The pathogens causing HAP, VAP, and HCAP vary depending on patient population, type of institution, diag­nostic methods, and length of the hospital stay. In early pneumonia (occurring up to 4 days after admission), pathogens associated with community-acquired pneumonia (see above), such as S. pneumoniae and H. influenzae, may be considered. Other potential pathogens include 5. aureus, Escherichia coli, Klebsiella spp., Enterobacter spp., Proteus spp., and Serratia spp. In patients with late-onset HAP (occurring 5 or more days after admission) or with risk factors for antibiotic- resistant organisms, pathogens in addition to those described for early HAP include P. aeruginosa, Klebsiella pneumoniae, Acinetobacter spp., extended-spectrum ^-lactamase (ESBL)-producing Enterobacteriaceae, methicillin-resistant 5. aureus (MRSA), and Legionella spp. In the medical ICU setting, common pathogens identified from NNIS surveillance data were P. aeruginosa (21%), 5. aureus (20%), Enterobacter spp. (9%), K. pneumoniae (8%), Acinetobacter spp. (6%), Candida albicans (5%), and other Enterobacteriaceae. Overall, gram-negative aerobic pathogens accounted for 64% of isolates collected from ICU patients with HAP. Because pathogens and antibiotic resistance rates may vary considerably from one institution to another, hospitals should be familiar with the local epidemiology of HAP-causing organisms, especially when Canadian and US epidemiologic data are compared. For example, S. aureus was reported as the most common cause of nosocomial pneumonia at 2 Canadian tertiary care centres.

Antibiotic Resistance

The rising emergence in hospitals of antibiotic- resistant pathogens such as MRSA, vancomycin-resistant enterococci, and MDR gram-negative organisms is of significant concern. Similar to the case for pathogens associated with CAP, the mechanism of antibiotic resistance may include a change in target site, production of antibiotic-modifying or antibiotic-inactivating enzymes, decreased penetration of the antibiotic, or efflux (antibiotic pump). Data from the NNIS indicate that rates of antibiotic-resistant nosocomial pathogens in the ICU setting continued to rise from 1997-2001 to 2002 (Table 7). The increasing prevalence of ESBL-producing Enterobacteriaceae, such as K. pneumoniae, in the ICU is also of concern. The rising rates of MRSA, vancomycin-resistant enterococci, and Enterobacteri­aceae resistant to fluoroquinolones or third-generation cephalosporins in the ICU are disturbing. As a result, older agents such as colistin and polymyxin have been used to treat MDR pathogens. As with PRSP, the rising prevalence of antibiotic-resistant HAP pathogens is associated with the volume of antibiotic use. In the United States, the rate of fluoroquinolone-resistant P. aeruginosa, for example, was associated with the volume of fluoroquinolone used in the hospital and surrounding community.
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Table 7. Rates of Antibiotic Resistance for Pathogens Typically Found in the Intensive Care Unit (National Nosocomial Infections Surveillance System)



Resistant in



Change (Compared with


Vancomycin-resistant enterococci




Staphylococcus aureus



Methicillin-resistant CNS




Escherichia coli




Klebsiella pneumoniae




Pseudomonas aeruginosa



P. aeruginosa






TGC-resistant Enterobacter



coagulase negative
staphylococci, TGC
third-generation cephalosporin.