The purpose of this retrospective study was to evaluate trends in antimicrobial resistance (AMR) in Klebsiella from milk samples submitted to the Wisconsin Veterinary Diagnostic Laboratory (WVDL) for bovine mastitis testing. At WVDL, the CMV1AMAF panel by the Thermo Scientific Sensititer system is used to run minimum inhibitory concentration (MIC) panels. The Sensititer database was reviewed for milk submitted for mastitis testing where Klebsiella species was determined to be the primary pathogen. The mastitis cases were finalized from January 1, 2008 through December 31, 2017. Clinical Laboratory Standards Institute (CLSI) guidelines were used to determine MIC breakpoints. Due to the lack of Klebsiella-specific breakpoints in CLSI, MIC cut-offs for antibiotics on the mastitis panel were used from E. coli; both bacteria are members of the Enterobacteriaceae family. The percent of samples resistant to the antibiotics was calculated; Klebsiella was found to be completely resistant to most antibiotics on the panel except for ceftiofur and tetracycline. For ceftiofur, resistance ranged from 0% (2010) to 22.7% (2017), and for tetracycline, from 13.8% (2011) to 40.9% (2013) (Table 1). Overall, there was no significant change in antimicrobial resistance of Klebsiella over the 10-year period of data. Table 1 (Abstr. 64). Percent of Klebsiella samples resistant to ceftiofur and tetracycline, 2008–2017

Antibiotic1	2008 (n=71)	2009 (n=59)	2010 (n=57)	2011 (n=29)	2012 (n=39)	2013 (n=44)	2014 (n=43)	2015 (n=40)	2016 (n=30)	2017 (n=75)
Ceftiofur2	1.4	6.8	0	0	7.7	11.4	7.0	7.5	3.3	22.7
Tetracycline2	21.1	23.7	26.3	13.8	41.0	40.9	25.6	30.0	26.7	25.3

¹CLSI breakpoint concentrations (µg/mL) used to classify samples as antibiotic resistant (R). ²Bacteria species/sample source for antibiotic: for ceftiofur, E. coli/bovine mastitis (R >4), and for tetracycline, Enterobacteriaceae family/humans (R >8).