There are two international organisations that set up guidelines and interpretive breakpoints for bacteriology and susceptibility testing. The CLSI or Clinical and Laboratory Standards Institute is a non-profit organisation with a mission to develop clinical and laboratory practices and promote their use worldwide. Their laboratory testing standards are based on input from and consensus among the industry, governments and healthcare professionals. The CLSI has a Veterinary Subcommittee that approves clinical interpretive breakpoints: this means that the breakpoints of sensitivity discs are related to the actual clinical cure for a certain disease.

The pathogens responsible for clinical and subclinical cases of mastitis that are most commonly observed are:

• "Contagious" pathogens:Staphylococcus aureus, Streptococcus agalactiae and Mycoplasma spp.
• Environmental "pathogens": non-agalactiae streptococci: Streptococcus uberis, Streptococcus dysgalactiae, Enterococcus faecalis and other streptococci; coagulase-negative staphylococci (CoNS) and coliform bacteria (Escherichia coli, Klebsiella pneumoniae, Enterobacter aerogenes).

Depending on its cause, mastitis can be either contagious or environmental, although the division between both is not entirely black and white. There are plenty of agents that can cause mastitis, but the main aetiological groups account for more than 95 % of the cases.