The emerging role of atypical pathogens in community-acquired pneumonia

Community-acquired pneumonia (CAP) constitutes a major cause of morbidity and mortality. Although Streptococcus pneumoniae remains the bacterium most commonly implicated in CAP, the atypical respiratory pathogens Mycoplasma pneumoniae, Legionella species, and Chlamydia pneumoniae are being isolated with increasing frequency. Contrary to previous beliefs, these agents are capable of causing severe as well as mild-to-moderate illness. Moreover, they can affect all age groups. Indeed, atypical pathogens are implicated in up to 40% of CAP cases and commonly occur as copathogens in mixed-infection CAP, an etiology associated with particularly high mortality (up to 25%). Laboratory methods for detecting atypical pathogens are slow, and there is significant overlap between atypical and typical CAP manifestations. For these reasons, accurate prediction of etiology cannot be made purely on clinical or radiologic grounds. Consequently, empiric antimicrobial therapy for atypical pathogens (with agents such as macrolides, fluoroquinolones, in some cases tetracyclines, or the new ketolides) warrants careful consideration and now is recommended for the treatment of CAP.