Final answer:
The leading cause of pneumonia in the United States that has shown increased penicillin resistance is Streptococcus pneumoniae, an organism that can also lead to other serious infections.
Step-by-step explanation:
The microbe of growing concern in the United States due to increased resistance to penicillin and which is a leading cause of pneumonia is Streptococcus pneumoniae.
Streptococcus pneumoniae, also known as pneumococcus, is a gram-positive bacterium that is often part of the normal microbiota of the human respiratory tract. When it does cause illness, it may lead to conditions like pneumonia, meningitis, and septicemia.
The pneumococci are significant because they can develop resistance to antibiotics, which complicates treatment efforts. The increased resistance to penicillin by Streptococcus pneumoniae is concerning because it leads to increased difficulty in managing infections caused by this organism, which can be serious, especially in immunocompromised individuals such as the elderly or those with preexisting conditions.