Final answer:
Salmonella and Shigella are known as facultative intracellular pathogens because they can live both inside and outside of a host cell. Unlike obligate intracellular pathogens, they are capable of surviving in the environment and do not rely exclusively on host cells for reproduction.
Step-by-step explanation:
The bacterial pathogens Salmonella and Shigella can live either inside a host cell or independently of a host cell. These bacteria are known as facultative intracellular pathogens. The term 'facultative' refers to their ability to adapt to different environments - they can reproduce both within a host cell, taking advantage of the nutrients there, and outside in the extracellular environment. In contrast, obligate intracellular pathogens can only survive and multiply inside host cells and are usually unable to live independently in the environment.
Pathogens like Rickettsia are examples of obligate intracellular pathogens because they cannot synthesize their own adenosine triphosphate (ATP) without a host cell. On the other hand, facultative intracellular pathogens such as Salmonella and Shigella use effector proteins and other mechanisms to invade host cells but can also thrive outside them. The ability to survive inside the host cells protects these pathogens from some immune system responses and provides a nutrient-rich environment.