Mycobacteria
are slender rods, sometimes show branching filamentous forms. In liquid
cultures they form a mold-like pellicle, hence the name mycobacteria, meaning
fungus-like bacteria. They do not stain readily, but once stained resist decolurisation
with dilute mineral acids, therfore they are called “Acid Fast Bacilli” or AFB.
They are aerobic, nonmotile, noncapsulated, nonsporing. Generally slow growth,
obligate parasites, opportunistic pathogens and saprophytes.
Important
pathogenic species are Mycobacterium
tuberculosis, Mycobacterium leprae
M. tuberculosis – A straight or
slightly curved rod, occuring singly, in pairs or small clumps. Tubercle
bacilli after staining with basic dyes they resist decolourisation by alcohol.
When stained with carbol fuchsin by the Ziehl-Neelson method, they resist
decolourisation by sulphuric acid and alcohol. Acid fastness is due to the
presence of mycolic acid. Electron micrographs show that the thick wall is
composed of three layers enclosing a trilaminar plasma membrane.
It grow slowly, the generation time
is 14-15 hours. Colonies appear in about 2 weeks, sometimes 8 weeks. Optimum
temperature is 370C, no growth below 250C or above 400C,
optimum pH is 6.4-7.0. It is an obligate aerobe. The addition of 0.5% glycerol
improves the growth of the tubercle bacilli, but not to other species. They are
sensitive to traces of toxic substances like fatty acids in culture media. The
toxicity is neutralised by serum albumin. The solid media containing egg,
Lowenstein-Jensen(LJ) medium without starch is the recommended media. It
consists of coagulated hen’s egg, mineral salt solution, asparagine, malachite
green(a selective agent inhibiting other bacteria). Liquid media generally not
employed for routine cultivation, but are used for sensitive testing, chemical
analysis, preparation of antigens, vaccines.
On solid media, M. tuberculosis forms dry, rough, raised, irregular colonies
with a wrinkled surface. They are creamy white, becoming yellowish or buff
coloured on further incubation. They are not easily emulsified. In liquid media
the growth begins at the bottom, creeps
up the sides and forms a prominent surface
pellicle which may extend along the sides above the medium.
Mycobacteria are killed at 600C
in 15-20 minutes. Survival is influenced by the material in which the bacteria
are present. Cultures may be killed by exposure to direct sunlight for 2 hours,
but bacilli in sputum may remain alive for 20-30hours. They are resistant to
chemical disinfectants, survive 5% phenol, 15% H2SO4, 3%
nitric acid, 5% oxalic acid, 4% NaOH. They are destroyed by tincture of iodine
in 5 minutes and by 80% ethanol in 2-10 minutes. Ethanol is a suitable
disinfectant for skin, gloves and
clinical thermometers.
Biochemical Reactions
Niacin test- Human tubercle bacilli form niacin when grown on an egg medium. When
10% cyanogen bromide and 4% aniline in
96% ethanol are added to a suspension of the culture, a canary yellow colour
indicates a positive reaction. The test is positive with human type, so useful
in identifying human strains.
Aryl
Sulphatase Test- Positive with atypical bacteria
only. The bacilli are grown in a medium containing 0.001 tripotassium
phenolphthalein disulphate, then 2N NaOH is added drop by drop to the culture,
a pink colour indicates a positive reaction.
Neutral Red
Test – Virulent strains of tubercle bacilli are
able to find neutral red in alkaline buffer solution, while avirulent strains
are unable to do so.
Catalase-
Peroxidase Test- to differentiate tubercle bacilli
from atypical mycobacteria, also provides an indication of the sensitivity of
the strain to isoniazid. Atypical mycobacteria are strongly catalase positive,
but peroxidase negative, while tubercle bacilli are weakly catalase positive
and peroxidase positive. Both these activities are lost when tubercle bacilli
become INH resistant. A mixture of equal volumes of 30vol.H2O2
and 0.2% catechol in distilled water is added to 5ml. Of the test culture and
allowed to stand for few minutes. Effervescence indicates catalase production
and browning indicates peroxidase activity.
Amidase Test- The ability to split amides has been used to differentiate
mycobacteria. Five amides such as acetamide, benzamide ,carbamide, nicotinamide
and pyrazinamide are tested.
A 0.00165M solution of the amide is incubated with the bacillary
suspension at 370C and 0.1ml of MnSO4.4H2O,
1ml of phenol solution and 0.5ml hypochlorite solution added. The tubes are
placed in boiling water for 20 minutes. A blue colour indicates a positive
test.
Nitrate
Reduction test- Here nitrate is reduced to nitrite
by the enzyme reductase, M. tuberculosis is positive with the
formation of red colour.
Antigenic Properties
Following infection of tubercle
bacilli delayed hypersensitivity is developed to the bacillary protein, tuberculin. Also polysaccharide, protein antigens present. They
also produce bacteriocins.
Pathogenicity
Tubercle bacilli causes natural infection in
humans, other primates, dogs. M. bovis produces tuberculosis in cattles, dogs
and cats, also in humans. The disease is acquired by the inhalation of droplet
nuclei of respiratory secretions or particles of dry sputum containing tubercle
bacilli. After inhalation they multiply very slowly inside WBC that have
phagocytized them. They stimulates a host response that includes neutrophil
infiltration and fluid accumulation within the alveoli of the lungs. The
organisms eventually rupture and destroy the neutrophils. Later macrophages and
lymphocytes move into the area. Alveolar macrophages also phagocytize the
living tubercle bacilli, which are again able to multiply within and destroy
their new hosts. Rupture of phagocytes releases infective organisms. No toxins,
but an acute inflammatory response occurs as additional cells are infected. A
large quantity of fluid is released, especially in lung tissue, where it
produces pneumonia-like symptoms. Sometimes lesions heal, if not massive tissue
necrosis or solidify to become chronic granulomas or tubercles. Tubercles consist of central accumulations of
macrophages, multinucleate Langerhans giant cells containing tubercle bacilli,
peripheral lymphocytes, macrophages and newly formed connective tissue. The
central portion of the tubercle undergoes destruction, giving it a cheesy
appearance. Some organisms gain access to the lymphatic and circulatory
systems. About 3-4 weeks after exposure, delayed hypersensitivity and cell
mediated immunity develop. On occasion host fails to respond immunologically.
Then uncontrollable multiplication of tubercle bacilli takes place in the
lungs, therefore numerous tubercles. Organisms spread by the circulatory system
to other body tissues, this condition is miliary tuberculosis(as it
resembles millet seeds). Lesions near blood vessels perforate the vessels and
cause hemorrhage, leads to bloody sputum, a major symptom.
Granulomas keep viable organisms
walled off by encapsulation for decades. When the immune system becomes
impaired by age or other infections , granulomas open and the disease can
be reactivated,eg., people with AIDS
have TB. TB on bones can cause extensive erosion, especially in the spine. The
urogenital tract , meninges, lymphatic system, peritoneum are prone to develop
extrapulmonary TB. In disseminated TB (frequent in AIDS patients) infected
cells' organelles and membranes destroyed by
multiplying bacilli and a clump of bacilli in the shape or cast of the
cell remains. When this process occurs in the intestine, a replica of the
intestine consisting of live Mycobacterium can be seen. Today digestive
tract TB occurs as a secondary infection site, when pathogen-laded sputum
coughed up from a primary lung infection is swallowed.
Laboratory Diagnosis
Laboratory diagnosis of TB may be
established by demonstrating the bacillus in the lesion, sputum culturing etc..
For pulmonary TB, the specimen tested is sputum, which is collected in
the morning before any meal. If scanty 24 hrs sample may be tested. Sputum
sampling on 3 days increases chances of detection. If sputum is not available,
laryngeal swabs, bronchial washings may be collected, and in small children
gastric lavage collected.
Microscopy
: Direct or concentrated smears of sputum examined. New slides should be used
for smears, and should not be reused, as AFB may not removed from slides by
cleaning. Smears are dried, heat fixed, stained by Ziehl- Neelson
technique,ie., the smear is covered with carbol fuchsin, gently heated to
steaming for 5-7 minutes(stain shouldn’t boil or dry), slide is then washed
with water and decolourised with 20% H2SO4, then with 95%
ethanol for 2 mts. After washing the smear is counterstained with Loeffler’s
Methylene Blue, 1% picric acid or 0.2% malachite green for 1 minute. Under the
oil immersion, AFB seen as bright red rods, while the background is blue, yellow
or green. At least 100 fields examined and report expressed as 1+ if 3-9
bacilli, 2+ if 10 or more and 3+ if 10 or more in oil-immersion.
Concentration methods : Petroff’s method-
sputum incubated with an equal volume of 4% NaOH solution at 370C
with frequent shaking till it becomes clear. It is then centrifugedat 3000rpm
for 20 mts, sediment neutralised with N/10 HCl and used for smear, culture,
animal inoculation.
Culture :
The concentrated material is inoculated onto LJ medium. Cultures examined for growth
after incubation at 370C for
4 days. Any growth is smeared, tested by ZN staining, confirmation by
biochemical studies. Culturing can be detected 10-100 bacilli/ml.
Sensitivity tests : are of 3 types-(a) Absolue
concentration method in which a number of media containing serial
concentrations of the drugs are inoculated, minimum inhibitory concentrations
calculated. (b) Resistance ratio
method in which 2 sets of media containing graded concentrations of the drugs
are inoculated, one set with test strain and other with a standard strain. (c) Proportion method which indicates
the average sensitivity of the strain.
Animal Inoculation : The concentrated material is inoculated intramuscularly into the
thigh of two healthy guinea pigs. Animals weighed before and after inoculation.
Progressive loss of weight is an indication of infection. Infected shows
positive tuberculin skin reaction. One killed after 4 weeks and autopsied,
other after 8 weeks.
Nucleic Acid Technology : PCR and LCR used as diagnostic techniques. Demonstration of
mutation in drug sensitivity genes is a useful indicator of drug resistance.
Immunodiagnosis : Antibody-Antigen reaction studied.
For extrapulmonary TB also microscopy,
culture are used for diagnosis. For the allergic tests tuberculin test is done.
Treatment & Prevention
In the prevention of TB,
measures such as adequate nutrition, good housing and health education are
important.
Immunoprophylaxis
is by intradermal injection of the live attenuated
vaccine developed by Calmette and Guerin in 1921, at Pasteur institute in
Paris,the Bacilli Calmette Guerin or BCG. This is a strain of M. bovis
attenuated.
Chemoprophylaxis is the administration of
antituberculous drugs such as isoniazid and rifampin for at least one
year. But atypical forms in AIDS patients are now resistant to isoniazid.
Antituberculous drugs are of two types, bactericidal and bacteriostatic
Bactericidal Drugs – Rifampicin, Pyrazinamide(sterilizing drugs- effectively kill
bacteria in the lesions). Isoniazid- effective only against replicating
bacilli. Streptomycin- only against extracellular bacilli.
Bacteristatic Drugs – Ethambutol
If both these are combined and given, it
is first line drugs in the therapy. The long course treatment has been replaced
by short course, ie., 7 months, which are effective and convenient. A very
serious consequence of unchecked drug resistance to the spread of Multidrug
Resistant TB(MDR-TB)
M. leprae – It causes leprosy or Hansen’s disease, recognised from vedic times
and Biblical times. Leprosy has always been held in superstitious, and the
person suffering from leprosy considered ‘unclean’ and a social outcaste. The
leprae bacilli was observed first by Hansen.
The
bacilli is a straight or slightly curved rod. Polar bodies and other intracellular
elements may be present. Clubbing, lateral buds,branching may be observed. It
is Gram positive and Acid Fast, but not as tubercle bacillus, So 5% H2SO4
employed for decolourisation. It
has not so far been possible to cultivate leprae bacilli either in
bacteriological media or in tissue culture. But recently they are found growing
in nine-banded armadillos, chimpanzees, mangabey monkeys, footpad of mice, made
the bacterium available for research. It reproduces very slowly,ie.,12 day
division cycle. The genome has been mapped and the genes for its major protein
antigens cloned and sequenced. It may now be possible to develop a vaccine.
They remain viable in a warm humid environment for 9-16 days and in moist soil
for 46 days. They survive exposure to direct sunlight for 2 hours and UV light
for 30 minutes..
Pathogenicity
Leprosy is a chronic granulomatous
disease of humans primarily involving the skin, peripheral nerves, nasal
mucosa, but capable of affecting any tissue or organ. The disease may be
classified into 4 types, which is a reflection of the immune status of the
host.
1.
Lepromatous : It is seen where the host resistance
is low. Superficial nodular lesions develop which consist of granulation tissue
containing a dense collection of vacuolated cells. The nodules ulcerate,
becomes secondarily infected and cause distortion and mutilation. Incubation
period is 9-12 years. Bacilli invade the mucosa of the nose, mouth and upper
respiratory tract and are shed in large numbers in nasal and oral secretions.
The bacilli are seen in large numbers inside lepra cells or extracellularly.
2. Tuberculoid
: Seen in patients with a high degree of resistance, also known as anesthetic
form, here areas of skin lose pigment and sensation. Bacilli are scanty in lesions.
Incubation time averages 2-5 years.
3. Dimorphous
: Lesions possessing characteristics of both tuberculoid and lepromatous types.
Depending on alterations in host resistance, they may shift to tuberculoid or
lepromatous type.
4. Indeterminate
: Not characteristic of tuberculoid or lepromatous type. This type of lesions
undergo healing spontaneously. In some, the lesions progress to tuberculoid or
lepromatous types.
Indian classification includes pure neuritic type, shows neural
involvement.
M. leprae destroys
skin and mucus membranes. The organism seen in ears, nose and fingers, except
central nervous system. Large numbers of bacilli are shed in respiratory
secretions and in pus discharged from lesions. The shedding of organisms
transmits the disease to those with in close contact with patients, such as
children of infected parents. As disease progresses, it deforms hand and feet.
Severe lepromatous disease erodes bone, fingers and toes become needle-like,
pits develop in the skull, nasal bones are destroyed, teeth fall from the jaw
as bone surrounding them is lost. Sometimes surgery can restore the use of
extremely crippled hands and feet.
Immunity- Infection with
lepra bacilli induces both humoral and cellular immune responses. Humoral
antibodies do not have deleterious effect on the bacilli, while cellular immune
mechanisms are capable of destroying them.
Lepromin test : This is a skin test for delayed type of hypersensitivity. The
original antigen, lepromin was boiled, emulsified and given as intradermal
injection. The test is employed to classify the lesions of leprosy patients.
The test is positive in tuberculoid, negative in lepromatous and variable in
other types.
Laboratory Diagnosis
Hansen’s
disease is diagnosed by PCR, or by finding the organism in acid-fast stained
smears and in scrapings from lesions or biopsies. For routine examination,
specimens are coleected from the nasal mucosa, skin lesions and ear lobules. A
blunt, narrow scalpel is introduced into the nose and the internal septum
scrapped sufficiently to remove a piece of mucus membrane, which is transferred
to a slide and made into a uniform smear. Samples from the skin should be
obtained from the edges of the lesion. In Ziehl-Neelson technique smears are
graded,
1-10 bacilli in 100 fields = 1+
1-10 bacilli in 10 fields = 2+
1-10 bacilli per field = 3+
10-100 per field = 4+
100-1000 per field = 5+
1000 = 6+
The
bacteriological index or BI calculated by totalling the number of pulses scored
in all smears, divided the number of smears.
Treatment –
Dapsone, Clofazimine, Rifampin are the chemotherapeutic agents used in therapy,
but dapsone resistant forms had appeared. Treatment reduces nodules of
lepromatous tissue, but cannot restore lost tissue. Vaccines not available.
Leprosy is not highly communicable. Case finding and adequate therapy are
essential for control.
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