Cause of TB & Its characteristics

 ETIOLOGY & Characteristics.

Etilogy is the study of cause of a particular disease. For tuberculosis the etiologic agent is Mycobacterium tuberculosis. Robert Koch discovered this microbe and announced its discovery on 24^th March 1882 and was awarded Nobel Prize for Medicine for his work on Tuberculosis[1]. He was the one who established a relationship between M.tuberculosis and the disease. Though many were skeptic that a Bacillus could be the cause and not merely an accompaniment of the disease, they soon accepted this fact. It was Koch who had first demonstrated the life cycle of Anthrax bacillus and proved for the first time then that a microorganism could be the cause of a disease.

M.tuberculosis

• Is a rod shaped bacterium.

• It belongs to the genus Mycobaterium.

• It is an Obligate aerobe

• It is non-motile

• It requires relatively simple nutrition- uses ammonia or amino acids as nitrogen sources and glycerol as a carbon source in the presence of mineral salts[2]. 
• It does not have endospores or capsules.

• It is a mesophile.

• It has Mycolic acid in its cell wall.

Properties conferred by Mycolic layer.

The organism's cell wall has high lipid content (40% of cell's dry weight is made up of Lipids)[4]. All Mycobacterium species share a characteristic cell wall, thicker than in many other bacteria, which is hydrophobic, waxy, and rich in mycolic acids/mycolates. Mycolic acid is a unique fatty acid which gives all species belonging to the genus of Mycobacterium specific characteristic. Mycobacteria do not fit in Gram positive category as their cell wall does not retain crystal violet stain well( normally, all gram positive bacteria have peptidoglycan cell wall, which has the ability to take up crystal violet stain well.) So they are put under the category Acid fast. Acid fast is the characteristic of Mycobacteria.
Acid fast bacteria have poor absorption and high retention property due to the presence of mycolic acid. That is, it difficult to stain this bacteria. But once stained, the cells retain the stain even upon treating with decolourizer like acid-alcohol(hydrochloric acid in alcohol). This bacteria is heated with carbolfuchsin for staining.

Importance of Mycolic layer in conferring virulence to the Bacteria.
Mycobacterial infections are notoriously difficult to treat[3]. It makes their cell surface hard. Due to its present the bacteria is neither gram negative nor gram positive. In addition, they are naturally resistant to a number of antibiotics that disrupt cell-wall biosynthesis, such as penicillin[2].Due to their unique cell wall, they can survive long exposure to acids, alkalis, detergents, oxidative bursts, lysis by complement, and many antibiotics[2].

Now that we saw the causative agent for Tuberculosis. Also see 

Mode of transmission of TB

Epidemiology

References:

[1]Sakula A.,1983, Robert Koch: Centenary of the Discovery of the Tubercle Bacillus, Canadian Veterinary Journal(Can Vet J), vol 24, no.4 ,pp. 127-131.

[2]Wikipedia,the free encyclopedia n.d ,Acid-fast , viewed 26 August 2012,

http://en.wikipedia.org/wiki/Acid-fast

[3]Wikipedia,the free encyclopedia n.d., Mycobacterium, viewed 26 August 2012, http://en.wikipedia.org/wiki/Mycobacterium#Microbiologic_characteristics

[4]Staley J.T.,  Gunsalus R.P., Lory S., and Perry J.J.,  Microbial life, second edition, chapter 28 microbial diseases of humans,Sinauer associatessin.