appear as singly or in small clumps from sputum and other clinical specimens
appear as twisted rope-like colonies termed serpentine cords when in liquid cultures
can grow on a wide range of enriched culture media but Lowerstein-Jensen (LJ) medium is the most widely used. The colonies grew on this medium are off-white (buff) in colour and often have a dry breadcrumb-like appearance
rather limited temperature range of growth (35-37 degrees) and fail to grow at 25 and 41 degrees
obligate aerobes
ability to reduce nitrates to nitrites
produce large amount of niacin
sensitivity to pyrazinamide
obligate pathogens but they do survive in milk and in other organic materials and on pasture land so long they are not explore to UV light due to sensitivity
heat sensitive and are destroyed in the process of pasteurization
more resistant than other bacteria to acids,alkalis and quaternary ammonium compounds
b) Pathogenesis
bacilli inhaled as droplet nuclei
in human,bacilli are engulfed by alveolar macrophages in which they replicate to form the initial lesion known as Ghon Focus or Tubercles
some bacilli are carried in phagocytic cells to the lymph nodes where additional foci of infection develop
both formation of tubercles and the enlarged lymph nodes are known as primary complex
within 10 days of infection,clones of antigen specific T lymphocytes are produced and release lymphokines which activate macrophages and cause them to form a compact cluster or granuloma around the foci of infection
the centre of the granuloma contains a mixture of necrotic tissue and dead macrophages,which from its cheese-like appearance and consistency is referred to as caseous lesion
the macrophages in the granuloma consume oxygen and the resulting anoxia and acidosis in the centre of the lesion kills most of the tubercle bacilli
granuloma formation is usually sufficient to limit the primary infection but not all bacilli are destroyed. Some remain in a poorly understand dormant form from which when reactivated,cause post primary disease
sometimes the tubercle lesions liquefy and form air-filled tuberculous cavities. From these cavities,the bacteria can spread to new foci of infections throughout the body. This spreading is often called miliary tuberculosis
reactivation may occur spontaneously or after an inter-current illness or other condition that lowers the host's immune responsiveness
reactivation often occurs in the upper lobes of the lungs
the same process of granuloma formation occurs but the necrotic element of the reaction causes tissue destruction and the formation of large areas of caseation termed tuberculomata
proteases liberated by activated macrophages cause softening and liquefaction of the caseous material and an excess of tumour necrosis factor and other immunological mediators cause the wasting and fevers characteristic of the disease
the interior of the tuberculoma is acidic and anoxic and contains few viable tubercle bacilli
eventually,however,the expanding lesion erodes through the wall of a bronchus,the liquefied contents are discharged and a well-aerated cavity is formed
once the cavity is formed,large number of bacilli gain access to the sputum and the patient becomes an open or infectiousm case
reactivation tuberculosis is particularly likely to occur in immunocompromised individuals including the elderly,transplant recipients and those who are HIV positive
c) Laboratory Diagnosis
by microscopy,cultural techniques or by Polymerase Chain Reaction (PCR)
specimens taken include sputum,bronchial washings,brushings or biopsies,larynegeal swabs and early morning gastric aspirates
d) Treatment
Divided into three phases :-
during the first week or two,the large no, of actively replicating bacilli in cavity walls are killed,principally by isoniazid but also by rifampicin and ethambutol. As a result,the patient rapidly ceases to be infectious
in the following few weeks,the less active bacilli within macrophages,caseous material and dense acidic,inflammatory lesions are killed by rifampicin and pyrazinamide
in the continuation phase,any remaining dormant bacilli are killed by rifampicin during their short bursts of metabolic activity. Any rifampicin-resistant mutants that start to replicate are killed by isoniazid
e) Control
by early detection and effective therapy of the open or infectious individuals in a commmunity
lowering the chance of infection by reducing overcrowding
by vaccination
Mycobacterium leprae
a) Description
not so acid-fast as tubercle bacilli
never been cultivated in vitro,so got limited information on the study of leprosy
a charcteristic surface lipid,peptidoglycolipid I (PGL-1) has been extracted from M.leprae and its unique carbohydrate antigenic determinant has been synthesized and used to develop serodiagnostic tests for leprosy
b) Pathogenesis
the principal target cell is the schwann cell and the resulting nerve damage is responsible for the main clinical features of leprosy which are anaesthesia and muscle paralysis
repeated injury to and infections of the anaesthetic extremities leads to their gradual destruction
first sign of leprosy is a non-specific or indeterminate skin lesionwhich often heals spontaneously
if the disease progresses,its clinical manifestation is determined by the specific immune responsiveness of the patient to the bacillus and there is a distinct immunological spectrum with hyper-raective tuberculoid (TT) leprosy at one pole and anergic lepromatous (LL) leprosy at the other. The intermediate position on the spectrum are classified as border-line tuberculoid (BT),mid-borderline (BB) or borderline lepromatous (BL)
the nasal bones are also involved in leprosy and their destruction may lead to collapse of the nose
blindness is another common and tragic complication of untreated leprosy
c) Laboratory diagnosis
by histological examination of skin biospies
by detection of acid-fast bacilli in nasal discharges,scrapings from the nasal mucous and slit-skin smears
d) Treatment
dapsone (4,4'-diaminodiphenyl sulphone or DDS) monotherapy was used to be the standard treatment for all forms of leprosy but due to increased incidence of depsone-resistant bacilli,multidrug therapy based on dapsone,rifampicin and clofazimine are introduced
clofazimine has the unfortunate property of causing skin decoloration,thus if the patient refuse to take this drug,prothionamide,ofloxacin or minocycline may be used instead
e) Control
detection and treatment of infectious cases
no living attenuated vaccines have been prepared but BCG vaccine seems to protect against leprosy in those regions where it protects against tuberculosis,strongly suggesting that protection is induced by common mycobacterial antigens
Glossary
tuberculosis - an infectious disease caused by mycobacterium and is characterised by the formation of tubercles and tissue necrosis,primarily as a result of host hypersensitivity and inflammation. Infection is usually by inhalation and the disease commonly affects the lungs
tubercles - a small rounded nodular lesion produced by mycobacterium
leprosy - a severe disfiguring skin disease caused by mycobacterium leprae
