Etiologic Agents

ARDS patients are at higher risk for developing VAP caused by S. aureus; nonglucose-fermenting Gram-negative bacilli, i.e., P. aeruginosa, Acinetobacter bau- mannii, and Enterobacteriaceae. Of note, in ARDS patients, antibiotics are profusely administered very early. As a result, antimicrobial causes a selective pressure over more susceptible bacteria, allowing potentially resistant bacteria to survive. In particular, Chastre et al. [6] compared VAP in patients with and without ARDS and found that methicillin-resistant S. aureus was isolated in 44% and 17% of the patients with or without ARDS, whereas Markowicz et al. [8] found that non-fermenting Gram-negative bacilli were more frequent in ARDS patients (47% vs. 33% in non-ARDS patients). Also, methicillin-susceptible S. aureus was less common in ARDS patients (3% vs. 15% in non-ARDS patients). In ARDS patients, VAP is often caused by multiple pathogens [44, 45]. Combes and colleagues [46] studied 124 ICU patients, of whom 65 (52%) had monomicrobial VAP and 59 (48%) had polymicrobial VAP. In most patients (34%), two different bacteria were isolated; however, up to four different bacteria coexisted in seven patients (6%). Interestingly, no differences were detected in mortality rates at 30 days between patients with polymicrobial or monomicrobial infection. Also, multiple episodes of VAP may occur in approximately 50% of the patients, which may be related to the prolonged stay in the ICU or substantial presence of ETT biofilm.