Staphylococcus aureus and Streptococcus salivarius bacteria have a thick PG layer and do not have an outer membrane. What color should a smear of either S. aureus or S. salivarius appear following Gram staining?

Purple

Explanation:

The cell wall of gram-positive bacteria has a thick peptidoglycan layer present outside the plasma membrane. The gram-negative bacteria have a thin peptidoglycan layer covered by a thicker outer membrane. The outer membrane is absent in gram-positive bacteria.

Given that Staphylococcus aureus and Streptococcus salivarius have a thick peptidoglycan layer and lack the outer membrane, they are gram-positive bacteria.

Due to the presence of a thick peptidoglycan layer, these gram-positive bacteria retain the purple color of crystal violet even after washing with ethanol or acetone.