NAPLEX (North American Pharmacist Licensure Examination) Practice Exam

Which organism is an Atypical bacterium that does not stain well with Gram staining?

1. Proteus mirabilis

2. Mycobacterium tuberculosis

3. Staphylococcus saprophyticus

4. Peptostreptococcus

The correct answer is: Mycobacterium tuberculosis

Mycobacterium tuberculosis is an Atypical bacterium that does not stain well with Gram staining. Other organisms such as Proteus mirabilis, Staphylococcus saprophyticus, and Peptostreptococcus are Gram-negative or Gram-positive bacteria and therefore stain well with Gram staining. Staining methods are used to differentiate bacteria based on the structure of their cell walls, and the cell wall of Mycobacterium tuberculosis contains a high content of unique lipids and mycolic acid, which makes it difficult to stain with the Gram staining technique. This unique structure also contributes to the pathogenicity and antibiotic resistance of Mycobacterium tuberculosis. Conversely, Proteus mirabilis is a Gram-negative bacterium that has a thin peptidoglycan layer, which makes it easier to stain with the Gram staining technique. Staphylococcus saprophyticus and Peptostreptococcus also have typical cell wall structures that can be easily stained with Gram staining. Therefore, these bacteria are not considered Atypical organisms.