Antimicrobial Resistance Profiles and Genes in Streptococcus uberis Associated With Bovine Mastitis in Thailand

Abstract

<i>Streptococcus uberis</i> is recognized as an environmental mastitis pathogen in dairy cattle. The varied success rate of antibiotic treatment for <i>S. uberis</i> intramammary infection may be associated with the antimicrobial resistance (AMR) of these bacteria. This observational study aimed to analyze 228 <i>S. uberis</i> strains associated with bovine mastitis in northern Thailand from 2010 to 2017. AMR and AMR genes were determined by the minimum inhibitory concentration (MIC) using a microdilution method and polymerase chain reaction, respectively. The majority of <i>S. uberis</i> strains were resistant to tetracycline (187/228, 82.02%), followed by ceftiofur (44/228, 19.30%), and erythromycin (19/228, 8.33%). The MIC50 and MIC90 of ceftiofur in 2017 were 2-4-fold higher than those in 2010 (<i>P</i> < 0.01). Resistance to tetracycline and ceftiofur significantly increased between 2010 and 2017 (<i>P</i> < 0.05). The most common gene detected in <i>S. uberis</i> was <i>tetM</i> (199/228, 87.28%), followed by <i>ermB</i> (151/228, 66.23 %) and <i>blaZ</i> (15/228, 6.58 %). The association between tetracycline resistance and <i>tetM</i> detection was statistically significant (<i>P</i> < 0.01). The detection rates of <i>tetM</i> significantly increased, while the detection rates of <i>tetO</i> and <i>ermB</i> significantly decreased during 2010-2017. AMR monitoring for bovine mastitis pathogens, especially <i>S. uberis</i>, is necessary to understand the trend of AMR among mastitis pathogens, which can help create an AMR stewardship program for dairy farms in Thailand.