Friday, August 05, 2005


A 3rd year resident had an interesting question. He wanted to see what others thought. He asks, "we are all taught that bacterial pneumonia is not 'catching'. You can put a patient with community-acquired pneumonia, i.e. strep pneumonia, in the same room as someone else and their roommate will not catch pneumonia. Why then is TB contagious, when this is ALSO a bacteria? Or to put it another way, why is TB the only type of bacterial pneumonia that is contagious?"

2 comments - CLICK HERE to read & add your own!:

Baleeiro said...

This is a cool question. The statement about pneumonia is true and interesting. Indeed, mice artificially given pneumonia may be housed with healthy mice with no dissemination from one to the other.
Airborne diseases such as TB depend on infectious particles forming smaller droplets: after a cough (or sneeze for some air-borne viral respiratory tract infections) millions of tiny droplets of water and mucus are expelled at about 200 miles per hour (100 metres per second). The droplets initially are about 10-100 micrometres diameter, but they dry rapidly to droplet nuclei of 1-4 micrometres, containing virus particles or bacteria. Coughing is more effective because the ratio favors smaller particles than sneezing. This smaller size allows for deposition at the smaller bronchioli and alveoli escaping mechanical clearance by the muco-ciliary apparatus. A few mycobacteria in a small droplet are thus more contagious than large amounts of bacteria/mycobacteria in larger droplets more easily cleared.
These small droplets usually will not carry enough bacteria to overwhelm the local alveolar macrophages and are less likely to cause “typical” pneumonia which is usually caused by aspiration of oropharyngeal contents.

Baleeiro said...

Other mycobacteria seem to have a different behavior: though also aerossolized (think of MAC and "hot-tub lung") they are usually not transmitted person-to-person. Check out previous postings on M. kansasii: