Mycobacterium tuberculosis are seen as acid-fast bacilli when stained with the Zeihl-Neelsen acid-fast stain and viewed at 1,000X magnification. These bacteria were seen in a sputum sample from a patient with active tuberculosis.

Mycobacterium tuberculosis is an aerobic intracellular pathogen, spread by aerosol droplets. M. tuberculosis is a slow growing organism and it takes weeks to incubate because of the complexity of the cell wall. The bacilli primarily infect the pulmonary system, but can spread to any body site for immunocompromised patients. M. tuberculosis bacilli inhabit unactivated macrophage in the alveoli (1). Patchy infiltrates form when the bacilli grow unchecked. Active tuberculosis is characterized by a continual cough, weight loss, night sweats and the presence of acid-fast bacilli in sputum.

Mycobacteria contain mycolic acids and have a high GC content in their DNA. A Gram stain cannot penetrate the waxy cell wall. The hydrophobic lipids cause the Gram stain to give no staining or a variable result. An acid-fast stain is used to visualize the bacilli. Mycobacteria are referred to as acid fast because they retain the acid-fast dye, Carbolfusion the primary stain of the acid fast procedure. As seen in the image the cells stain bright pink with this procedure.

The mycolic acids are comprised of free lipids, glycolipids and peptidoglycolipids. The host’s immune system is trigged by the presence of the mycolic acids. Once activated, the immune system usually clears the presence of the bacterium within a few weeks post exposure. However, the onset of the active disease largely depends on the competency of the host’s immune system. The active form of the disease is seen in approximately 5% to 10% of infected individuals (1).

Skin tests are used to determine whether or not the individual has been exposed to M. tuberculosis. The skin tests are made of purified protein derivatives (PPD) from the bacterium. Sputum cultures, chest x-rays showing infection (abscess formation or cavities) in the upper lungs and a positive skin test are all used to diagnose tuberculosis (1).

The cell wall structure is an important virulence factor, which can confer antibiotic resistance. Therefore, M. tuberculosis infections require multiple drug treatments for extended periods of time. Rifampin and Isoniazid are two of the most commonly prescribed antibiotics for active tuberculosis. Treatment with these drugs usually lasts at least nine months and may include other antibiotics.

Reference

1. Murray, P.R, K.S. Rosenthal, G.S. Kobayashi, and M.A. Pfaller. 2002. Medical Microbiology, Fourth Edition, p. 366-370. Mosby Inc., St. Louis.

Legend written by:
Jaimie VanBuren
Colorado State University
Fort Collins, Colorado