Determination of minimum inhibitory concentrations of gamithromycin against European swine food-borne pathogen and commensal intestinal field isolates

To address concerns of antibiotic (AB) resistance in both human and animal health, approval of veterinary antibiotics requires an assessment for potential impact on resistance development in food-borne pathogens (FBPs). During treatment for Swine Respiratory Disease (SRD), populations of FBPs and commensal organisms from the genera Escherichia coli (Ec), Salmonella (Sa), Enterococcus (En) and Campylobacter (Cb) may be impacted. Isolates from the respective genera may vary in susceptibility to various ABs, depending on selective pressures in place at the time of collection.

Susceptibility is measured by determining the Minimum Inhibitory Concentration values (MICs) for each isolate against the test antibiotic. The risk for development of co- or cross-resistance is assessed by comparing MICs for gamithromycin (GAM) with other ABs. The in vitro activity of GAM, a macrolide of the sub-class azalide, was evaluated against European swine intestinal isolates taken at the time of slaughter.

240 isolates (60 isolates each of Ec, Sa, En and Cb) were sourced from the EASSA III collection coordinated by the European Animal Health Study Centre (CEESA). Recent isolates were collected from healthy swine in eight European Countries (Belgium, Denmark, France, Germany, Hungary, Netherlands, Spain and UK). MICs were generated using Clinical & Laboratory Standards Institute (CLSI) methods (VET01-A4, 2013). Based on these results, 52 isolates (12 Ec, 16 Sa, 14 En, 10 Cb) were evaluated for co- and cross-resistance using GAM and a panel of 13 common ABs.

The GAM MIC distribution (in µg/mL) for Ec was 2-8; for 95% of Sa isolates (4-8); En (0.12 to > 32); and 94% of Cb (either 0.06-0.25 or > 32). Resistance patterns of isolates were evaluated using isolates with high MICs. GAM MICs were strongly correlated to other macrolides (erythromycin, azithromycin, tulathromycin) and lincomycin. All En and Sa isolates with raised GAM MICs were resistant to tetracycline. No correlation was seen between the MIC of GAM and any of the other non-macrolide ABs. The few isolates with raised GAM MICs did not demonstrate linked co-resistance with other antimicrobial classes in the test panel

MICs for GAM were determined for recent swine food borne pathogens and commensals isolated from the intestine of healthy swine from distinct areas of the EU. This study revealed a very high susceptibility of Ec and Sa to GAM, along with expected susceptibility patterns to En and Cb. Based on these studies and literature, the use of GAM to treat SRD is not considered to influence development of resistance and will have no impact on human food safety.

Disclosure of Interest: J. Harriman Conflict with: Merial, Inc., A. Richard-Mazet Conflict with: Merial S.A.S., J. Hayward: None Declared, P. Silley: None Declared, J. Gerhart Conflict with: Merial, Inc.