Laboratory-based surveillance of current antimicrobial resistance patterns and trends among Staphylococcus aureus: 2005 status in the United States

Abstract

Background: The virulence, antimicrobial resistance, and prevalence of S. aureus underscores the need for up-to-date and extensive insights regarding antimicrobial susceptibility trends. One approach to meet this need is analysis of clinical laboratory-based surveillance data.

Methods: Data from The Surveillance Network-USA (TSN), an electronic surveillance network that collects microbiology data from 300 clinical microbiology laboratories across the United States, were used as the source for analysis that included prevalence of S. aureus in clinical specimens, MRSA and multi-drug resistance phenotype rates and trends according to patient location, geographic distributions, and specimen source.

Results: S. aureus was the most prevalent species isolated from inpatient specimens (18.7% of all bacterial isolates) and the second most prevalent (14.7%) from outpatient specimens. In March 2005 MRSA rates were 59.2%, 55%, and 47.9% for strains from non-ICU inpatients, ICU, and outpatients, respectively. This trend was noted in all nine US Bureau of Census regions and multi-drug resistance phenotypes (resistance to > or = 3 non-beta-lactams) was common among both inpatient MRSA (59.9%) and outpatient MRSA (40.8%). Greater than 90% of multi-drug resistant MRSA were susceptible to trimethoprim-sulfamethoxazole, linezolid, and vancomycin.

Conclusion: Prevalence of MRSA among both inpatient and outpatient specimens continues to increase with multi-drug resistance as a common phenotype. Continued emergence of outpatient MRSA that exhibit multi-drug resistant phenotypes has important implications for developing and evolving outpatient treatment guidelines.

Figure 1

Relative frequency of bacterial species/groups encountered in clinical specimens from inpatients. Data is cumulative data: 1998 – March 2005 and based on a total of 3,209,413 bacterial isolates.

Figure 2

Relative frequency of bacterial species/groups encountered in clinical specimens from outpatients. Data is cumulative data: 1998 – March 2005 and based on a total of 3,209,413 bacterial isolates.

Figure 3

MRSA trends (1998 – YTD 2005) according to patient location. Data is cumulative data: 1998 – March 2005. Red line, all patients; yellow line, ICU patients; green line, inpatients; blue line, outpatients.

Figure 4

Inpatient (IP) and outpatient (OP) MRSA rates according to US Census Bureau Regions. Data is cumulative data: 1998 – March 2005.