Final answer:
Salmonella and Shigella are facultative intracellular pathogens because they can reproduce both inside and outside a host cell. They use specialized mechanisms, such as effector protein-induced membrane ruffling, to invade host cells, but are also capable of living independently.
Step-by-step explanation:
The bacterial pathogens Salmonella and Shigella can live either inside a host cell or independently of a host cell, which classifies them as facultative intracellular pathogens. They are not restricted to a host cell for reproduction, unlike obligate intracellular pathogens like Mycoplasma and Chlamydia, which cannot reproduce outside a host cell. By residing within host cells, facultative intracellular pathogens are able to evade some immune responses and utilize host resources for survival.
Salmonella and Shigella use effector proteins that cause membrane ruffling to facilitate entry into host cells. Once inside, they can also survive independently, contrasting with rickettsias and other obligate intracellular pathogens, which are metabolically inactive outside of host cells and cannot synthesize their own ATP, thus relying exclusively on the host for energy.