Colonization and Infection with Pseudomonas aeruginosa in Intensive Care: Endogenous or Exogenous Origin?

Abstract

Ventilator-associated pneumonia (VAP) is the most common nosocomial infection among mechanically ventilated patients [1], and has been associated with increased morbidity, longer hospital stay, increased health care costs and higher mortality rates [2]. Pneumonia is defined as VAP when diagnosed in an intubated, mechanically ventilated patient after more than 48 hours of ventilation. On the basis of time of diagnosis, two types of VAP are distinguished: early-onset VAP, occurring within the first four days of mechanical ventilation, and late-onset VAP, occurring thereafter. Early-onset VAP is mainly caused by Streptococcus pneumoniae, Staphylococcus aureus and Haemophilus influenzae, pathogens that presumably already colonize the respiratory tract at the time of intubation. Late-onset VAP is caused by nosocomial pathogens such as Enterobacteriaceae, S. aureus and Pseudomonas aeruginosa. Because these commensal bacteria may cause serious infections under certain circumstances, they are usually grouped and labeled as potentially pathogenic microorganisms (PPMO). In many studies, colonization and infection with PPMO is analyzed instead of analysis of the separate species. However, it should be kept in mind that each species has its own characteristics with regard to preferred site of colonization, routes and vectors of transmission, and clinical spectrum. Incidence rates of VAP among intensive care unit (ICU) patients depend on the type of ICU, the severity of illness of patients studied and the criteria for diagnosis. In a number of studies aiming to ascertain incidences of VAP, or to evaluate modalities to diagnose VAP, incidences range from 8.6 to 78%. Studies using quantitative cultures of bronchoalveolar lavage (BAL) and/or protected specimen brush demonstrate that approximately 60% of all cases of VAP are associated with Gram-negative bacteria, mainly P. aeruginosa (20%), and 35% with Gram-positive bacteria [3–6], Although the proportional distribution of species causing VAP, as well as their antibiotic susceptibility, may vary considerably between hospital settings, patient populations and countries, P. aeruginosa are most prevalent in most studies. In two risk factor analyses, P. aeruginosa VAP was associated with chronic obstructive pulmonary disease (COPD), prolonged mechanical ventilation, and prior use of antibiotics [7, 8]. P. aeruginosa is considered as a high-risk pathogen, associated with increased attributable mortality [2]. Prevention of this infection is, therefore, a major challenge for intensive care medicine.