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Modern laboratory methods such as next generation sequencing and MALDI-TOF allow identification of novel bacterial species. This can affect surveillance of infections and antimicrobial resistance. From 2017, increasing numbers of medical microbiology laboratories in Switzerland differentiated from complex using updated MALDI-TOF databases, whereas many laboratories still report them as or complex.


Our study explored whether separate reporting of and the complex affected the ANRESIS surveillance database.


We analysed antibiotic susceptibility rates and specimen types of and non-- complex isolates reported by Swiss medical laboratories to the ANRESIS database (Swiss Centre for Antibiotic Resistance) from January 2017 to June 2022.


Analysis of Swiss antimicrobial resistance data revealed increased susceptibility rates of compared with species of the complex other than in all six antibiotic classes tested. This can lead to underestimated resistance rates of complex in laboratories that do not specifically identify . Furthermore, strains were significantly more often reported from blood and primarily sterile specimens than isolates of the complex other than indicating increased invasiveness of .


Our data suggest that refined differentiation of the complex can improve our understanding of its taxonomy, susceptibility, epidemiology and clinical significance, thus providing more precise information to clinicians and epidemiologists.


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