A quarter is seldom infected with more than one organism at the same time. However, it is possible to detect organisms from the teat skin or teat canal in addition to those actually infecting the quarter.
Isolation of contagious organisms such as Streptococcus agalactiae ( Strep. ag. ) or Staphylococcus aureus (Staph. aureus) indicates infection of the gland.
Environmental organisms such as Strep. species, coliforms ( E. coli or Klebsiella sp. ), Staph. species (coagulase negative Staph. species , Staph. species other than Staph. aureus ), Pseudomonas sp. , Corynebacterium sp. , yeast, and fungi are equally likely to have come from the skin or teat canal as from the gland. However, these organisms can infect the gland and produce clinical or sub-clinical mastitis.
It is difficult to determine the relative importance of these bacteria in heavily contaminated milk samples. More information on a particular cow's history beyond the simple isolation of an organism is needed to determine if the quarter is actually infected.
Factors such as increased somatic cell count, presence of clinical mastitis, and/or the absence of isolation of contagious organisms indicate that a quarter may be infected with an environmental pathogen.