Microbial etiology and bacterial resistance.

The nature of the causative organisms involved in community-acquired pneumonia has always attracted the interest of investigators. Despite multiple studies using different methodological approaches, it remains a matter of controversy because a reliable cause cannot be obtained in a significant percentage of cases, even when using more sophisticated diagnostic procedures. The recent discovery of new pathogens such as the Legionella species or Chlamydia pneumoniae, highly prevalent worldwide shows the limitations of our knowledge in that field. Recently, common respiratory pathogens such as the pneumococcus or Haemophilus influenzae, among others, showed a progressive tendency to develop resistance to penicillins and other antibiotics. Although this phenomenon has a variable impact among different countries, its growing importance is changing the classical therapeutic approach to community-acquired pneumonia.