case23-mycobacteriumTB - Case 23 Lydyard Peter Cole Michael...

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1. What is the causative agent, how does it enter the body and how does it spread a) within the body and b) from person to person? Causative agent This patient is infected with Mycobacterium tuberculosis . It is a weakly gram- positive mycobacterium classified as an ‘acid-fast bacillus’ because the dye that is used to stain it is resistant to removal by acid. The stain used to identify mycobacteria is called the Ziehl-Neelsen (ZN) stain and charac- teristically stains mycobacteria red while all other organisms stain green (Figure 2). Mycobacterium is the only genus of medical importance that stains red with ZN stain. In common with most other bacteria the cell wall contains peptidoglycan. Overlying this is a layer of arabinogalactan, which is covalently linked to the outer layer composed of mycolic acid, long chain fatty acids specific for the mycobacterial genus, with other components such as glycophospho- lipids and trehalose dimycolate (also called cord factor as on staining the organism it has the appearance of cords). Running vertically through the whole of the cell wall and linked to the cytoplasmic membrane is lipoara- binomannan (Figure 3). Case 23 Mycobacterium tuberculosis A 63-year-old man lived in a hostel for the homeless and sold magazines outside a railway station. He had been finding it difficult to cope with this recently, as he had been feeling weak, had lost weight, and often had a fever at night. One month ago, he started coughing up blood and feeling breathless, which had really worried him. He was not registered with a primary health-care provider but a friend told him about a walk-in practice for homeless people. Next day he went to the practice and was seen by the physician on duty, who found that the patient had a low-grade fever and detected bronchial breathing when he listened to his chest. The doctor sent him for a chest X- ray and asked him to return for the results. When the X-ray result came back, it showed that he had apical shadowing and large cavitation consistent with tuberculosis (TB). The X-ray is shown in Figure 1. A sputum sample was taken since the doctor suspected that the patient had tuberculosis and the patient was started an antituberculosis therapy. Figure 1. Chest X-ray of patient showing typical apical consolidation with possible cavities . There is also some consolidation adjacent to the mediastinum on the right. The bases are relatively spared. The medial aspect of the right hemi-diaphragm is just visible. Figure 2. Ziehl-Neelsen stain of sputum: note the red bacilli (arrowed) against a green background stai n. Note that this is the only genus of medically important bacteria that stains red with the Ziehl- Neelsen stain. © Lydyard, Peter; Cole, Michael; Holton, John; Irving, Will; Porakishvili, Nino; Venkatesan, Pradhib; Ward, Kate, Jan 01, 201
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This note was uploaded on 09/14/2011 for the course PHARM mb taught by Professor Staff during the Spring '11 term at UCSD.

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case23-mycobacteriumTB - Case 23 Lydyard Peter Cole Michael...

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