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Mycobacteria
Definition:
Microscopic appearance
AFB
Ziehl-Neelsen stain Smear is fixed, stained with carbol-fuchsin (a
pink dye), and decolorized with acid-alcohol. The smear is
counterstained with methylene-blue or certain other dyes. Acid-fast
bacilli appear pink in a contrasting background.
Kinyoun staining cold staining
Mycobacterium cell wall composition
60% lipids +mycolic acids +polysaccharides and peptides =waxy cell
surface that makes mycobacteria strongly hydrophobic and accounts
for their acid fast staining characteristic
Virulence factor
M.TB. has special mechanisms for cell entry
High lipid concentration in cell wall
M.TB. can grow intra-cellularly.
Predisposing factors for TB infection include:
• Close contact with large populations of people, i.e., schools, nursing
homes, dormitories, prisons, etc.
• Poor nutrition
• iv drug use
• Alcoholism
• HIV infection is the #1 predisposing factor for M.TB. infection. 10
percent of all HIV-positive individuals harbor M.TB. This is 400-times
the rate associated with the general public
Only 3-4% of infected individuals will develop active disease upon
initial infection, 5-10% within one year. These percentages are much
higher if the individual is HIV+.
Pathogenesis
The organism is highly infectious, but low virulence, infection may be pulmonary
T.B or extra pulmonary affecting any organ of the body ( skin Tb , urinary ,
meningitis, genital organ , bone, lymph nodes, cold abscess, intestinal tb
Habitat: no carrier strain
There are 41 species, most common one is
Mycobacterium TB
Mycobacterium leprae
M. ovium
M. fortuitum
atypical mycobacterium
M. chelonei
M. kansasii
Mycobacterium TB
M. africanum
obligate pathogens
M. bovis
Tuberculin reaction
Tuberculin reaction test is a manifestation of delayed hypersensitivity to
protein antigens of M. tuberculosis, they don’t confirm that the patient
currently has active disease
Mantoux test: purified protein derivative (PPD)
Skin Testing is performed as the tuberulin or Mantoux test. PPD
(purified protein derivative) is employed as the test antigen in the
Mantoux test. PPD is generated by boiling a culture of M.TB.,
specifically Old Tuberculin (OT). 5 TU (tuberculin units), which equals
0.000lmg of PPD, in a O.1 ml volume is intra-cutaneously injected in the
forearm. The test is read within 48-72 hours.
The test is considered positive if the diameter of the resulting lesion is
10 mm or greater=10 mm or greater are currently infected with M.TB.
or have been previously exposed to M.TB.
15 mm or greater are currently infected with M.TB. or have been
previously exposed to M.TB.
False positive
False negatives
Culture:
Atmosphere: strictly aerobic
Temperature: 37c , Co2, in presence of humidity
•Dorset egg
•LÖwenstein-jensen (Lj) medium ( egg, glycerin, malachite green) in a
slope
•Glycerin inhibits bovine strains
•Malachite green inhibits all other bacteria
•Middle brook 7H10 agar or 7H9 agar, 7 H12 agar
•Radioactive fluid media BACTEC the radioactive elements is carbon 14
palmitic acid)
Colonies: raised, dry colonies, cream (buff) colonies, it is slow growing
after 2-3 weeks you are going to see growth but not discard plates
before 6 weeks. Yellow pigments,It is very difficult to emulsify to make
a film.
Bactac system radiometric monitoring: the organism multiply
in the broth and metabolize the 14C containg palmatic acid,
producing radioactively labeled 14Co2 contaning atomospher and
measures the amount of radioactivity present
Biochemical reaction
Niacin test
BCG vaccination bacilli calmette Guerin
This is vaccine against TB containing living attenuated bovine strains of
Mycobacteria TB and is giving directly after birth in the left forearm and
is given to adult only tuberculin negative
Antibiotic sensitivity test:
There are resistant mutant: treatment with one of antibiotic will select
this resistant mutant,
So combination of antibiotic is important
Streptomycin
Ethambetoul
Cycloserine
Rimactan
PAS paraaminosalycilic
INH isonicotinin acid hydrolyzed
Anti-mycobacterium susceptibility test:
If less than 1% considered to be sensitive
Tb inoculated in to control LJ medium without drug and a number of lj
medium with a drug at 37 for 2 weeks to 2 months the no. of colonies in
each off control and drug lj are counted so
No of colony of drug/no of colony on control
Then multiply by 100
Sensitive less than 1% we give antibiotic
Resistant more than or 1% we don’t give antibiotic
Atypical Mycobacteria:
It is differ from mycobacterium that it is more resistant to antibiotic able
to produce pigments in culture, rapid in growth and cable to grow at room
temperature above 4 c
Runyoun into 4 groups
Atypical Mycobacteria classified according to
Photochrmogenic : light pigment formation
Scotochromogen dark pigment production
Rapid growth within 7 days
Non chromogen non pigmented when grow in the dark and remain so even after
light exposure (M. ovium
All these called MOTT mycobacterium other than TB
When there is rich growth its called eugonic
Poor growth dysgunic
When add tween 80 it will wet the surface increase permeability and then
increase in speed of growth