| ANAEROBES AND
PSEUDOMONAS - OPPORTUNISTIC INFECTIONS
Dr Alvin Fox
Microbiology, MBIM 650/720
OUTLINE OF MAJOR POINTS
of anaerobic bacteriology
Gram negative and Gram positive non-spore-formers
Gram positive spore-formers (clostridia)
Pseudomonas (a strict aerobe)
Obligate anaerobes are bacteria
that cannot survive in the presence of a high oxidation-reduction
potential) / high oxygen content. During metabolism bacteria can
produce toxic bi-products from oxygen (including superoxide
radicals and hydrogen peroxide). Strict anaerobes lack certain enzymes
dismutase and catalase)
that detoxify these products.
Strict anaerobes cannot grow in
healthy tissues due to their oxygen content. When tissue injury occurs
with limitation of the blood (and oxygen) supply, conditions are created
for opportunistic growth of obligate anaerobes. Often more than one
species will infect the same site. Simultaneous infection with a
facultative anaerobe (which uses up the already diminished oxygen
supply) also encourages growth of obligate anaerobes.
Endogenous versus exogenous
Most anaerobes in the normal
flora are non-spore formers and anaerobic infections often occur from
this source. However, contamination of wounds can also occur with
anaerobic spore-formers (e.g. clostridia) which are common in the
environment (e.g. soil). Non-spore-formers rarely produce exotoxins
in contrast to spore-formers.
Sites of anaerobes in normal
Strict anaerobes are present in
large numbers in the intestine (95-99% of total bacterial mass), but
also in the mouth and genitourinary tract. The most common infections
resulting from abdominal surgery or other gut injury are Enterobacteriaceae
(facultative anaerobes) and Bacteroides fragilis (see
below). These are minor components of the gut flora and demonstrate the
important point that certain organisms more readily produce
opportunistic infections than others.
Problems in identification of
1. They are often derived from
the normal flora. One must be confident that one has not isolated a
2. If air gets into the sample
during sampling or transportation to the clinical laboratory, then the
organism may not be isolatable.
3. Slow growth of the organism
(due to inefficiency of fermentation) means isolation takes several days
Identification in the clinical
laboratory after isolation
Two systems are commonly used:
Biochemical systems and/or gas
chromatographic identification of volatile fermentation products
(short chain fatty acids / alcohols)
ANAEROBIC NON-SPORE-FORMERS OF CLINICAL IMPORTANCE
rods (Bacteroides [e.g. B. fragilis] and Fusobacterium)
2. Gram-positive rods (Actinomyces, Arachnia, Eubacterium,
Bifidobacterium, Lactobacillus, Propionibacterium)
3. Gram-positive cocci (Peptostreptococcus and Peptococcus)
cocci (Veillonella, Acidominococcus)
1. The most
important strict anaerobic non-spore-former causing clinical disease.
2. Prominent capsule involved in pathogenesis (i) anti-phagocytic
(ii) directly involved in abscess formation.
which differs in composition from typical endotoxin and is of low
ANAEROBIC SPORE-FORMERS (CLOSTRIDIA))
reservoir: found in the environment (particularly soil) but also
intestine of man.
This baby has neonatal tetanus. It is completely rigid.
Tetanus kills most of the babies who get it. Infection usually
happens when newly cut umbilical cord is exposed to dirt CDC
This baby has tetanus. He cannot breast feed or open his mouth
because the muscles in his face have become so tight WHO
This adult has a severe case of tetanus. The muscles in his
back and legs are very tight. Muscle spasms can break
bones in the person's body CDC
1. Commonly found in
the soil; thus contamination of wounds can lead to colonization and
2. The exotoxin (tetanospasmin)
binds to ganglioside
receptors on inhibitory neurones in CNS (where glycine
is commonly the neurotransmitter) and stops nerve impulse transmission
to muscle leading to spastic paralysis. Continued severe muscle
contractions and spasms result which can be fatal.
3. The organism is
non-invasive and thus remains in the local wound.
4. Vaccination of
infants with tetanus toxoid
have almost eliminated this disease in the US.
tetanus manual (requires Acrobat)
1. Causes wound
gangrene) after soil, and to a lesser extent intestinal tract,
contamination. Primarily seen in time of war. The term gas gangrene
refers to swelling of tissues due to release of gas, as fermentation
products, of clostridia.
2. The organism
produces several tissue degrading enzymes (including lecithinase
[alpha toxin], proteolytic and saccharolytic enzymes). Necrosis and
destruction of blood vessels and the surrounding tissue result. This
creates an anaerobic environment in adjacent tissue and the organism
spreads systemically. Death can occur within 2 days. Nowadays, treatment
(including anti-toxin, antibiotic therapy, debridement)
is extremely effective and amputation and death is rare.
3. The determination
of production of lecithinase is important in laboratory identification
of the organism.
4. A significant
cause of food poisoning by enterotoxin
Clostridium botulinum - rod prokaryote. Vegetative (yellow
arrow) and spore (blue arrow) stages : note the flagella on
the vegetative cells. Causes botulism. SEM x15,400,
© Dr Dennis
Kunkel, University of Hawaii. Used with permission
Clostridium botulinum - rod prokaryote. Vegetative (yellow
arrow) and spore (blue arrow) stages: note the flagella on the
vegetative cells. Causes botulism. SEM x12,800 ©
Dr Dennis Kunkel, University of Hawaii. Used with
1. Botulism (a rare
but fatal form of food poisoning) is caused by a potent exotoxin (botulinum
toxin). This toxin binds to receptors on peripheral nerves, where
acetylcholine is the neurotransmitter. The toxin inhibits nerve impulses
and flaccid paralysis results and often death (from respiratory and/or
cardiac failure). The organism does not grow in the gut, but pre-formed
exotoxin from prior germination of spores may be present in inadequately
autoclaved canned food (usually at home).
2. Wound botulism
can occur but is even rarer.
3. C. botulinum
does not readily grow in the adult intestine due to competition with the
normal flora. In the neonate, where the flora is not established,
colonization with C. botulinum can occur. Infant botulism,
although uncommon, is now the predominant form of botulism.
includes administration of anti-toxin and (for neonates) antibiotic
information on botulism
botulism manual (requires Acrobat)
facts about botulism (from WHO)
When the normal
flora of the intestine is altered by antibiotic therapy this organism
can colonize. It produces an enterotoxin and pseudomembanous
colitis can result. Therapy includes discontinuation of the
implicated antibiotic (e.g. ampicillin). Severe cases require specific
antibiotic therapy (e.g. with vancomycin).
Scanning Electron Micrograph of Pseudomonas aeruginosa CDC
gram-negative rods, polar flagella, oxidase positive (in contrast to Enterobacteriaceae).
2. Among the genus Pseudomonas
the majority of human infections are caused by P. aeruginosa,
although other related organisms also cause disease.
3. Reservoir: soil,
water and air. Commonly infects compromised host e.g. a) Burns and
wounds - destruction of blood vessels limits access of phagocytes b)
After use of cytotoxic drugs in cancer therapy (which destroys the
immune system) c) Alteration of the respiratory epithelium in cystic
fibrosis commonly allows colonization and development of pneumonia.
includes (a) pigments: pyocyanin (blue-green) and fluorescein
(green-yellow, fluorescent) and (b) biochemical reactions. Cultures have
5. Slime layer is
6. Toxin A - ADP
ribosylates EF2 similar action to diphtheria toxin
toxins: Friend or Foe (from CDC)