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A. Staphylococcus aureus: Mannitol Salt Agar (MSA) is both selective and differential. It contains high salt concentration, which inhibits most bacteria except staphylococci, and mannitol with a pH indicator to detect fermentation. Staphylococcus aureus ferments mannitol, producing acid that lowers the pH and turns the medium yellow.

B. Enterococcus faecalis: Enterococcus faecalis can tolerate salt but does not typically ferment mannitol on MSA, so the medium generally remains red or pink. It is more commonly identified using other selective media like bile-esculin agar.

C. Salmonella enterica: Salmonella species do not grow well on high-salt MSA and do not ferment mannitol, so they would not produce yellow colonies on this medium. They are typically detected using selective media like XLD or Hektoen enteric agar.

D. Pseudomonas aeruginosa: Pseudomonas aeruginosa is inhibited by the high salt concentration in MSA and does not ferment mannitol. While it may grow slowly under some conditions, it will not produce yellow colonies, making it unlikely to be the pathogen here.

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.