Intracellular bacteria are common causes of community-acquired
pneumonia that grow poorly or not at all on standard
culture media and do not respond to
beta-lactam antibiotic therapy. Apart from well-established agents of
pneumonia such as Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae, Chlamydia psittaci and Coxiella burnetii, some new emerging pathogens have recently been recognized, mainly Parachlamydia acanthamoebae and Simkania negevensis, two Chlamydia-related bacteria. Most of them are causes of benign and self-limited
infections. However, they may cause severe
pneumonia in some cases (i.e.,
Legionnaires' disease) and they may cause outbreaks representing a public health problem deserving prompt recognition and appropriate
therapy. Although extrapulmonary manifestations are often present, no clinical features allow them to be distinguished from classical bacterial agents of
pneumonia such as Streptococcus pneumoniae. Thus, specific molecular diagnostic tools are very helpful for early recognition of the offending bacteria, whereas serology often only allows retrospective or late diagnosis.
Macrolides remain the best empirical treatment of intracellular respiratory pathogens, although some observational studies suggest that
quinolones may be superior for the treatment of
legionellosis.