A 52-year-old man comes to the office with 3 weeks of cough, night sweats, and occasional hemoptysis. The patient also has diabetes and hypertension. He emigrated from Vietnam 15 years ago to start a job as an associate professor at a university. He has a 20-year smoking history but quit 5 years ago. Sputum cultures grown on a selective medium demonstrate mycobacteria that appear as parallel chains (serpentine cords) under microscopy. This observed growth pattern in culture medium most strongly correlates with which of the following?

Question

A 52-year-old man comes to the office with 3 weeks of cough, night sweats, and occasional hemoptysis.  The patient also has diabetes and hypertension.  He emigrated from Vietnam 15 years ago to start a job as an associate professor at a university.  He has a 20-year smoking history but quit 5 years ago.  Sputum cultures grown on a selective medium demonstrate mycobacteria that appear as parallel chains (serpentine cords) under microscopy.  This observed growth pattern in culture medium most strongly correlates with which of the following?

Quizplus · Accepted Answer

Mycobacterium tuberculosis has a classical morphologic appearance under microscopy. The cells are arranged in long, slender, serpentine cords due to the presence of a hydrophobic surface glycolipid called cord factor. Cord factor is composed of two mycolic acid molecules bound to the disaccharide trehalose (trehalose dimycolate). In vivo, cord factor forms cylindrical micelles that surround the organism and prevent macrophage-mediated destruction within the phagolysosome. It also forms a highly toxic crystalline monolayer on hydrophobic surfaces (oil-water/air-water interface [eg, edge of cavitation]) that helps drive the formation of caseating granulomas (a hallmark of M tuberculosis infection). (Choice A) Mycolic acid is the primary component of the cell wall of all Mycobacterium species and is the source of their acid-fastness (ability to retain dye in the presence of acid). Although it is a component of cord factor, mycolic acid alone does not cause M tuberculosis to grow in a serpentine pattern. Other less virulent mycobacteria (eg, M avium) are also acid-fast but have lower concentrations of cord factor and do not typically form serpentine chains. (Choice B) M tuberculosis antibiotic resistance is due largely to mutations in the genes responsible for the synthesis of mycolic acid (isoniazid) and mycobacterial RNA polymerase (rifampin). Antibiotic resistance does not cause the bacteria to grow in a serpentine pattern. (Choice C) The slow growth rate of M tuberculosis contributes to the need for prolonged antibiotic regimens due to decreased antibiotic responsiveness. However, it does not affect the ability of the organism to form serpentine cords. (Choice D) Pigment production is characteristic of some mycobacteria, especially nontuberculous organisms such as M marinum, but is not associated with cord formation. Educational objective: Mycobacterium tuberculosis grows in long, serpentine cords due to the presence of cord factor, a surface glycolipid, on the cell wall. Cord factor is a primary virulence factor of M tuberculosis; it prevents macrophages from being bactericidal due to the inhibition of phagolysosome acidification and also leads to the formation of caseating granulomas. __________ References: Multiple roles of cord factor in the pathogenesis of primary, secondary, and cavitary tuberculosis, including a revised description of the pathology of secondary disease. (http://www.ncbi.nlm.nih.gov/pubmed/17127724)

A 52-Year-Old Man Comes to the Office with 3 Weeks