It is abundantly clear that surveillance programs are necessary to identify changes in the spectrum of microbial pathogens causing serious infection and to monitor trends in antimicrobial resistance patterns in nosocomial and community-acquired infections (1
). The information gleaned from surveillance efforts is integral to the design of empirical approaches to the therapy of serious infection and also to defining appropriate control measures for antimicrobial-resistant pathogens (1
). Such information has been provided in recent years by programs such as the National Nosocomial Infection Surveillance (NNIS) system (4
), the Surveillance and Control of Pathogens of Epidemiologic Importance (SCOPE) program (10
), and the Intensive Care Antimicrobial Resistance Epidemiology (ICARE) project (1
). These programs have been limited by their focus on nosocomial infections (NNIS and SCOPE) and by the lack of validated identification and antimicrobial susceptibility testing performed in a central laboratory (NNIS and ICARE). The SENTRY program was initiated in January 1997 and was designed to monitor the spectrum of microbial pathogens and antimicrobial resistance trends for both nosocomial and community-acquired infections on a broad geographic scale by using validated reference quality identification and susceptibility testing methods performed in a central laboratory. Because rapid communication and dissemination of information is an important component of any surveillance program, we are summarizing the first 6 months of data for BSI in the present report.
The early findings of the SENTRY Program for BSI (objective A) underscore the prominence of gram-positive cocci and E. coli
as etiologic agents of both community-acquired and nosocomial BSI (Table ). These findings are consistent with the recent observations of Weinstein et al. (22
) and also confirm those of the NNIS (3
) and SCOPE (10
) programs regarding the importance of staphylococci and enterococci.
The most striking finding of the SENTRY program was the degree of antimicrobial resistance among key pathogens in the United States (Table ). Resistance to oxacillin among staphylococci, to vancomycin among enterococci, and to ceftazidime among Enterobacter
spp. was observed for nosocomial and community-acquired pathogens alike. It is notable that in almost every instance the percentage of resistant strains was higher among nosocomial isolates than among those from the community (Table ). Unfortunately, this analysis is compromised by the fact that for 44% of the BSI the status of nosocomial or community acquired could not be confirmed. Thus, although the apparent increased resistance among nosocomial BSI isolates compared to those from the community is consistent with that reported by Archibald et al. (1
) for the ICARE program, these findings must be tempered by the fact that a large number of isolates were excluded from the analysis.
Of note, in the current survey, among 209 bloodstream isolates of S. pneumoniae in the United States, 59.0% were susceptible to penicillin; in Canada, 69.5% of 59 isolates were susceptible. In objective B of the SENTRY study (data not shown), the percentage of penicillin-susceptible strains among 845 outpatient respiratory tract isolates of S. pneumoniae collected during roughly the same period in the United States was 56.2%; among 202 such isolates in Canada, 69.2% were susceptible. In other words currently there exist only minor differences in rates of penicillin resistance among bloodstream versus respiratory tract isolates of S. pneumoniae in North America. Comparisons of resistance rates with other antimicrobial agents revealed the same degree of concordance between bloodstream and respiratory tract isolates of the pneumococci.
Although the rank orders of pathogens in the United States and Canada were very similar (Table ), distinct differences were observed in the antimicrobial susceptibilities of several pathogens. Overall, U.S. isolates were considerably more resistant than those from Canada. This was most apparent with S. aureus, Enterococcus spp., and Enterobacter spp. For oxacillin-resistant S. aureus, vancomycin-resistant enterococci, and ceftazidime-resistant Enterobacter spp., the percentages of resistant U.S. isolates were 26, 17.7, and 30.6%, respectively; the corresponding values for resistant Canadian isolates were 2.7, 0, and 6.2%, respectively. This comparison dramatically emphasizes the differences and points to the relative lack of specific antimicrobial resistance genes (mecA, vanA, and vanB) in the Canadian microbial population. The in vitro susceptibilities of U.S. and Canadian E. coli, Klebsiella spp., and P. aeruginosa were generally comparable, although the Canadian P. aeruginosa isolates were somewhat less susceptible than U.S. isolates to carbapenems and fluoroquinolones. The reasons for these differences in antimicrobial susceptibility are unclear but may be due to differences in antimicrobial utilization practices.
Although high rates of antimicrobial resistance were observed in this survey, there were several encouraging observations regarding specific antimicrobial agents. First, we did not observe any vancomycin-resistant or -intermediate strains among 1,829 U.S. and Canadian isolates of staphylococci. Second, the investigational agent quinupristin-dalfopristin appeared to be quite active against oxacillin-resistant staphylococci, vancomycin-resistant E. faecium, and penicillin-resistant S. pneumoniae. Third, the extended-spectrum cephalosporin cefepime and the carbapenems imipenem and meropenem were highly active against the major gram-negative BSI pathogens, including those strains of Enterobacteriaceae that were resistant to ceftazidime and other extended-spectrum β-lactam and monobactam agents.
In conclusion, ongoing surveillance of microbial pathogens and their resistance profiles is essential on national and international scales. The SENTRY program will continue to monitor these trends in both nosocomial and community-acquired BSI. Comparisons of the results both internally within the SENTRY program and with other ongoing programs such as NNIS and ICARE will enhance our knowledge base regarding the problem of antimicrobial resistance and will serve as a basis for policies and practices that might serve to limit the scope and magnitude of this problem.