MRSA resistance to TMP-SMX increases
A recent study reported an increase in the percentage of MRSA in the United States that was resistant to trimethoprim-sulfamethoxazole from 2012 through 2017.
“We were initially contacted by one of our partners at a state health department regarding reports of increased trimethoprim-sulfamethoxazole (TMP-SMX) resistance among [patients with] MRSA from several hospitals within their jurisdiction,” D. Cal Ham, MD, MPH, deputy lead of the Antimicrobial Resistance Team in the CDC’s Prevention and Response Branch, told Healio. “Based on this we initiated the analysis to determine if similar increases were being seen in other jurisdictions and to describe national resistance patterns over the last several years.”
Ham and colleagues used the Cerner Health Facts EMR and the Premier Healthcare Database to identify Staphylococcus. aureus infections among inpatients discharged from acute-care hospitals between Jan. 1, 2012, and Dec. 31, 2017.
After identifying cases of MRSA or methicillin-susceptible S. aureus (MSSA), they calculated the pooled mean percentages of MRSA and MSSA isolates that were not susceptible to TMP-SMX by year using weighted estimates.
Between 2012 and 2017, they identified 232,460 MRSA-incident cultures and 227,900 MSSA-incident cultures were identified. Based on weighted estimates, 98.3% of all MRSA cultures underwent antimicrobial resistance testing (AST) for TMP-SMX, giving an estimate of 2,155,794 cases, among which 5.3% were not susceptible to TMP-SMX. Additionally, within the same period, a projected 98.5 of MSSA cultures received AST for TMP-SMX, and 1.4% of cases were not susceptible to TMP-SMX.
According to the study, the estimated proportion of cases that were not susceptible to TMP-SMX increased 124.5% during the study period among MRSA cases (adjusted 5-year change; 95% CI, 87.7%-168.6%) and 51.3% among MSSA cases (adjusted 5-year change; 95% CI, 19.2%-92.1%).
Additionally, among MRSA cases, the trend in percentage of cases not susceptible to TMP-SMX increased among sterile site, nonsterile site, hospital-onset and community-onset categories.
“Increasing TMP-SMX resistance among MRSA has the potential to limit its effectiveness as a current first-line therapy and bears careful monitoring,” Ham said. “Health care providers should ensure that TMP-SMX remains active against MRSA in their area, select appropriate antibiotic regimens based on local resistance patterns, and monitor patients for treatment failure.”