A 19-year-old college athlete presents with a painful, red abscess on his thigh. A wound swab cultured on Mannitol Salt Agar. The plate shows growth with a yellow zone surrounding the colonies. What does this indicate?

A. Pink: Pink indicates Gram-negative bacteria, which do not retain the crystal violet stain during the decolorization step and are counterstained with safranin. Gram-positive bacteria are not pink after Gram staining.

B. Red: Red is another way to describe the color of Gram-negative bacteria after counterstaining with safranin. Gram-positive bacteria do not take up the red counterstain because they retain the primary crystal violet stain.

C. Purple: Gram-positive bacteria appear purple after Gram staining. Their thick peptidoglycan cell walls trap the crystal violet-iodine complex even after alcohol or acetone decolorization, allowing them to retain the purple color and be distinguished from Gram-negative bacteria.

D. Colorless: Colorless cells would indicate a failure to stain or a decayed sample. Gram-positive bacteria are never colorless after a proper Gram staining procedure.

A. To differentiate between Gram-positive and Gram-negative bacteria: Differentiating Gram-positive from Gram-negative bacteria is the purpose of Gram staining, a differential staining technique. Simple staining does not distinguish between bacterial cell wall characteristics or species based on Gram reaction.

B. To determine bacterial motility: Bacterial motility is assessed using methods such as the hanging drop technique or motility agar. Simple staining does not provide information about motility because it stains all cells uniformly and immobilizes them on the slide.

C. To visualize the size, shape, and arrangement of bacteria: The primary purpose of simple staining is to enhance the contrast between bacterial cells and the background, allowing clear observation of cellular morphology, including size, shape (cocci, bacilli, spirilla), and arrangement (chains, clusters, pairs). This information is fundamental for bacterial identification and classification in microbiology.

D. To identify bacterial endospores: Endospore identification requires special staining techniques, such as the Schaeffer-Fulton method using malachite green and safranin. Simple staining cannot differentiate spores from vegetative cells because all structures take up the basic dye similarly.