CLSI validates Interpretive Criteria for the Susceptibility Testing of BI´s Ubrolexin® against Bovine Mastitis Pathogens

Ubrolexin®, a combination of kanamycin and cefalexin has been marketed for the intramammary treatment of clinical mastitis in lactating dairy cows in 27 countries by Boehringer-Ingelheim since 2008.  The combination broadens the spectrum of activity attributed to either agent alone, and synergy between the two agents has been demonstrated for target mastitis pathogens1,2.

Methods for testing the susceptibility of mastitis pathogens to the kanamycin and cefalexin combination, including recommendations for interpretive criteria and quality control ranges for broth microdilution and disk diffusion susceptibility testing, were the subject of prior peer-reviewed publications3,4.

Revised interpretive criteria for the combination for broth microdilution and disk diffusion susceptibility testing were recently approved by the Clinical and Laboratory Standards Institute (CLSI) Veterinary Antimicrobial Susceptibility Testing (VAST) Subcommittee (table blow).

The revised recommendations take into consideration additional susceptibility data collected on recent bovine clinical mastitis field isolates and should be used for testing kanamycin in combination with cephalexin against bovine mastitis pathogens going forward.

Disk diffusion criteria were adjusted accordingly based on the approved broth criteria and resulting error-rate bounding analysis in line with CLSI M23 guidelines5 and were also approved by VAST.

Quality control ranges were previously approved by VAST for the testing of kanamycin:cephalexin (10:1 fixed ratio) by broth microdilution and for the testing of kanamycin:cephalexin (30/15 µg) by disk diffusion6.

With these interpretation criteria now made available to milk quality labs worldwide it will now be easier for vet practitioners to accurately test the susceptibility of mastitis pathogens to the cephalexin-kanamycin combination, hence contributing to prudent use of antimicrobials.


Source – Boehringer Ingelheim


1 Ganiere JP, Denuault L.  Synergistic interactions between cefalexin and kanamycin in Mueller-Hinton broth medium and in milk.  J Appl Microbiol.  2009;107:117.

2 Maneke E, Pridmore A, Goby L, Lang I.  Kill rate of mastitis pathogens by a combination of cefalexin and kanamycin.  J Appl Microbiol.  2011;110:184.

3 Pillar CM, Goby L, Draghi D, Grover P, Thornsberry C. Evaluating the in vitro susceptibility of bovine mastitis pathogens to a combination of kanamycin and cefalexin: recommendations for a disk diffusion test. J Dairy Sci. 2009;92:6217..

4 Silley P, Goby L, Pillar CM.  Susceptibility of coagulase-negative staphylococci to a kanamycin and cefalexin combination.  J Dairy Sci. 2012;95:3448.

5 CLSIDevelopment of in vitro susceptibility testing criteria and quality control parameters,5th edition.  CLSI guideline M23.  CLSI, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2008.

6 CLSI.  Performance standards for antimicrobial disk and dilutions susceptibility tests for bacteria isolated from animals, 5th edition.  CLSI supplement VET01S.  CLSI, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2020.

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