Dissemination of arr-2 and arr-3 is associated with class 1 integrons in Klebsiella pneumoniae clinical isolates from Portugal.

\nKlebsiella pneumoniae is a common pathogen of healthcare-associated infections expressing a plethora of antimicrobial resistance loci, including ADP-ribosyltransferase coding genes (arr), able to mediate rifampicin resistance. The latter has activity against a broad range of microorganisms by inhibiting DNA-dependent RNA polymerases. This study aims to characterise the arr distribution and genetic context in 138 clinical isolates of K. pneumoniae and correlate these with rifampicin resistance. All isolates were subjected to whole-genome sequencing for species identification, typing and AMR genes identification, along with the determination of the minimum inhibitory concentration (MIC) of rifampicin. Molecular detection of arr genes and class 1 integrons was performed for rifampicin-resistant isolates. Efflux activity was investigated as a possible determinant of rifampicin resistance in isolates devoid of known genetic determinants. Twelve isolates exhibited high rifampicin MICs (≥ 64 mg/L), 124 showed intermediate MICs (16-32 mg/L) and two displayed low (8 mg/L) MICs. Two arr allelic variants, arr-2 and arr-3, were found across one and nine K. pneumoniae isolates, respectively, all within class 1 integrons, including a newly described integron, and all associated with high rifampicin MICs  (≥ 64 mg/L). Elevated resistance levels were additionally linked to increased arr-2/3 expression and closer proximity to the promoter. No arr gene or rpoB mutations were found across the remaining two isolates and no correlation between efflux activity and high-level rifampicin resistance was found for both isolates. In conclusion, this study demonstrates that arr genes confer high levels of rifampicin resistance in K. pneumoniae highlighting its widespread dissemination within class 1 integrons.