42 Identification of mycobacteria
42.1 Identification of mycobacteria
A number of different mycobacterial species can cause disease in humans, perhaps most notably Mycobacterium tuberculosis (the causative agent of tuberculosis). Some mycobacterial species are challenging for the clinical microbiologist, in that they are extremely slow-growing - for some species, it can take weeks for cultures to grow.
Mycobacteria are therefore often cultured on selective media such as Löwenstein-Jensen medium, which contains high levels of malachite green that inhibit the growth of most other organisms.
Mycobacteria can sometimes stain weakly Gram-positive, but the structure of their cell walls is normally rather impervious to the stains used in Gram staining. Instead, an acid-fast stain, which you will learn about in the next section, is commonly used.
CASE STUDY 7. Tuberculosis?
Patient history: fever, persistent cough, weight loss, loss of appetite. The patient is immunocompromised and has recently travelled extensively outside the UK.
The patient is positive for the Tb skin test, but has had the BCG vaccine and so this may be a false positive. The clinician suspects that it may in fact be an infection by Mycobacterium tuberculosis, and takes a sputum sample and sends it to the lab for analysis. The sample was plated on Löwenstein–Jensen medium and colonies were then isolated into pure culture.
Your task is to identify the bacterial species causing this infection (Task 5C).