Which of the following organisms is considered Gram-Positive?

Staphylococcus is considered a Gram-positive organism due to its cell wall structure, which has a thick layer of peptidoglycan that retains the crystal violet stain during the Gram staining process. This thick peptidoglycan layer is characteristic of Gram-positive bacteria and leads to the appearance of a purple color under a microscope after the staining process.

In contrast, the other organisms listed are Gram-negative. For example, Escherichia coli and Salmonella possess thinner peptidoglycan layers surrounded by an outer membrane, which does not retain the crystal violet stain, resulting in a pink appearance. Similarly, Neisseria meningitidis has a Gram-negative cell wall structure that also does not hold onto the crystal violet stain. Therefore, Staphylococcus stands out as the Gram-positive choice due to its distinctive cell wall composition.