Friday, April 25, 2014
Staphylococcus aureus: here, there and everywhere
To spend any time on our infectious diseases consult service is to be knee-deep in invasive, difficult-to-treat Staphylococcus aureus disease. Most S. aureus (including MRSA) disease is caused by a strain previously colonizing the host, and given that up to a third of the human population carries S. aureus it is easy to understand why the disease is so common. Exposure to health care is a major risk factor for invasive S. aureus disease, simply because so many health care interventions (surgery, device use, antibiotic exposure) provide opportunities for the organism to invade.
Nonetheless, the conventional wisdom still holds that a large proportion of health care-associated S. aureus disease results from patient-to-patient transmission events—the corollary being that prevention of S. aureus disease should focus primarily on preventing transmission (including active detection and isolation).
However, a careful assessment may demonstrate that interventions solely designed to interrupt transmission are responsible for only a small portion of disease reduction in observational studies (for an example, see my previous post on the VA MRSA directive). This is important, as it should focus our attention on preventing disease among those at risk for colonization (everyone), via such horizontal measures as device-associated infection prevention bundles, chlorhexidine bathing, and suppression/eradication of the carrier state during high-risk intervals.
A study by UK investigators published in Clinical Infectious Diseases provides further evidence that patient-to-patient S. aureus transmission is a relatively uncommon event, even as an explanation for S. aureus ”acquisition” events in the ICU. Using whole genome sequencing, the investigators found that only 7 of 37 ICU patients who “newly acquired” S. aureus were colonized with strains that were closely related to other patients who had an overlapping ICU stay.
There are several limitations to the work, most of which are outlined by the authors in their discussion and by the excellent accompanying editorial by David and Daum. The limitation most concerning to me is the assumption that a single nares + perineum culture plated directly to solid agar media (chromogenic agar and Columbia CNA) is a sensitive method for detection of S. aureus carriers. It isn’t. Failure to perform a throat culture or to use broth enrichment probably reduced sensitivity by 30-50% (explaining their overall carriage rate of only 16.7%, when most published studies demonstrate S. aureus colonization rates of closer to 30%).
We recently performed a study wherein we cultured 500 pregnant women at 5 body sites. The table below is taken from our presentation at the Decennial meeting in 2010 in Atlanta.
The bottom line? Using only nares and perineal cultures directly plated to solid agar media would have missed more than half of our S. aureus carriers. Achieving 90% sensitivity (using a positive culture at any of the 5 body sites as the gold standard) required sampling both the nares and the throat and using overnight broth enrichment.
So what about all those “acquirers” carrying isolates that didn’t match other ICU patients? In addition to implicating other potential reservoirs (personnel, visitors, etc.), I’d wager that some were prior carriers who were newly detected due to sampling issues, increase in CFU associated with healthcare exposure, etc. Intermittent detection of S. aureus carriage is well described, and would be magnified using the microbiological techniques in this study.
Finally, while I agree with the authors that whole genomic sequencing is the new gold standard for assessing genetic relatedness, the use of spa typing as the “conventional method” comparator is too easy. As we found in our recently published study, roughly half of all MRSA isolated from epidemiologically unrelated clinical infections in 43 U.S. hospitals were from a single spa type! (How’s that for discriminatory power?) If you’re going to write off the conventional methods, at least use a method with better discrimination, such as pulsed-field gel electrophoresis!
Daniel J. Diekema, MD, FACP, practices infectious diseases, clinical microbiology, and hospital epidemiology in Iowa City, Iowa, splitting time between seeing patients with infectious diseases, diagnosing infections in the microbiology laboratory, and trying to prevent infections in the hospital. This post originally appeared at the blog Controversies in Hospital Infection Prevention.
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Ira S. Nash, MD, FACP, is the senior vice president and executive director of the North Shore-LIJ Medical Group, and a professor of Cardiology and Population Health at Hofstra North Shore-LIJ School of Medicine. He is Board Certified in Internal Medicine and Cardiovascular Diseases and was in the private practice of cardiology before joining the full-time faculty of Massachusetts General Hospital.
Zackary Berger, MD, ACP Member, is a primary care doctor and general internist in the Division of General Internal Medicine at Johns Hopkins. His research interests include doctor-patient communication, bioethics, and systematic reviews.
Controversies in Hospital
Run by three ACP Fellows, this blog ponders vexing issues in infection prevention and control, inside and outside the hospital. Daniel J Diekema, MD, FACP, practices infectious diseases, clinical microbiology, and hospital epidemiology in Iowa City, Iowa, splitting time between seeing patients with infectious diseases, diagnosing infections in the microbiology laboratory, and trying to prevent infections in the hospital. Michael B. Edmond, MD, FACP, is a hospital epidemiologist in Richmond, Va., with a focus on understanding why infections occur in the hospital and ways to prevent these infections, and sees patients in the inpatient and outpatient settings. Eli N. Perencevich, MD, ACP Member, is an infectious disease physician and epidemiologist in Iowa City, Iowa, who studies methods to halt the spread of resistant bacteria in our hospitals (including novel ways to get everyone to wash their hands).
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Robert M. Centor, MD, FACP, contributes short essays contemplating medicine and the health care system.
Suneel Dhand, MD, ACP Member
Suneel Dhand, MD, ACP Member, is a practicing physician in Massachusetts. He has published numerous articles in clinical medicine, covering a wide range of specialty areas including; pulmonology, cardiology, endocrinology, hematology, and infectious disease. He has also authored chapters in the prestigious "5-Minute Clinical Consult" medical textbook. His other clinical interests include quality improvement, hospital safety, hospital utilization, and the use of technology in health care.
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Toni Brayer, MD, FACP, blogs about the rapid changes in science, medicine, health and healing in the 21st century.
Vineet Arora, MD, FACP, is Associate Program Director for the Internal Medicine Residency and Assistant Dean of Scholarship & Discovery at the Pritzker School of Medicine for the University of Chicago. Her education and research focus is on resident duty hours, patient handoffs, medical professionalism, and quality of hospital care. She is also an academic hospitalist.
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Ryan Madanick, MD, ACP Member, is a gastroenterologist at the University of North Carolina School of Medicine, and the Program Director for the GI & Hepatology Fellowship Program. He specializes in diseases of the esophagus, with a strong interest in the diagnosis and treatment of patients who have difficult-to-manage esophageal problems such as refractory GERD, heartburn, and chest pain.
Mike Aref, MD, PhD, FACP, is an academic hospitalist with an interest in basic and clinical science and education, with interests in noninvasive monitoring and diagnostic testing using novel bedside imaging modalities, diagnostic reasoning, medical informatics, new medical education modalities, pre-code/code management, palliative care, patient-physician communication, quality improvement, and quantitative biomedical imaging.
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Alexander M. Djuricich, MD, FACP, is the Associate Dean for Continuing Medical Education (CME), and a Program Director in Medicine-Pediatrics at the Indiana University School of Medicine in Indianapolis, where he blogs about medical education.
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Neil Mehta, MBBS, MS, FACP, is interested in use of technology in education, social media and networking, practice management and evidence-based medicine tools, personal information and knowledge management.
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Other blogs of note:
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Also known as the Green Journal, the American Journal of Medicine publishes original clinical articles of interest to physicians in internal medicine and its subspecialities, both in academia and community-based practice.
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One of the most popular anonymous blogs written by an emergency room physician.