Community-associated MRSA epidemic linked to fluoroquinolone usage

Since the late 1990s, USA300, a strain of methicillin-resistant Staphylococcus aureus (MRSA), has spread across the United States, contributing to an epidemic of community-associated [CA] MRSA. The strains can persist in residential homes after infections, and appear to be linked to high usage of fluoroquinolones, according to a study published in the Proceedings for the National Academy of Sciences.

Anne-Catrin Uhlemann, MD, PhD, of Columbia University Medical Center in New York City, and colleagues sequenced the genomes of 387 USA300 isolates from an epidemiological network of community-acquired MRSA infections of 161 people between 2009 and 2011 in northern Manhattan, where the strain is believed to be a leading cause of the epidemic.

Phylogenetic analysis showed that the strain may have diverged from its most recent ancestor around 1993 and was likely introduced multiple times into northern Manhattan rather than rapidly diversifying from a single local progenitor. More importantly, Dr. Uhlemann and colleagues noted, most USA300 isolates became endemic in households, which likely act as reservoirs for colonization, diversification, and transmission.

“We used comparative whole genome sequencing of a large collection [~400] of epidemiologically well-defined group of isolates from patients with community-associated infections and their household members. We found that the clone was introduced multiple times,” said Dr. Uhlemann. “The most closely related isolates were detected within households, but we were also able to identify several links between households based on the sequence information. However, even larger studies are needed to identify how USA300 is introduced into households.”

High fluoroquinolone usage

“Lastly, we also identified the emergence and subsequent expansion of a fluoroquinolone-resistant subpopulation of USA300 at the time of high fluoroquinolone usage. This highlights the potential impact of antibiotic usage on selecting for resistant bacteria,” she said.

The study was community-based and recently hospitalized patients were excluded.

“However, USA300 is now also present in hospitals,” said Dr. Uhlemann. “We don't know yet how exactly hospital- and community-USA300 isolates are related to each other. I would say the observation regarding fluoroquinolone-resistance might be of most importance to hospital decision-makers. It highlights the importance of antibiotic stewardship-helping choose the right drug-and antibiotic usage only when indicated.”

The spread of resistant strains during a decade beginning in early 2000 when fluoroquinolone usage among outpatients in the United States temporarily rose by nearly 50%. The observation that a fluroquinolone-resistant sub-clone has developed highlights the potential side effects of antibiotic use, according to Dr. Uhlemann. “Antibiotics should be used only when indicated,” she said. 

“We conducted the study to understand the recent evolution and spread of this clone as it is the major driver of the CA-MRSA epidemic in the United States,” she said. “We further wanted to analyze how isolates from within households and in the wider community were related to each other.”

The authors recommended an approach targeting not only infected patients but their household members and home environments might be necessary to curb the spread of MRSA outbreaks.