Mycobacteria

Tuberculosis

discovered in 1882 by Robert Hooke (also discovered Antrax)

1.7 billion infected, 8 million new infections per year (7.6 million in developing countries)

kill 3 million people per year worldwide, more than malaria, AIDS, tropical diseases, and leprosy combined

most cases are in Southeast Asia, Africa, South America (especially the Andes), and Haiti

steady drop in U.S. and Canada from 1950 (35,000) to 1984 (20,000) – then increased

caused by AIDS, homelessness, drugs, alcohol, and migration (1/3 of U.S. cases are from foreign-born people)
down again since 1993 – now 18,000

spread by air – inadequate ventilation increases transmission (hospitals change air 10-25 times per hour)

Microbiology of Mycobacteria

genus Mycobacterium (within the order Actinomycetales) is related to corynebacteria and fungi

many species (at least 30 described, many more) – only a few cause disease; habitats = water and soil

M. tuberculosis – cause of tuberculosis

transmitted by airborne nuclei containing one or a few bacteria

M. leprae – the cause of leprosy – never been cultured in vitro (propagates in armadillos and mouse foot pads)

transmission requires close and prolonged contact

M. avium (also called M. intracellulare) – many strains, found in most water – disseminated disease in AIDS

causes chronic pulmonary disease, adenopathy in children

many less common: M. kansasii, M. szulgai, M. ulcerans
occasionally pathogenic: M. scrofulaceum, M. xenopi, M. mariunum
newly discovered – M. malmoesi and M. haemophilum

staining must be done with "acid fast" stain due to waxy capsule (mycolic acid)

stain red with carbol-fuchsin, and are not decolorized by 3% HCl in ethanol (as are other bacteria)
do not grow very dense, so hard to see
small, highly pleomorphic rods, usually curved, occurring singly or in clumps or branched chains. Usually stained

cultivation can be done on minimal media – produces rough, irregular, dry, buff-colored colonies, like bread crumbs

slow generation time (doubling every 22 hours) – require months for mature culture (most bacteria = 20 minutes)
in liquid culture grows as a surface pellicle with formation of serpentine cords

M. avium – mucoid colonies, liquid can be transparent or opaque

no toxins – virulence mediated by immunoactive materials on cell wall (mycolic acid, arabinogalactin, murinodipeptide)

severity of lesions determined primarily by the state of the patient

Pathogenesis of Mycobacteria

M. tuberculosis – divided into primary and reinfection types

(1) primary tuberculosis (first infection tuberculosis, childhood tuberculosis)

deposited in apical lung – local lesion of nonspecific inflammatory response, but TB survives inside macrophage

genetics determine susceptibility – identical twins more likely to transmit
age affects susceptibility – most susceptible during infancy and post-pubescence – delayed if between

to regional lymph node – carried by macrophages via lymphatics to regional (hilar) lymph node, to bloodstream

enlarged lymph node may cause bronchial obstruction in children

disseminated to kidney, vertebrae, meninges – delayed hypersensitivity within 3-10 weeks (test positive)
95% heal and never relapse (form Ghon complex, small lung lesion with an enlarged hilar lymph node)
5% relapse (mostly elderly); a few progress straight to active TB

(2) reinfection tuberculosis (adult tuberculosis) – endogenous source – usually respiratory symptoms or asymptomatic

usually affect lung apex - granuloma formation/caseous necrosis, Langhans giant cells (able to kill TB)
follow blood or lymphatics to urinary tract, bone, meninges, and other organs
without treatment, 50% die, life expectancy 2.5 years

M. avium – usually only immunocompromised individuals, through GI tract – enteritis, adenitis, disseminated disease

bronchiectasis, cystic fibrosis, and preexisting pneumonia predisposes to infection

M. leprae –spectral, bipolar disease – state of the host determines its pathogenesis

tuberculoid leprosy – much cell-mediated hypersensitivity, florid granuloma formation with only a few bacilli

cutaneous and neural lesions are prominent

lepromatous leprosy – only humoral immune responses, no granuloma formation, many bacilli

infiltrative lesions of the skin and mucous membranes.

borderline leprosy – intermediate state

Immunology of M. tuberculosis

facultative intracellular organism – replicates within macrophages (attaches to Fc and complement receptors)

most opsonized by complement – taken up by CR1, CR3, and (in alveolar macrophages CR4 receptors)
some unopsonized – cell wall lipoarabinomannan interacts with macrophage fucosyl mannose receptors

evades destruction – prevents lysosome fusion

LAM inhibits reactive oxygen cascade, NH₄ neutralizes acid, glycoproteins inactivate lysosome,

reactive nitrogen is produced by arginine Þ citrulline – uncertain effect

invasion via fibronectin, possibly invasin; phagocytosis may also facilitate cell entry

polysaccharide arabinogalactan and lipoarabinomannan act as immunosuppressive and anti-inflammatory agents

scavenge reactive oxygen, deactivate T-cells, and induce host cytokines (TGF-b , IL-10 deactivate macrophages)

double edged sword

CD4 cells activate IL-12, IL-1, TNF-a , and IL-6 (enhance response), but also TGF-b and IL-10 (reduce response)

TGF-b causes fibrosis but deactivates macrophages, TNF-a activates macrophages but causes necrosis

IFN-g and IL-2 are necessary to control the infection, TNF-a controls replication (by granuloma formation)

host immune response causes most symptoms (granuloma, caseous necrosis, cavitary lung disease)

some manefestations due to reaction of organism to antigen (TB pleurisy and TB meningitis)

fever, drenching sweats, loss of appetite, inanition, and debility TB are due to TNF-a and other cytokines

can enter latency – not well understood (involves CD4 to maintain latency – AIDS patients reactivate)

experiments show infection with TB confers immunity against exogenous TB (perhaps mediated by 30 kD a -antigen)

not same as delayed type hypersensitivity (tuberculin skin test reactivity) – different CD4 cells, requires live TB
several antigens for possible vaccine

Immunosuppression – less active CD4 lymphocytes are observed in later stages of TB – parallels disease progression

IFN-g is 10-fold lower than normal in active infection; gradually increases with treatment

TB test – identify infection, not disease – induration after injection of purified protein derivative (PPD)

mean infected is 16 mm – lower than 5 mm usually means uninfected
however, some places have cross reactivity to antigens of environmental mycobacteria, causing false positive (i.e. BCG)
also some infected are unreactive (anergy) leading to false negative

TB-HIV Co-infection – very common (60,000 in U.S., 15.3 million worldwide)

depression of CD4 cells increases TB reactivation by 120-fold; also fewer giant cells, no IFN-g

CD4 counts parallel disease progression – pulmonary TB at 300T, TB meningitis at 100T, anergic miliary TB at 50T

called spectral disease (spectrum of manefestations)
normal CD4 counts are at least 500,000

leads to increased viral loads and increased HIV transcription (activated by TNF-a ) at higher CD4 counts

Immunology of M. avium

common in AIDS patients – envelope protein of gp120 promotes intracellular replication of M. avian

organism is complement-opsonized and ingested by macrophages as for TB

virulence assayed by culture – transparent colonies are more virulent

more virulent phagocytosed less well and produce less cytokines, but grow better inside the cell

like TB, macrophages inactivated by TGF-b and IL-2

Immunology of M. leprae (ML)

rare in U.S., declining in prevalence worldwide

produces leprosy, a highly variable and spectral disease – course determined genetically

tuberculoid leprosy – vigorous T-cell reactivity, well-formed granulomas, mostly Th1 cytokines

localized skin disease

lepromatous disease – depressed T-cell reactivity, poorly-formed granulomas, mostly Th2 cytokines

extensive skin and neural disease