US study finds shifting resistance patterns in Staph aureus

Methicillin-resistant Staphylococcus aureus

National Institute of Allergy and Infectious Diseases

An analysis of Staphylococcus aureus samples from US outpatient settings reveals a shift in antibiotic-resistance trends, researchers reported late last week in JAMA Network Open.

The study of S aureus isolates from more than 268,000 US Veterans Health Administration (VHA) patients found that the proportion of methicillin-resistant S aureus (MRSA) declined significantly from 2010 through 2019. But significant increases in resistance to non–beta-lactam antibiotics like tetracyclines and trimethoprim-sulfamethoxazole (TMP-SMX) were observed, particularly in southern US states.

The findings are noteworthy, because the emergence and spread of MSRA infections beginning in the 1990s has influenced empiric antibiotic prescribing habits across the country, with clinicians in outpatient settings often choosing oral non–beta-lactam antibiotics for empiric treatment because of concerns about MRSA infection.

The authors of the study say the shifting resistance patterns suggest outpatient clinicians may need to tailor antibiotic prescribing for S aureus infections.

Decline in proportion of MRSA 

To evaluate geographic and temporal resistance trends in S aureus, which is a leading cause of skin and other soft-tissue infections, researchers from the University of Iowa and the Iowa City Veterans Affairs Health Care System analyzed isolates collected from outpatient VHA settings in 48 states and Washington, DC, from 2010 through 2019, focusing on resistance to lincosamides (clindamycin), tetracyclines, sulfonamides (TMP-SMX), and macrolides. Spatiotemporal variation of S aureus resistance was stratified by MRSA or methicillin-susceptible (MSSA) status and subdivided by US region.

A total of 382,149 S aureus isolates from 268,214 unique patients (mean age, 63.4 years; 94.3% male) were analyzed. Of the S aureus isolates, 173,118 (45.3%) were classified as MRSA. The proportion of MRSA isolates declined from 53.6% in 2010 to 38.8% in 2019, with the decline observed across all US regions, though MRSA rates remained consistently higher in the South.

Among the MRSA isolates, tetracycline resistance rose from 3.6% in 2010 to 12.8% in 2019, and TMP-SMX resistance rose from 2.6% to 9.2%. There was also a modest, non-significant increase in clindamycin resistance (from 24.2% to 30.6%) and a significant decrease in macrolide resistance (from 73.5% to 60.2%). 

Among MSSA isolates, significant increases in resistance to clindamycin (from 13.1% to 18.7%), tetracyclines (3.7% to 9.1%), and TMP-SMX (0.9% to 2.7%) were observed, while macrolide resistance saw a non-significant increase (25.4% to 28.9%).

Regional stratification revealed that MRSA isolates from the South had higher rates of resistance to tetracyclines and TMP-SMX, while the Northeast had slightly higher rates of macrolide resistance. Co-occurrence of very high levels of resistance (above 50%) to both tetracyclines and TMP-SMX was rare.

Several factors could be at play

The authors say the higher rates of resistance in S aureus isolates from patients in the South could be linked to several factors that have been documented in previous studies.

"Prior work has suggested that the combination of sociodemographic factors (eg, crowding, poverty), climate factors (eg, heat, humidity), and antibiotic prescribing patterns have contributed to consistently higher rates of antimicrobial resistance in the South," they wrote. "Climate factors and changing behavior in patient population also likely play a role in the seasonality of S aureus infections observed in the patient cohort."

They add that clinicians in the South should be aware of how susceptibility patterns are changing in their patient population and adjust their prescribing accordingly.

"Geographically specific antibiograms, particularly for outpatient facilities that serve a large patient catchment area, could better inform empirical therapy decisions, although evidence suggests that such antibiograms need to incorporate seasonality and patient-level information," they concluded. "Additionally, regional variation in resistance trends suggests that greater geographic representation in surveillance populations, particularly in more rural areas of the US, is needed to gain a full understanding of antimicrobial resistance among S aureus in community settings."

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