Bacteriology
Microbiology of skin and soft tissue infections in the age of community-acquired methicillin-resistant Staphylococcus aureus,☆☆

https://doi.org/10.1016/j.diagmicrobio.2013.02.020Get rights and content

Abstract

The objectives of this study were to determine the etiology of skin and soft-tissue infections (SSTIs) in a general population, and to describe patient characteristics, SSTI types, frequency of microbiologic testing, and the role of methicillin-resistant Staphylococcus aureus (MRSA) over time. Using electronic databases, we identified SSTI episodes and microbiologic testing among members of a large US health plan. Between 2006 and 2009, 648 699 SSTI episodes were identified, of which 23% had a specimen, of which 15% were blood. A pathogen was identified in 58% of SSTI cultures. S. aureus was the most common pathogen (80% of positive cultures). Half of S. aureus isolates were MRSA. Among cellulitis and abscess episodes with a positive blood culture, 21% were methicillin-sensitive S. aureus, 16% were MRSA, 21% were beta-hemolytic streptococci and 28% were Gram negative bacteria. Between 1998 and 2009, the percentage of SSTIs for which a culture was obtained increased from 11% to 24%. In SSTI episodes with a culture-confirmed pathogen, MRSA increased from 5% in 1998 to 9% in 2001 to 42% in 2005, decreasing to 37% in 2009. These data can inform the choice of antibiotics for treatment of SSTIs.

Introduction

Skin and soft tissue infections (SSTIs) occur with wide-ranging clinical manifestations, from mild to life-threatening (Dryden, 2008, Ki and Rotstein, 2008). In many SSTI episodes, specimens for culture are not obtained, and so the most common causes of SSTIs remain uncertain, although S. aureus and beta-hemolytic streptococci are often suggested as being the most important (Dryden, 2009, Eron et al., 2003, Jeng et al., 2010, Vinh and Embil, 2005). Among culture-confirmed SSTIs in the United States, the most common cause is S. aureus, although Pseudomonas aeruginosa, Enterococcus spp., Escherichia coli, and beta-hemolytic streptococci have also been identified as important causes (Jenkins et al., 2010, Ki and Rotstein, 2008, Lipsky et al., 2007, Moet et al., 2007, Rennie et al., 2003, Zilberberg et al., 2009).

Infections due to S. aureus present a significant health problem in the United States (Fridkin et al., 2005, Klein et al., 2007, Klevens et al., 2007). Treatment of these infections has become more difficult in the last decade due to the emergence and rapid spread of methicillin-resistant S. aureus (MRSA) (Klein et al., 2007, Klevens et al., 2007, Shorr, 2007). MRSA is now endemic in many US hospitals, long-term care facilities and communities (Klein et al., 2007). Between 1995 and 2005, there was a dramatic increase in community-associated MRSA, most commonly associated with SSTIs (Adam et al., 2009, Crum et al., 2006, Dietrich et al., 2004, Farley, 2008, King et al., 2006, Miller and Kaplan, 2009, Morin and Hadler, 2001). There are, however, some indications that the rate of MRSA has stabilized or declined in the last few years. One recent national study found that the incidence of healthcare-associated invasive MRSA declined during the period from 2005 to 2008 (Kallen et al., 2010), and an analysis of S. aureus isolates from outpatient pediatric patients with SSTIs found that the percent of isolates that were MRSA was lower in 2008–2009 compared to 2005–2007 (Diamantis et al., 2011).

There are few recent large studies of the pathogens associated with both hospitalized and non-hospitalized SSTIs, especially covering the years after the rapid rise of MRSA. In order to shed light on the etiology and potential burden of SSTIs, it is important to know the proportion of SSTIs for which a culture was obtained and the results of those cultures, both of which may depend on the infection site. The absence of these data has been noted in the literature (Miller & Kaplan, 2009).

This study reports on the microbiologic evaluation of SSTIs in the period 2006–2009. Using the member population of a large, integrated health plan, we report the number of SSTIs diagnosed over the 4-year period, the proportion of SSTI episodes with a culture, and the pathogens identified. In addition, due to special interest in S. aureus and MRSA, we report on trends in S. aureus-related SSTIs over the 12-year period from 1998 to 2009, spanning the years before, during, and after, the rapid increase in MRSA observed in this setting.

Section snippets

Setting

Kaiser Permanente of Northern California (KPNC) is a nonprofit, integrated health care delivery system providing care to over 3 million members. The member population reflects the general population in the Northern California region, although, as an insured population, it under-represents persons with very low levels of education and income (Gordon, 2012). KPNC provides services in more than 15 counties and operates more than 40 outpatient clinics and 18 hospitals throughout Northern

Results

During the four-year period from 2006 to 2009, we identified 648 699 SSTI episodes among 495 458 unique patients (Table 1). Patients were, on average, 41 years old at the start of their SSTI episode, and 6% of episodes included a hospitalization. A microbiology specimen was obtained for 23% of all episodes and 15% of the specimens were blood. Although Table 1 allows multiple SSTI episodes per patient, when we restricted the analyses to unique patients, the percentages by age, gender and race were

Discussion

In this setting, we found that between 2006 and 2009 - the era after the rapid increase in MRSA - 80% of culture-positive SSTIs were S. aureus, and half of those were MRSA. Other important pathogens identified were beta-hemolytic streptococci (9%), E. coli (4%), and P. aeruginosa (3%). Although S. aureus was the most common pathogen for all types of SSTIs, there were differences in the pathogen distribution by SSTI type. Carbuncle/furuncle was the most likely SSTI type to test positive for

Conclusions

Our study shows that S. aureus remains the most commonly identified pathogen in SSTIs. Bacteriological testing of SSTIs doubled in a 12-year period, as the percent of S. aureus SSTIs that were MRSA increased six-fold. By 2006, the percent of S. aureus that was MRSA peaked, and MRSA as a cause of SSTIs may have stabilized or even declined in recent years. In our analysis of blood cultures in patients with SSTIs, which controls for some of the bias in testing while concentrating on the most

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  • Cited by (0)

    This work was supported by GlaxoSmithKline. Potential conflicts of interest: GTR has received research support from GlaxoSmithKline, Pfizer and Merck. RB has received research funding from GlaxoSmithKline, Pfizer, Merck, Sanofi Pasteur, and Novartis. JAS is an employee of GlaxoSmithKline.

    ☆☆

    Some results included in this paper were presented at the October, 2011, Interscience Conference on Antibiotics and Antimicrobials (abstract K-875), Chicago, IL, USA.

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