A $20-$year$-$old student with a nonproductive cough, sore throat, and fever is seen at the university health clinic. He is treated with penicillin but is not improving and seeks treatment at an outpatient clinic the following week where a chest x$-$ray and sputum sample are collected. The chest x$-$ray shows a diffuse intersitial infiltrate. The Gram stain shows a moderate amount of PMNs but no organisms. He is diagnosed with atypical pneumonia and given doxycycline. He recovers within a few days.

What microorganism do you suspect?

Why was penicillin ineffective in treating the infection?