Mycobacteria Flashcards
BCG vaccine
Bacille Calmette Guerin
Isolated from mycobacterium bovis
attenuated 231 times over a period of 13 years in potato broth with added beef broth
gradual loss of virulence in mice
first given as a vax to kids in 1921. now over 3 billion doses given.
NB: immunocompromised humans shouldn’t get BCG d/t risk of disseminated disease.
Mycobacteria
many species consisting of major pathogens, opportunist pathogens, harmless saprophytes- pathogens are fairly rare
Cause granulomatous lesions (tubercles) in tissues of a wider range of domestic and wild animals and humans
Specialized granulomas- an attempt to wall off infection from uninfected tissue.
Development of mycobacterial disease in a host depends on the ability of the bacteria to survive and multiply WITHIN the host macrophages.
General characteristics of mycobacteria
Aerobic, non-spore-forming, non-motile
Acid fast (gram positive but don’t stain well with Gram stain)- bind phenol based dyes (carbol fuschin) and resist acid alcogol decolation (Z-N stain)
Very unique because of hardy cell wall.
Catalse positive
Many produce pigments on culture
relatively simple growth reqs (except m. leprae can’t grow in lab on solid media)
Fairly slow growing compared to other bacteria- esp. pathogenic forms
Unique cell walls- lipid rich- acid fastness related to presence of peptidoglycan, but particularly glycolipids
Lipids in cell wall related to pathogenicity, particularly survival in phagolysosome of macrophages, resists drying, extreme pH and other stresses.
Mycobacterium tuberculosis
TB in humans, but also cats, dogs, pigs, chickens, parrots, canaries, guinea pigs and mice (rare in cattle)- humans can transmit via aerosol to domestic animals
Mycobacterium bovis
Bovine TB, also TB in man (badgers, deer), other ruminants, pigs and more rarely in horses, dogs, cats and sheep
Mycobacterium avium subspecies paratuberculosis
Johne’s disease in cattle, sheep, goats and deer- chronic wasting disease causing diarrhea
Mycobacterium avium complex (incl. subspecies avium)
TB in birds, poultry very susceptible, Pigs susceptible but not cattle. Sporadic cases in horses, dogs and cats. Opportunist in man (AIDS- M. avium intracellulare).
M. leprae
Leprosy in man, mice and armadillos
M. lepraemurium, M. ulcerans, M. kanasii, M. fortuitum and M. chelonae
skin ulceration and LN involvement in many different species. Chronic RTI
Acid-fast staining of Mycobacteria
Red rods are m. tuberculosis, intracellular location via active phagocytosis in macrophage
Mycobacterial acid-fast cell wall
Acid-fast cell wall is unique to mycobacteria- glycolipids, wax molecules, arabinan molecules- basiclaly like being covered by a bar of soap
Virulence factors of mycobacteria
largely cell wall-related.
Cell wall components: survives in a hostile environment
Mycolic acids: resist phagocytic digestion
Sulfatides: prevent pahgocytic activation and phagosome-lysosome fusion
Trehalose di-mycolate (cord factor): inhibit phagocytic chemotaxis, activation, phagolysosome fusion and digestion– form serpentine cords on staining
Lipoarabinomannan (LAM): prevents phagocyte activation and digestion within the phagocytes
Mycoside: prevent intracellular killing and digestion
Other factors include superoxide dismutase and heat shock protein.
Cell wall antigens
In general, they induce delayed type hypersensitivity (DTH). TB exists in macrophages. after several weeks, immune system ramps up and on stimulation with IF-gamma, marcophages can kill TB by forming phagolysosomes and NO radicals. However, hyper-activated macs secrete TNF which recruits multiple monocytes which differentiate into epitheliod histiocytes (activated macs). These wall of the infected cells, but at the cost of significant inflammation and local damage.
Epithelioid cells
activated macrophages- essential characteristic of granulomas, which means by definition, TB lesions are granulomatous.
Outcomes of infection of M.bovis
ability to mount an effective activated macrophage response determines the outcome of an encounter with pathogenic mycobacteria. Less than 10% of those infeced develop disease.
1) infects, killed by IR, no disease
2) infects, lies dormant for years, no disease (most common)
3) infects, lies dormant for years, reactivates and causes acute disease
4) infectes causes rapid acute disease, may disseminate (young, immunocompromised, HIV)
Pulmonary TB lesions in man
most prominent when environment is most oxygenated (cranial portions of the lung). Lymph nodes-caseating material, break down of tissue. lesions can liquefy into bronchi, aerosols disseminate disease.
Immune response to mycobacterial infections
1) antibody response irrelevant to protection: Th1 (CMI) required to limit the disease and provide protection
2) immune status of host: active response results in lymphocyte infiltration, central necrosis of the lesion, tubercle may be limited by fibrin capsule. granuloma walls of lesion become fibrosed and most often contains it and it stops growing but mycobacteria are still alive. response may kill it, but often only able to restrict disease. reactivation occurs with stress/immunosuppression.
3) IF-gamma from CD4 lymphocytes activate macs to kill intracellular mycobacteria. CD8 lymphocytes become cytotoxic, killing myobacterial infected cells. CD1 restricted T-cells recognized glycolipids (of cell wall)
TB lesion- what does it look like histologically?
central necrosis, surrounded by a region of lymphocytes and macrophages (often Langhan’s cells- multinucleate giant cells formed by fusion of epithelioid cells). also see some fibrosis surrounding.
Spread of Mycobacterium bovis
spread promoted by high stocking densities and immune suppresion. generally primary respiratory infection leads to tuberlces in lung and associated LNs (bronchial and retropharyngeal)
Closed lesions are less infectious than animals actively coughing out M. bovis.
Spread to intestine via sputum and serosal surface
Pleural TB lesions= pearl’s disease
Further spread to liver, spleen, kidney and brain (usually hematogenous).
Vertical transmission possible after spread to mammary glands and uterus.
Presentation of mycobacterium bovis
multifocal to coalescing caseous granulomas.
Epidemiology of bovine TB
Cattle transmit infection to cattle via infected resp. droplets. Badgers are wildlife reserve. Badgers transmit via respr route and by biting. mums transmit to cubs, but not by milk, rather via close personal contact. Cattle may get TB from badgers via grazing on contaminated pasture (badger feces, urine, pus etc) or badgers urinate and defecate, or dried badger saliva in cattle feeders.
Diagnosis of Mycobacterial infection
immunological detection (tuberculin)
lab diagnosis: Z-N staining
Culture
ID by PCR
Comparative intradermal test
Two sites prepared on animal neck (13cm apart). Hair clipped in 2 cm radius and skin fold is measured. Inject purified protein derivative (containing different antigens). Use M. avium at preparation in upper site and M. bovis on lower site. Re-measure fold after 72 hours. If reaction to M. bovis, PPD is 5mm greater than M. avium. If 1-4mm greater, retest within 40-60 days.
This test measures lymphocyte infiltration into site. If previous exposure, increased size.
TB specific lymphocytes in lesion lead to the increased size.
Interfron gamma assay
whole blood assay or assay that takes T-cells. whole blood or t-cels exposed and then IFN gamma response is measured. Can get a negative response ot TB skin tests with a massive/severe infection (i.e. T-cells too busy responding to real disease). Increased pathogens, increased bacterial load, decreased TB skin test and decreased IF gamma.
Mycobacterium avium
widest host range among mycobacteria.
M. avium ssp. avium serovars 1,2, and 3 isolated from TB lesions in avian species.
other avium serovars produce minimal disease.
Non-human primates, cattle and pigs infection. Avium is confined to LN infection (spoils carcass–> mycobacteriosis in pigs).
M. avium ssp. paratuberculosis- Johne’s disease
transmissible chronic and progressive enteritis (diarrhea and ileal inflammation) in cattle, sheep and goats. Infection usually occurs within 1st month, but may take 6 months to 5 years.
Clinical course of Johne’s disease
starts with general signs of illness (weight loss, intermittent diarrhea) followed by severe diarrhea, emaciation and death. Impaired intestinal function due to chronic inflammation. Evidence of diffuse granulomatous changes. accumulation of lymphocytes and epithelioid cells in the lamina propria and submucosa.
Pathogenesis of Johne’s disease
Organism ingested–> penetration of ileal mucosa –> phagocytosis by local macrophages–> inflammatory response (IR and DTH (granulomatous response)–> granuloma formation (ileum/colon/mesenteric LNs)–>proliferation of sub-epithelial macs –> thickening of intestine, increased permeability (tight junctions are leaky now) –> protein losing enteropathy–> diarrhea.
thickened and corrugated infected ileum.
Cutaneous mycobacterial infections
M. leprae: leprosy in man and primates, cutaneous lesions and nerve damage. also in armadillos and mice. peripheal nerves- leprosy likes lower temp of skin
M. lepramurium- skin lesions–> more acute than leprosy- cats and rodents
M. ulcerans- skin nodules (granulomas) which can ulcerate. ulcers undermine tissue, become susceptible to secondary infection (humans)
M. Marinum: fatal infections common in frogs and fish. colonies skin abrasions in man
Tuberculoid leprosy
strong IR to organism, few organisms in tissue; IR destroys tissue–> little disfigurement. Few organisms in well-contained granulomatous lesions in tissue
Lepromatous leprosy
disfigurement, nodular swellings full of bacteria, anesthesia, shortening of toes and fingers in response to repeated, un-felt trauma. No IR to organisms. No cell-mediated IR, massive number of organisms
Transmission of leprosy
via aerosol, but NOT very contagious
Ridley-Jopling spectrum of bacterial load/ CMI
Tuberculoid leprosy: high CMI, low bacterial load
Lepromatous leprosy: low CMI, high bacterial load
Can fall anywhere on the spectrum.
