A. Typical organism: In tissue, tubercle bacilli
are thin straight rods measuring about 0.4 x 3 um. On artificial media, coccoid and filamentious forms are seen with
variable morphology from one specie to another.
Mycobacterium can be classified as
either gram- positive or gram- negative. Once stained by basic dyes they cannot
be decolorized by alcohol regardless of treatment with iodine. True tubercle
bacilli are characterized by “acid- fastness” which means 95% ethyl alcohol
containing 3% hydrochloric acid can quickly decolorize all bacteria except the
mycobacterium. Acid fastness depends on the integrity of the waxy envelope.
The
ziehl-Neelsen technique of staining is employed for identification of acid fast
– bacteria (Jawetz et al, 1998).
B
Culture: The
media for primary culture of mycobacterium should include a non selective
medium and a selective medium. Selective media contain antibiotics to prevent
the overgrowth of contaminating bacteria and fungi (Burrel, 2005).
C.
Growth Characteristics:
Mycobacterium is obligate aerobes and derives energy from the oxidation of many
simple carbon compounds and increased CO2 tension enhances growth.
Biochemical activities are not characteristic, and the growth rate is much
slower than that of most bacteria. The doubling time of tubercle bacilli is
about 18 hours. Saprophytic forms tend to grow more rapidly well at 22-230C
to produce more pigment and to be less acid- fast than pathogenic forms
(Burrel, 2005).
D.
Pathogenicity of mycobacterium:
There are marked differences in the ability of mycobacterium to cause lesions
in various host species. Humans and guinea pigs are highly susceptible to m. tuberculosis infection, whereas fowl
and cattle are more resistant. M.
tuberculosis and m. bovis are
equally pathogenic to humans and the route of infection determines the pattern
of lesions (NTBLCP, 2005).
E. Constituents of tubercle bacilli: The constituents found in cell walls of tubercle bacilli include
lipids, proteins and polysaccharides. Mycobacterium cell walls can induce
delayed hypersensitivity and some resistance to infection and can replace whole
mycobacteria cells in freunds adjuvant. Mycobacterium cell wall contents only elicit
delayed hypersensitivity reactions in previously sensitized animals.
F.
Pathogenesis: Mycobacterium
is a droplet of 1-5µm in diameter when inhaled and reach alveoli. The disease
results from establishment of virulent organisms and interactions with the
host. (Bayer, 1995). Body developes certain level of cell mediated immunity
that protects the individual. This immunity is destroyed by HIV/AIDS, leading
to reactivation hence increase of cases. Destruction of lung parenchyma and
cavitation (tubercle) is important feature of adult tuberculosis. There is also
cell death, necrosis. (K. Parks, 2009.
G. Pathology: The production and development of lesions and
their healing or progression are determined mainly by:
(1). the number of
mycobacterium in the inoculums and the subsequent multiplication.
(2). the resistance and
hypersensitivity of the host (Allewelt, 2002).
H. Immunity and hypersensitivity: Unless a host dies during the first
infection with tubercle bacilli, a certain resistance is acquired and there is
an increased capacity to localize tubercle bacilli, retard their
multiplication, limit their spread and reduce lymphatic dissemination. This can
be attributed to the development of cellular immunity during the initial
infection with evident ability of mononuclear phagocyte to limit the
multiplication of ingested organism and even to destroy them. Antibodies form against
a variety of the cellular constituents of the tubercle bacilli. The presence of
antibodies can be determined by many different serologic tests. None of these
serologic reactions bears any unequivocal relations to the immune state of the
host, but high titers of IgG antibody is detectable by EIA test with
polysaccharides which exists in many patients with active pulmonary
tuberculosis. (Atlas, 1995).
I. Clinical features: This includes coughing for up to three months or more with or without sputum,
weight loss, fever especially at nights, tiredness, chest pain, blood in sputum etc.