In-vitro susceptibility of Brachyspira spp. clinical isolates to tiamulin and narasin

Brachyspira spp. are a group of enteric pathogens prevalent worldwide in most swine rearing geographies. To determine appropriate therapy for Brachyspira disease, minimum inhibitor concentrations (MIC) of antibiotics are a common first step. Narasin is a recently FDA approved ionophore antibiotic to improve growth in swine in the USA. The objective of this study was to determine the MIC of tiamulin and narasin using Brachyspira isolates from clinical cases.

Iowa State University, Veterinary Diagnostic Laboratory, used thirty-one (31) isolates of Brachyspira spp: (10 B. hyodysenteriae, 11 B. hampsonii, and 10 B. murdochii) for MIC testing of narasin and tiamulin. These isolates had been collected from 2008 until 2015 and all isolates were from clinical disease herds from North American origin. The isolates were evaluated in an agar dilution MIC antimicrobial testing method using narasin and tiamulin as test material. Four Brachyspira spp. controls were used. The MIC ranges used in this study were 0.015625 – 8.0 µg/ml for tiamulin and 0.0078125 – 1.0 µg/ml for narasin. These ranges were chosen to include the MIC value of all test organisms. There were 2 MIC replicates for each isolate. Diluent and PBS agar dilution controls were included to exclude growth inhibition from the antibiotic diluent used.

The B. hyodysenteriae isolates had a MIC range of 0.03125 – 4 µg/ml for tiamulin and a point dose MIC of 0.015625 µg/ml for narasin. These isolates had a MIC50 of 0.125 µg/ml and a MIC90 of 1.00 µg/ml for tiamulin.

The B. hampsonii isolates had a MIC range of 0.03125 – 8 µg/ml for tiamulin and a point dose MIC of 0.015625 µg/ml for narasin. These isolates had a MIC50 of 0.0625 µg/ml and a MIC90 of 0.125 µg/ml for tiamulin.

The B. murdochii isolates had a MIC range of 0.03125 – 0.25 µg/ml for tiamulin and a point dose 0.015625 µg/ml for narasin. These isolates had a MIC50 of 0.03125 µg/ml and a MIC90 of 0.25 µg/ml for tiamulin.

No inhibition of growth from diluents was observed.

Brachyspira spp. MIC’s for tiamulin determined in this study were similar to Clothier et al previous MIC study. Narasin, a new antibiotic for US swine, inhibited the growth of all Brachyspira isolates at the 0.015625 µg/ml MIC level. These data support the continued evaluation of narasin for swine Brachyspiral colitis.

Disclosure of Interest: T. Marsteller Conflict with: employee.