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Whole-genome sequencing indicates importation of USA300 CA-MRSA in Geneva
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A recently published whole-genome sequencing analysis suggested that the recent prevalence of ST8-USA300 community-associated MRSA in Geneva, Switzerland, is the result of multiple importation events from the Americas, as opposed to clonal spreading.
“In the [U.S.], USA300 (ST8-SCCmec IVa) isolates are the predominant cause of [community-associated (CA-MRSA)] infections, accounting for approximately 90% of infections in some areas,” Elodie Von Dach, PhD, of the infection control department at Geneva University Hospitals, Switzerland, and colleagues wrote. “In 2003, the first USA300 strain was identified at Geneva University Hospital in a female patient who had recently traveled to New York City. Since then, our molecular surveillance system revealed a few sporadic USA300 cases until 2013, when we detected a sudden increase of CA-MRSA isolates (ST8PVL-positive).”
To explore factors influencing the strain’s recent prevalence, Von Dach and colleagues performed a molecular epidemiologic investigation of MRSA cases identified at their institution using whole-genome sequencing analysis. Isolates from CA-MRSA patients colonized and infected with USA300 were compared with isolates without the strain. Whole-genome sequencing results were examined alongside case and patient contact data to explain discriminating molecular features, as well as the epidemiologic spread of the disease.
Among the 594 MRSA isolates characterized in 2013, 46 were CA-MRSA. Twenty-two of these isolates were USA300, of which 14 were clinically infected and eight were cases of asymptomatic carriage. The median age of USA300 patients was 32 years. Previous exposure to the health care system was uncommon, while documented international travel was frequently observed.
Familial transmission events were rare, and many isolates appeared distant, the researchers wrote, suggesting that many strains originating in the same country may not be directly related. In addition, different mobile elements co-localized in the chromosome likely represent different acquisition events. Together, these findings led Von Dach and colleagues to conclude that importation from the Americas was responsible for USA300’s prevalence in the Geneva region.
“Absence of close relatedness in our collection clearly suggests that our area is exposed to numerous importation events rather than clonal spreading of an adapted clone,” they wrote. “Active surveillance of these epidemiological parameters appears important to limit the spread of these isolates known to be particularly successful in the Americas.”
In a related editorial, Nathalie L. van der Mee-Marquet, PhD, of the microbiology department at Centre Hospitalier Régional Universitaire, Tours, France, noted the importance of whole-genome sequencing use in epidemiologic studies such as these, as the sole use of conventional typing tools could produce misleading evidence.
“These results confirm the particular complexity encountered in the epidemiological evolution of the USA300 clone, and demonstrate the limitations of conventional typing tools and their inadequacy for micro-epidemiological studies of CA-MRSA USA300,” she wrote. “Further investigations of this clone will clearly require the use of genomic tools.” – by Dave Muoio
Disclosure: The researchers report no relevant financial disclosures
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