A patient with a cavitary lung lesion coughs up sputum that contains thin, acid-fast positive rods. Which of the following features would most likely be associated with these bacteria?

Mycobacteria, such as the causative organism of this patient's tuberculosis, are "acid fast" because they have an envelope that contains large amounts of lipids and even true waxes (unlike envelopes of other types of bacteria) that prevents the acid-fast stain (carbol fuchsin) from leaking out. The other characteristics listed in the answers are commonly tested features of specific bacteria: Nutritional requirement for factors V and X is a feature of Haemophilus influenzae. Streptokinase is a feature of Streptococci. Toxic shock syndrome toxin is a feature of Staphylococcus aureus. Ref: Ray C.G., Ryan K.J. (2010). Chapter 27. Mycobacteria. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.