacterial species, a possible alteration of the anatomical barriers allows
the penetration of many microorganisms, often giving rise to mixed infections
with numerous species of anaerobes associated with optional or microaerophilic
bacteria. Mixed infections of this type occur at the head and neck (chronic
sinusitis, chronic average otitis, Ludwig's angina and periodontal abscesses).
The cerebral abscess and the subdural empyema represent the most common
anaerobic pathologies affecting the central nervous system. Anaerobes are also
responsible for pleuropulmonary infections, such as aspiration pneumonia,
necrotizing pneumonia, abscesses and empyema. Anaerobes play an important role
in the genesis of various abdominal infections, including peritonitis,
intra-abdominal abscesses and hepatic abscesses. These bacteria are then
frequently isolated in infections of the female genital tract such as
salpingitis, pelviperitonitis, tubal abscess, septic abortions and endometritis;
finally, they are often found in infections of the skin, soft tissues, bones and
in septicemia.
The current taxonomy of this group has changed, so as to now include several species, such as Porphyromonas gingivalis, Porphyromonas asaccharolytica and P. melaninogenica itself. Ascolytic and pigment producing Porphyromonas species sometimes cause systemic infections in humans. In infections of the female genital tract Prevotella bivia and Prevotella disiens are the most often isolated bacteria, although B. fragilis is also common. Fusobacteria can also be isolated from foci of infection, including abscesses and necrotizing pneumonia. Bilophila wadsworthia is a Gram-negative, anaerobic bacterium, often resistant to various antibiotics, including imipenem, cefoxitin and other beta-lactams. This microorganism can cause serious infections such as bacteraemia, necrotizing fasciitis and abscesses. Infections caused by anaerobic bacteria are often polymicrobial, being able to be determined either by several species of anaerobic bacteria or by a combination of anaerobic and aerobic microorganisms, which act in synergy with each other.
All of these bacteria voraciously consume mouth debris
containing proteins that, once digested, produces volatile sulfur compounds.
These compounds are comprised of:
Methyl mercaptan
Dimethyl sulfide
Hydrogen sulfide
Cadaverine
Skatole
Isovaleric acid
Putrescine