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Microbiology and Immunology 2500A/B Lecture Notes - Necrosis, Debridement, Phospholipid

Microbiology and Immunology
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Lecture 7: Clostridia 09/21/2012
Gram-positive, rod shaped, endospore formers
Strict anaerobes, vegetative cells killed by O2 oxygen is toxic, cannot grow in the presence of
Endspores resistant structure, critical for the development of diseases they cause
Generally found in soil and intestinal tracts of animals
Responsible for several unrelated diseases
More than 80 species of Clostridia
Important human pathogens:
o Clostridium difficile - pseudomembranous colitis
o Clostridium tetani - tetanus
o Clostridium botulinum - botulism
o Clostridium perfringens food-borne illness and gas gangrene
Can cause life threatening diseases mediated by exotoxins
Endospore formation:
o “endo” means “within”
o These structures are formed from within the cell
o Endospores are highly resistant to heat, drying, harsh chemical and nutrient depletion
o Function as “survival structures”
o Also used for dispersal through air, water and intestinal tracts
o Clostridia produce terminal endospores
o Best studied in Bacillus and Clostridium species also gram
positive rods
o Sporulation due to lack of nutrients or stress
o A dormant stage in the bacterial life cycle
o Endospores can remain dormant for years but can revert back
to vegetative cells rapidly (within minutes)
o Vegetative cells do not have endospores
o Occurs through a complex series of events
o Exosporium and spore coat are composed of protein
o Core wall is peptidoglycan
o The cortex contains DNA, cytoplasm, ribosomes, etc.
o Dipicolinic acid complex with with Ca++ helps to dehydrate the
cortex consistency of a gel but very resistant to heat, chemicals,
o Pasteurization (63-72°C) will not kill endospores
o Autoclaving at 121°C will kill endospores
The Pathogen: Clostridium difficile (“C. diff”)

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Cause of pseudomembranous colitis, also called antibiotic-associated diarrhea
Can exist as:
o Asymptomatic carrier state does not cause any disease
o Cause of mild to moderate diarrhea
o Cause of life-threatening pseudomembranous colitis
C. difficile can be harbored in the large intestine in low numbers
o About 3% of healthy adults are colonized
Often found in nursing homes and hospital environments
o A nosocomial pathogen (HA pathogen)
Under harsh conditions reverts to the highly resistant spore
Spores can be very difficult to eradicate from the environment
o Cultured from floor, bed pans, toilets, hands and clothing of medical personnel
Mode of transmission is through the spore: fecal-oral route
The Disease: pseudomembranous colitis
Most symptomatic patients have recently received an antimicrobial agent
o This is the single most important risk factor
Antibiotics are used to cure infections, but they also kill the normal microbiota
Suppression of normal flora + persistence of C. difficile endospores
In response to the antibiotic, C. difficile enters the endospore state
After the antibiotic is stopped, spores germinate, overgrowth of C. difficile occurs with
production of toxins
C. difficile does not invade, but the toxins damage the intestinal lining of the large intestine
The Pathogen: Clostridium difficile (“C. diff”)
C. difficile produces A-B toxins called the large
clostridial cytotoxins
“A-B” serves to designate two domains:
o The A domain denotes the active portion
of the toxin that carries the enzymatic activity responsible for inducing damage
o The B domain denotes the portion of the toxin molecule responsible for binding and
uptake by the host cell
“A” domain functions to inactivate key regulatory proteins (called G proteins) of host cells
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