While doing your rounds, you meet Josh, an HIV-positive 25-year-old man who presents with a 2-month history of non-productive cough. He also describes poor appetite and significant weight loss, fever, night sweats, and excessive tiredness. He denies dyspnea or hemoptysis. Physical examination is unremarkable. A PPD intradermal test was performed and it was negative. His chest X-ray showed a peri-hilar lesion with central necrosis and calcification as well as lymphadenopathy of nearby nodes.
Now, this person seems to suffer from tuberculosis, or TB for short. But first, a bit of microbiology. Mycobacteria tuberculosis are slender, rod-shaped, Gram positive bacteria that need oxygen to survive, in other words, they’re “strict aerobes”. One piece of high-yield information is that although they are classified as Gram positive - meaning they have an outer cell wall, it is the same wall that makes the bacteria special. This is because Mycobacterium have an unusually waxy cell wall made of mycolic acid, which is composed of long chains of branched lipids, which won't stain with Gram. This makes them “acid-fast” so the Ziehl-Neelsen stain has to be applied, a dye that will not be washed away by acids, giving the bacteria a bright red color. The wall also makes the bacteria incredibly hardy, and allows them to resist weak disinfectants, antibiotics, and allow them to survive on dry surfaces for months at a time.
Okay, so Tuberculosis is a type of pulmonary infection caused by Mycobacterium tuberculosis, sometimes just called TB bacteria. Before we start, you need to know that there are a few high-yield risk factors for TB. These include immunosuppression, like in people with HIV; iatrogenic immunosuppression, like in people who undergo treatment with corticosteroids; systemic diseases such as COPD, diabetes, and end-stage renal disease; extremes of age; substance abuse; and populations with an increased risk of exposure, like the prison populations, homeless people, those born in an endemic country, and health care workers.