A 53-year-old man with three months of productive cough, six kilograms of weight loss and night sweats has come to see you in a chest clinic. He has a pleasant appearance and you learn that he has recently immigrated to Hong Kong from China. In the initial investigation, a chest X-ray shows a cavitary mass in the right upper lobe, and the acid-fast bacilli (‘AFB’) smear shows positive on sputum. After six weeks, the AFB culture also shows positive.

a) Name the respiratory disease that is most likely caused by acid-fast bacteria. (2 marks)

Tuberculosis is caused by Mycobacterium tuberculosis.

Cough for an extended period of time, weight loss, night sweats, and the cavitary mass in the chest x-ray are signature signs and symptoms of tuberculosis.

'Acid Fast' refers to the microbiology technique of acid staining and acid fast bacteria resist decolourisation by acids. 'Bacilli' refers to the shape of the bacteria as being rod shaped.

Mycobacteria tuberculosis

MTB is one of the most difficult bacterial infections to eradicate.

• It is a slow growing organism which makes it resistant to many antibiotics which work better on fast dividing bacteria
• Plus it is able to become dormant (non-growing persisters)
• It has a thick cell wall which makes it impermeable to many drugs
• It is able to reside and replicate inside macrophages  
• Very good at developing resistance to single drug therapy

Macrophages are white blood cells which engulf particles and destroy them. They are stationed throughout the body, including the alveoli. However when MTB is engulfed by macrophages it has a unique ability to evade its killing mechanisms.

The immune system tackles this by surrounding the infected macrophage within a ball of macrophages and other immune cells, forming a granuloma (tubercles), which shows up as a mass on the chest X-ray.

As long as the immune system is healthy, the MTB can be held within the granuloma indefinitely. Spontaneous resolution may even be possible.

MTB is also able to stay dormant for extended periods of time - these are the non-growing persisters. Most antibiotics do not have activity against non-growing persisters (exception pyrazinamide).

TB and HIV are synergistic.

Although the primary source of infection is the respiratory system, MTB is able to proliferate into other parts of the body, especially in immunosuppression.

Simplified diagram showing a TB granuloma. It shows two free bacteria in the middle and two which have been engulfed by macrophages. The MTB is able to evade the internal killing mechanisms of the macrophage and reside within it for an extended period of time as it is a slow grower and also able to become dormant. The immune system forms the granuloma to contain the infected macrophages and the MTB indefinitely.