What is a differential stain used for?

A differential stain is primarily utilized to distinguish between different types of bacteria based on their structural differences, particularly in their cell walls. The most widely known differential stain is the Gram stain, which classifies bacteria into two groups: Gram-positive and Gram-negative, based on their ability to retain the crystal violet stain. This classification is crucial in microbiology as it helps determine the appropriate treatment for bacterial infections, informs the choice of antibiotics, and provides insight into the bacteria's potential behavior and pathogenicity.

The other options pertain to different aspects of microbiological analysis. Measuring bacterial growth typically involves techniques like colony counting and turbidity measurements rather than staining. Identifying viruses generally requires different methods, such as serological tests or PCR, since viruses do not possess cell walls and do not stain in the same manner as bacteria. Counting cells under a microscope, while an important task in microbiology, would not specifically involve differential staining techniques. Thus, the use of differential stains is distinctly focused on the classification and distinction of bacterial types.