In vitro evaluation of gentamicin activity against Spanish field isolates of Brachyspira hyodysenteriae

Background: The treatment of swine dysentery (SD) has become constrained in recent years due to the limited availability of effective drugs combined with a rise in antimicrobial resistance. Gentamicin, an aminoglycoside antibiotic, is authorised for the control of this disease in several European countries but has not been extensively used so far. In this study, the in vitro susceptibility of fifty-six Brachyspira hyodysenteriae field isolates was evaluated against gentamicin using a broth microdilution test. The molecular basis of decreased susceptibility to gentamicin was also investigated by sequencing the 16S rRNA gene. Results: : Most B. hyodysenteriae isolates presented low minimum inhibitory concentration (MIC) values, with a mode of 2 µg/mL, a median or MIC 50 of 4 µg/mL and percentile 90 or MIC 90 of 16 µg/mL. The distribution of these values over the period studied (2011-2019) did not show a tendency towards the development of resistance to gentamicin. Differences in susceptibility among isolates could be explained by two point mutations in the 16S rRNA gene, C990T and A1185G, which were only present in isolates with high MICs. Analyses of co-resistance between gentamicin and antimicrobials commonly used for the treatment of SD revealed that resistance to tiamulin and valnemulin was associated with low MICs for gentamicin. Conclusions: : The results provide an accurate characterisation of antimicrobial sensitivity to gentamicin and possible mechanisms of resistance in Spanish B. hyodysenteriae isolates . These findings allow us to propose gentamicin as an alternative in the antibiotic management of SD, particularly in outbreaks caused by pleuromutilin resistant isolates.