Week 6 Pathogen Parade & Reading Flashcards
Clostridium perfringes characteristics
Anaerobic Gram-positive rods;
spore forming, but spores rarely seen in infected material.
More tolerant of oxygen than other clostridia.
Clostridium perfringes lab ID
Haemolytic colonies on blood agar incubated anaerobically.
Identification confirmed by demonstration of alpha-toxin (lecithinase) production in the Nagler’s test.
Germination of heat-resistant spores (with subsequent toxin production) may be responsible for food poisoning.
Five types of C. perfringens (A–E) identified on the basis of toxins produced; type A strains can be further divided into several serotypes.
Clostridium perfringens disease
Gas gangrene resulting from infection of dirty ischaemic wounds.
Food poisoning following ingestion of food contaminated with enterotoxin-producing strains.
Clostridium perfringens transmission
Spores and vegetative organisms widespread in soil and normal flora of humans and animals.
Infection acquired by contact; may be endogenous (e.g. wound contaminated from patient’s own faecal flora) or exogenous (e.g. contamination of a wound with soil, ingestion of contaminated food).
Clostridium perfringens pathogenesis
In ischaemic wounds, production of numerous toxins and tissue-destroying enzymes allows organism to establish itself and multiply in wound.
Local action of toxins produces necrosis thereby further impairing blood supply and keeping conditions anaerobic, and aiding spread of organism into adjacent tissues.
Food poisoning results from the ingestion of large numbers of vegetative cells, which sporulate in the gut and release enterotoxin
Clostridium perfringens treatment
Gangrene requires rapid intervention with extensive debridement of the wound.
Penicillin plus clindamhycin or tetracycline are examples of appropriate therapies.
Hyperbaric oxygen may also be helpful.
Food poisoning does not usually require specific treatment.
Clostridium tetani characteristics
Gram-positive spore-forming rod with terminal round spore (drumstick).
Strict anaerobe.
Clostridium tetani lab ID
Grows on blood agar in anaerobic conditions as a fine spreading colony; ‘ground glass’ appearance (hand lens inspection of cultures important).
Has very little biochemical activity useful for identification purposes.
Demonstration of toxin in a specimen is possible in a two-mouse model in which one animal is protected with antitoxin, the other unprotected (performed in Public Health reference laboratories).
Clostridium tetani disease
Tetanus (lockjaw).
Severe disease characterized by tonic muscle spasms and hyperflexia, trismus, opisthotonos and convulsions.
Clostridium tetani transmission
Organism widespread in soil.
Acquired by humans by implantation of contaminated soil into wound.
Wound may be major (e.g. in war, in road traffic accident) or minor (e.g. a rose-thorn puncture while gardening).
No person-to-person spread.
Clostridium tetani pathogenesis
Tetanus results from neurotoxin (tetanospasmin) produced by organisms in wound.
Toxin genes are plasmid-encoded.
The organism is non-invasive, but the toxin spreads from site of infection via bloodstream and acts by binding to ganglioside receptors and blocking release of inhibitory neurotransmitters.
Causes convulsive contractions of voluntary muscles.
Clostridium tetani treatment
Antitoxin is available (hyperimmune human gamma globulin; tetanus immune globulin).
Metronidazole and spasmolytic drugs indicated.
Prevention readily available and effective in form of immunization with toxoid. Usually given in childhood, but if immunization status of injured patient is unknown, toxoid is given in addition to antitoxin.
Clostridium botulinum characteristics
Anaerobic Gram-positive rods.
Not easily cultivated in competition with other organisms.
Produces &&most potent toxins known to man.**
Seven immunologically distinct toxins (A to G) produced by different strains of C. botulinum.
Types A, B, E and F are most commonly associated with human disease: serotypes A and B linked to a variety of foods (e.g. meat), serotype E especially associated with fish.
Clostridium botulinum lab ID
Requires strictly anaerobic conditions for isolation.
Grows on blood agar, but very rarely isolated from human cases of disease.
Detection of the toxin or organisms in the food or detection of the toxin or organisms in the serum or faeces of the patient, respectively, is the way of confirming the diagnosis.
Clostridium botulinum disease
Major pathogen of birds and mammals, rare in humans.
Botulism acquired by ingesting preformed toxin.
Disease entirely due to effects of toxin.
Infant botulism results from ingestion of organisms and production of toxin in infant’s gut.
Associated with feeding honey contaminated with spores of C. botulinum.
Wound botulism: toxin produced by organisms infecting a wound. Extremely rare.
Clostridium botulinum transmission
Soil is the normal habitat.
Intoxication most often by ingestion of toxin in foods that have not been adequately sterilized (e.g. home-preserved foods) and improperly processed cans of food.
Toxin is associated with germination of spores.
There is no person-to-person spread
Clostridium botulinum pathogenesis
Toxin released from organism as inactive protein and cleaved by proteases to uncover active site.
It is acid stable and survives passage through stomach.
Taken up through stomach and intestinal mucosa into bloodstream.
Acts at neuromuscular junctions inhibiting acetylcholine release. Results in muscle paralysis and death from respiratory failure.
Clostridium botulinum treatment
Supportive therapy is paramount.
Trivalent antitoxin is available.
In the rare cases of infant and wound botulism (i.e. when the organism is growing in vivo), penicillin and metronidazole are effective.
Prevention relates to good manufacturing practice. The toxin is not heat stable, therefore adequate cooking of food before consumption will destroy it.
Clostridium difficile characteristics
Slender Gram-positive anaerobic rod
Spore former
Motile.
Clostridium difficile lab ID
Difficult to isolate in ordinary culture because of overgrowth by other organisms; selective medium CCFA (cycloserine–cefoxitin–fructose agar) may be helpful; however, mere presence of the organism is not indicative of infection.
Diagnosis by detection of toxin in faeces (e.g., immunoassay) or molecular detection of toxin genes.
Clostridium difficile disease
Pseudomembranous colitis (antibiotic-associated diarrhoea).
Can be rapidly fatal especially in the compromised host
Clostridium difficile transmission
Component of normal gut flora; flourishes under selective pressure of antibiotics.
May also be spread from person to person by the faecal–oral route.
Clostridium difficile pathogenesis
Toxin-mediated damage to gut wall.
Produces both an enterotoxin (toxin A) and cytotoxin (toxin B)
Clostridium difficile treatment
Oral vancomycin or metronidazole.
Other antibiotics should be withheld if possible.
Prevention of cross-infection in hospitals depends upon scrupulous attention to hygiene.
Mycobacterium characteristics
Aerobic rods with a Gram-positive cell wall structure, but stain with difficulty because of the long-chain fatty acids (mycolic acids) in the cell wall.
Acid fastness can be demonstrated by resistance to decolorization by mineral acid and alcohol (Ziehl–Neelsen stain). Mycobacteria grow more slowly than many other bacteria of medical importance, but the genus can be divided into: rapid growers (form visible colonies within c. 3–7 days); slow growers (form visible colonies only after c. 2 weeks to 2 months’ incubation).
Mycobacteria are widespread both in the environment and in animals. The major human pathogens are M. tuberculosis and M. leprae, but awareness of the importance of other species (e.g. M. avium complex)
Mycobacterium lab ID
Staining and microscopic examination of specimens for acid-fast rods are important because of the time required for culture results.
All species except M. leprae can be grown in artificial culture, but they require complex media. Identification is based on rate of growth (rapid or slow), optimum temperature of growth and pigment production. Scotochromogenic species produce pigment in the absence of light whereas photochromogenic species require exposure to light before pigment becomes apparent. Further biochemical tests are required for full specification. Polymerase chain reaction methods, DNA probes and sequence-based approaches are available for identification purposes.
Mycobacterium disease
M. tuberculosis causes tuberculosis in humans and animals.
M. leprae is restricted to humans and causes leprosy.
Mycobacteria other than tuberculosis (MOTT) are associated with a range of conditions, usually in immunocompromised hosts.
M. avium–intracellulare (M. avium complex) has important associations with AIDS patients in the USA; in Africa M. tuberculosis is more common
Mycobacterium transmission
Droplet spread aided by ability of organisms to survive in the environment (M. tuberculosis, M. leprae).
Unpasteurized milk from cattle infected with M. bovis has been responsible for human infections in the past.
Social and environmental factors and genetic predisposition all have a role.
Leprosy requires close and prolonged contact for spread.
Mycobacterium pathogenesis
Both M. tuberculosis and M. leprae are intracellular parasites surviving within macrophages.
They give rise to slowly developing, chronic conditions in which much of the pathology is attributable to host immune responsiveness rather than to direct bacterial toxicity.
Mycobacterium treatment
Prolonged treatment with combinations of antimycobacterial drugs is required.
Bacille Calmette-Guérin (BCG) vaccination is valuable for prevention in endemic areas. Isoniazid prophylaxis used for contacts of cases of tuberculosis.
Pasteurization of milk and improvement of living conditions have played a major role in prevention.
Listeria monocytogenes characteristics
Short Gram-positive rods, often coccobacillary in clinical material (must avoid confusion with streptococci in chains); frequently Gram variable.
Motile at 25°C with a characteristic ‘tumbling’ movement; non-motile at 37°C.
Listeria monocytogenes lab ID
Haemolytic on sheep or horse blood agar.
Selective medium aids recovery of these organisms, especially from food samples (fish, chicken and cheeses).
Cold enrichment at +4°C for several weeks is also an effective selective technique. On translucent, non-blood-containing agar, colonies appear green-blue in oblique light.
Catalase positive, nitrate reduction negative; coupled with motility at room temperature these results are useful identifying features.
Biochemical and serological tests provide definitive identification.
Listeria monocytogenes disease
Meningitis and sepsis in neonates.
Infections in the immunocompromised (particularly meningitis) and in pregnant women.
Listeria monocytogenes transmission
Widely distributed in nature, survives well in cold.
Reaches food chain via silage as well as more directly via for example vegetables.
Excreted in large numbers in cows’ milk.
Humans may carry Listeria in gut as normal flora.
Infection may be acquired by ingestion or transplacentally to the baby in utero. While 13 different serotypes exist, pulsed-field gel electrophoresis and other molecular techniques are routinely used to investigate outbreaks.
Listeria monocytogenes pathogenesis
Virulent strains produce internalins (cell attachment factors), haemolysins, and a motility protein; organism can survive in phagocytes.
Listeria monocytogenes treatment
Treatment with penicillin or ampicillin, often in combination with gentamicin.
Widespread distribution of organism in nature makes prevention of acquisition difficult.
Pregnant women have been advised against eating uncooked food thought to be of particular risk (e.g. coleslaw, pâté, soft cheese, unpasteurized milk).
How quickly does a bacterium multiply?
1 hr
How do microorganisms gain entry?
Attach and penetrate body surface
Biting by arthropods
Introduced via skin wounds or animal bites
What is myxomatosis
Rabbit story.
In S. American rabbits, not deadly
In Europe deadly
Introduced into Australia and virus mutated to survive at a less pathogenic level.
How many microbes cause infection?
100
Kochs postulates (4)
Microbe must be present in every case of the disease
Microbe must be isolated from the diseased host and grown in pure culture
The disease must be reproduced when a pure culture is introduced into a non-diseased susceptible host
The microbe must be recoverable from an experimentally infected host
