Tuberculosis (TB) is an infectious disease caused by bacteria whose scientific name is Mycobacterium tuberculosis .
It was first isolated in 1882 by a German physician named Robert Koch who received the Nobel Prize for this discovery .
TB most commonly affects the lungs but also can involve almost any organ of the body.
Over 8 million new cases of TB occur each year worldwide. In the
United States, it is estimated that 10-15 million people are infected
with the TB bacteria, and 22,000 new cases of TB occur each year.
How is at Risk?1-People who live with individuals who have an active TB infection, (Crowded living conditions). 2-Poor or homeless people, 3-People who are in contact with foreign- individuals from countries that have a high prevalence of TB. 4- People with diabetes, certain cancers, and HIV infection ( AIDS ). 5- Health-care workers.
Microscopic Examination of Mycobacterium:
The Mycobacteria are aerobic acid fast, non-motile, non-spore forming rods.
The most common pathogens that cause disease to man are: M. tuberculosis, M. bovis, M. africanum, M.microti, and M. leprae.
M. tuberculosis is a non-spore forming, non capsulated straight or slightly curved rod, measuring 1-4µm x 0.2-0.6 µm.
Although M. tuberculosis does not Gram stain well due to its waxy surface, the organism has a Gram positive cell wall.
The cell envelope of Mycobacterium tuberculosis.
M. tuberculosis is best demonstrated by using the Ziehl -Neelsen technique or a fluorescence technique. When stained by ZN, they appear as thin pink rods arranged singly or in groups. Once stained, they resist decolorization with 2% H2SO4 and alcohol or 3% HCl in 95% ethyl alcohol (Acid-fast, Alcohol fast).
Ziehl- Neelsen stain of a sputum sample from a tuberculosis patient (1000x).
COMMENT ON THE RESULT
If the organisms are seen the smear; it will be reported as AFB positive and given a + symbol that indicates the number of the bacteria per high power felid (oil immersion lens):
More than 10 AFB/Field report +++ 1 - 10 AFB/Field report ++ 10 -100 AFB/100 Field report + 1- 9 AFB/100 Field report the exact number
Cultural Characteristics and colony morphology:
M. tuberculosis is strict aerobic, grows very slowly (2-4 weeks) on egg enriched medium as Lowenstein-Jensen medium, giving dry creamy colored colonies.
Other selective media as Middlebrook agar or broth may be used for primary isolation and for antibiotic susceptibility testing.
M. tuberculosis is a non chromogen ;does not grow on media contain p-nitrobenzoic acid, these characteristics help to differentiate them from Mycobacteria other than tubercle (MOTT) as M. avium, M. intracellulare,and M. kansasi.
The optimum temperature for growth of M.T.C( Mycobacterium tuberculosis complex) is at 37C. The culture must be incubated up to 12 weeks.
M.tuberculosis produces rough, and tough colonies on glycerol and pyruvate egg media. They are niacin producers and grow under aerobic conditions at 37 C.
Cultivation and culture characteristics of Tubercle bacilli on Lowenstein Jensen medium (raised dry cream colored colonies)
+ + + Aerobic Pathogenic
M. bovis _ + _ Microaerophilic
_ + +_
M. microti +_
+ + Aerobic None
Disease transmission:TB is spread by aerosol droplets expelled by people with the active disease of the lungs when they cough, sneeze, speak, or spit .
These infectious droplets are 0.5 to 5 micrometer in diameter and about 40,000 can be produced by a single sneeze.
Pathogenesis:TB infection begins when the Mycobacteria reach the pulmonary alveoli , where they invade and replicate within alveolar macrophage.
The microbe will multiply inside the macrophage causingGranulomas. Granulomas are cellular infiltration contain active microbes and present inside lung tissue. Bacteria are picked up by dendritic cells which transport the bacilli to local mediastinal lymph nodes. Further spread throughout blood stream to organs such as kidney, brain, and bone.
Symptoms:In the patients where TB becomes an active disease, 75% of these cases affect the lungs, where the disease is called pulmonary TB. Symptoms include a productive, prolonged cough of more than
three weeks duration .
Systemic symptoms include ,
coughing up blood, chest pain ,
night sweats, fever, chills, and weight loss .
1- Isolation of Mycobacterium species by L.J medium, or microscopic positive results for acid-fast bacilli.2- Identification of microbial genetic material in clinical specimens by molecular methods such as PCR.3- The Tuberculin skin test. 4- Chest X-ray; Radiology.