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SUMMARY
Forty-four Paenibacillus larvae subsp. larvae isolates cultured from larval smears and honey samples from Australia and honey imported from Argentina were typed using pulsed-field gel electrophoresis (PFGE). Their minimum inhibitory concentrations (MICs) to oxytetracycline (OTC) were also determined using an agar dilution method. PFGE fingerprints of P. l. larvae DNA generated by the restriction endonuclease Xbal produced 12 distinct PFGE types. Types I, II and VIII comprised the majority of strains (28/44: 63.6%). Type I (9/44: 20.5%) consisted only of isolates cultured from Argentinean honey and Argentinean honey blended with Australian honey, and all these isolates had a MIC to OTC of 0.6 μg/ml. Type II, comprising 22.7% of total (10/44), consisted of isolates from New South Wales (3), Queensland (6) and one isolate originating from a mixture of Argentinean and Australian honey. All these isolates had MICs to OTC of 0.04 or 0.05 μg/ml. Type VIII consisted of 9 (20.5%) isolates, all originating from New South Wales, with MICs to OTC of 0.04 or 0.05 μg/ml. The other PFGE types consisted of 3 or fewer isolates (Type XI, 3 isolates; Types III, V, VI, VII and IX, 2 isolates each; Types IV, X and XII, 1 isolate each). P. l. larvae isolates of Australian origin and Argentinean origin were all very sensitive to OTC. However, the isolates of Argentinean origin and Argentinean/Australian honey blend origin were less sensitive to OTC (MIC of 0.6 μg/ml) than isolates of Australian origin (MICs of 0.04 or 0.05 μg/ml). Collectively these data indicate that PFGE is an affective means of discriminating between P. l. larvae strains and could be used to determine whether OTC resistant strains are from local or imported P l. larvae strains.
