**1. Introduction**

Pneumonia is a serious public health problem associated with high morbidity and mortality rates that leads to a significant increase in healthcare costs. It results from an infectious process of the lower airways through aspiration or inhalation of pathogenic microorganisms. It can be acquired in the community or in the hospital environment, after 48 h of admission [1].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Hospital-acquired infections usually have a high mortality rate (approx. 20%) when compared to the community acquisition (10%), this rate increases even more when it is associated with mechanical ventilation [2].

According to the guidelines of the American Thoracic Society, hospital pneumonia is divided into ventilator-associated pneumonia (VAP), which develops after 48–72 h of endotracheal intubation and the one that occurs in nonhospitalized patients, but that have constant contact with health services [3]. VAP is the infection of the pulmonary parenchyma with onset after 48–72 h of endotracheal intubation. Early-onset VAP occurs during the first 4 days of mechanical ventilation, whereas late-onset VAP occurs on 5 or more days of mechanical ventilation [4–7]. VAP corresponds to 70–80% of cases of hospital-acquired pneumonia in intensive care units [1].

VAP is characterized by the presence of new or progressive pulmonary infiltrates, systemic alterations such as fever and leukocyte alterations, altered sputum, and diagnosis of an infectious agent [8]. Mortality due to VAP is high, principally because of the association with multidrug-resistance (MDR) bacteria [9]. In pediatrics and neonatology, the frequency of VAP is 3–19%, with a mortality rate ranging from 10 to 20% of patients [10].

Many microorganisms can be involved in VAP. In this chapter, data on microbiology of VAP are reviewed, including risk factors, etiology, virulence features of main pathogens contributing to VAP severity, antimicrobial susceptibility, and laboratory diagnosis.
