Macrolide resistance in in adult patients.

is one of the most significant pathogens responsible for respiratory infections in humans. Macrolides are recommended as the first-line treatment for infection. The prevalence of macrolide-resistant has increased significantly in recent decades, particularly in China. The mechanisms of resistance in to macrolides have been extensively studied in pediatric patients. However, a paucity reports regarding the resistance characteristics and mechanisms exhibited in adults. The aim of this study was to elucidate the resistance of to macrolides and the underlying mechanisms in adult patients. Pharyngeal swab specimens were collected from adult patients presenting with subacute cough or community-acquired pneumonia at our hospital from January 2011 to June 2017 to identify and isolate strains. The antimicrobial susceptibility of these isolates to 3 macrolide antibiotics was assessed using broth microdilution method. The genes of macrolide-resistant strains were sequenced, and the presence of target methylation genes (, , and ), efflux pump genes (, , , and ), and the macrolide resistance gene was identified through polymerase chain reaction (PCR) testing. Additionally, MICs were determined with and without the efflux pump inhibitor reserpine. A total of 72 strains were isolated from adult patients, with 41.7% (30/72) exhibiting macrolide resistance. Among the 3 macrolides tested, the 16-membered-ring midecamycin exhibited the greatest activity (MIC: 16 µg/ml) against . All macrolide-resistant strains harbored mutations at the 2063 site in domain V of the gene. Two macrolide-resistant clinical isolates were found to harbor the efflux pump genes and . The efflux pump inhibitor reserpine reduced the MIC for azithromycin in these two strains to a quarter of their original values. In summary, macrolide-resistant is commonly observed among adults in Beijing. Point mutations are the primary mechanism responsible for macrolide resistance in adults with . Additionally, the efflux pump mechanism may contribute partially to this resistance. Midecamycin presents a promising alternative drug for treating infections, particularly in cases of azithromycin-resistant infection in young children.