Which of the following bacteria is known to be an example of opportunistic infection?

Staphylococcus aureus is recognized for its potential to cause opportunistic infections, particularly in individuals who have compromised immune systems or underlying health conditions. This bacterium can enter the body through cuts or openings in the skin and may lead to infections that are particularly serious in hospital settings. Conditions such as diabetes, HIV/AIDS, or even certain invasive medical procedures can increase the likelihood of Staphylococcus aureus infections, as the body's defenses are not strong enough to fend off pathogens that would normally be harmless.

Klebsiella, Listeria, and Enterobacter can also cause opportunistic infections, but they do not have the same widespread recognition as Staphylococcus aureus in this context. For instance, Klebsiella pneumoniae is often associated with lung infections and can infect patients with weakened immune systems but is less commonly known compared to Staphylococcus aureus. Listeria monocytogenes primarily affects pregnant women, newborns, and individuals with weakened immune systems but is not as broadly opportunistic as Staphylococcus aureus in general populations. Enterobacter species can lead to infections, particularly in healthcare settings, but Staphylococcus aureus is more widely noted for being a significant opportunistic pathogen in many risk scenarios.