Which of the following is an example of a Gram-positive cocci?

The correct choice is Staphylococcus aureus, which is a classic example of a Gram-positive cocci. Gram-positive bacteria are characterized by their thick peptidoglycan layer in the cell wall, which retains the crystal violet stain used during the Gram staining procedure, resulting in a blue-purple appearance under the microscope.

Staphylococcus aureus is known for being spherical in shape (cocci) and often appears in clusters resembling grapes due to its division in multiple planes. It is a significant pathogen associated with various infections, from skin infections to more severe conditions such as pneumonia and sepsis.

Streptococcus spp. is also a Gram-positive cocci, but Staphylococcus aureus is the more widely recognized example in a clinical context. Other options, such as Escherichia coli and Salmonella enterica, are Gram-negative rods (bacilli), thus do not fit the category of Gram-positive cocci. These differences in cell wall structure and resulting bacterial morphology are fundamental in microbiology, emphasizing the distinction between Gram-positive and Gram-negative organisms.