Final answer:
The main culprits for pneumonia in adults vary by setting; Streptococcus pneumoniae is the lead cause of community-acquired pneumonia, while other opportunistic bacteria like Klebsiella pneumoniae and Pseudomonas aeruginosa are more common in healthcare-associated cases, including ventilator-associated pneumonia and potentially aspiration pneumonia.
Step-by-step explanation:
When identifying the main causes of pneumonia in adults, it's necessary to consider both community and healthcare-associated contexts. For community-acquired pneumonia, the most common bacterial cause is Streptococcus pneumoniae, often leading to a productive cough with bloody sputum due its virulence factors. In contrast, health care-associated pneumonia can be caused by a variety of opportunistic pathogens including Klebsiella pneumoniae, Staphylococcus aureus, and various proteobacteria which are more likely in immunocompromised individuals or those with preexisting conditions. Specifically, Klebsiella pneumoniae can cause severe pneumonia characterized by lung necrosis and 'currant jelly sputum'. While Pseudomonas aeruginosa is associated with ventilator-associated pneumonia due to contaminated equipment, leading to serious infections especially in patients with cystic fibrosis. Aspiration pneumonia results from inhaling bacteria into the lungs, which could include multiple pathogens.