Abstract
BackgroundThe anaerobic spirochetes Brachyspira hyodysenteriae and Brachyspira pilosicoli cause diarrheal diseases in pigs. Their fastidious nature has hampered standardization of methods for antimicrobial susceptibility testing. For monitoring of antimicrobial susceptibility wild type cutoff values are needed to define where the wild type distribution of MICs ends and no approved cutoffs are available for Brachyspira spp. In this study antimicrobial susceptibility data for both species (in total 906 isolates) were compiled and analyzed and wild type cut off values for B. hyodysenteriae proposed.MethodsThe MICs of tiamulin, valnemulin, tylosin, tylvalosin, doxycycline and lincomycin were determined by broth dilution in brain heart infusion broth supplemented with 10% fetal calf serum.ResultsThe compiled MICs from the broth dilution tests of the B. hyodysenteriae type strain, B78T (ATCC® 27164T), showed that the method yields reproducible results. In an international perspective the frequencies of isolates with decreased antimicrobial susceptibility were low among both B. hyodysenteriae and B. pilosicoli. However, in B. pilosicoli a constant level of 10-15% isolates with tiamulin MICs >4 μg/ml was detected between 2002 and 2010 and in B. hyodysenteriae a gradual increase in tiamulin MICs was seen between 1990 and 2003 although this increase has ceased during the last years. The wild type cutoff values proposed for B. hyodysenteriae are: tiamulin >0.25 μg/ml, valnemulin >0.125 μg/ml, tylosin >16 μg/ml, tylvalosin >1 μg/ml, lincomycin >1 μg/ml and doxycycline >0.5 μg/ml.ConclusionsThe broth dilution method used in this study has over the years generated tightly grouped MIC populations for the field isolates and reproducible results for the control strain B78T and is therefore a suitable antimicrobial susceptibility test method for monitoring of Brachyspira spp. Here we propose wild type cutoff values for six antimicrobial agents for B. hyodysenteriae tested by broth dilution based on MIC distributions and the current knowledge on mechanisms of resistance in this species. There are few studies on antimicrobial resistance mechanisms and MIC distributions in B. pilosicoli but to some extent the cutoff values proposed for B. hyodysenteriae may be applicable also for monitoring of antimicrobial susceptibility in B. pilosicoli.
Highlights
The anaerobic spirochetes Brachyspira hyodysenteriae and Brachyspira pilosicoli cause diarrheal diseases in pigs
All tests were performed with the identical broth dilution method and in 96 separate tests the Minimum inhibitory concentration (MIC) for the control strain were within proposed ranges [6]
When viewed from an international perspective the frequencies of isolates with decreased susceptibility to the tested antimicrobial agents are low among Swedish B. hyodysenteriae and B. pilosicoli (Figure 1 and 2)
Summary
The anaerobic spirochetes Brachyspira hyodysenteriae and Brachyspira pilosicoli cause diarrheal diseases in pigs. Their fastidious nature has hampered standardization of methods for antimicrobial susceptibility testing. Brachyspira pilosicoli causes a milder diarrheal disease in growing pigs, porcine intestinal spirochetosis, which is usually nonfatal but impairs growth rate [2]. Antimicrobial agents such as pleuromutilins, macrolides and lincosamides are important in the control of infections with both B. hyodysenteriae and B. pilosicoli in pigs. Antimicrobial susceptibility tests of Brachyspira spp. are not always performed on a routine basis because of the fastidious nature of these anaerobes and there are no generally approved or recommended standards available. No criteria for interpretation of the results from susceptibility tests of Brachyspira spp. have been officially established
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