Antimicrobial susceptibility of porcine Brachyspira hyodysenteriae and Brachyspira pilosicoli isolated in Sweden between 1990 and 2010

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.

Minimum inhibitory concentration of Brazilian Brachyspira hyodysenteriae strains

“…The strains from São Paulo and Minas Gerais were more sensitive than those from Santa Catarina and Mato Grosso, among all tested antimicrobials. Only one isolate from Minas Gerais was sensitive to all of the tested antimicrobials (tiamulin = 0.063 µg/ml; valnemulin = 0.031 µg/ml; doxycycline = 0.5 µg/ml; tylvalosin = 1 µg/ml; lincomycin = 2µg/ml; tylosin = 4µg/ml), showing similar or lower results compared with the microbiological cut-off proposed by Pringle et al (2012).…”

“…The MIC values obtained using the six antimicrobial agents in 22 B. hyodysenteriae isolates are shown in Figure 2. Twenty-one isolates exhibited higher MIC values for all active principles than the previously established microbiological cut-off (Pringle et al 2012). Comparisons between the MIC 90 values and cut-offs are shown in Table 1 and 2.…”

“…When the obtained results were compared with the cut-offs described by Pringle et al (2012) for the wildtype strain (Table 3), the proportion of strains showing low sensitivity was determined to be 90.9% for tiamulin, 91% for valnemulin, 95% for doxycycline, 95% for tylvalosin, 100% for lincomycin and 95% for tylosin.…”

“…After four days of incubation, all wells were analysed for possible contaminants that could interfere with the test results. The obtained results were compared with microbiological cut-offs described by Pringle et al (2012).…”

“…The presence of Brachyspira spp. strains with a low response to previously effective antimicrobial agents has been demonstrated in different studies in several countries (Karlsson et al 2004, Lobová et al 2004, Duinhof et al 2008, Pringle et al 2012. Thus, evaluation of the antimicrobial susceptibility of clinical isolates is essential to obtain a better understanding of this situation.…”