Dear Erminio, I would like to answer in the following way: In modern dairy herds we have to some extent S.aureus. Yet, concerning  other pathogens, the so called environmental pathogens (e.g. E. coli; Str. uberis, etc.) are the most important bacteria  causing mastitis. Sometimes also coagulase-negative Staphs. can evoke severe mastitis cases..

All the best, yours sincerely Jörn

I recently ran across a herd dealing with protothecal mastitis. Since prototheca is not susceptible to antibiotics, and has a huge impact on the dairy, I would submit that this would be one to consider aside from the usual pathogens.

Both S.aureus and   other pathogens, ( E. coli; Str. uberis, etc.) are very important bacteria  causing mastitis and their patogenity might be favourized by oculte viral infections that decrease immunity, like BVD and also by any stressfull conditions. Endometritis is a source of infection

Protothecal mastitis reports are increasing.

Subclinical hypocalcemia decrease the tonus of mamelonar sphincter facilitating the colonization with fecal germs

Dear Erminio

Recently S. aureus. s.aglactiae, E.coli. s. uberis, Pseudomonasis etc. are important  for causing mastitis in cow

As far as I know the among many other pathogens S. aureus is the most important one, as it has negative impact on different levels including animal health, human health in addition to the economic impact

mastitis can cause by 3 main ways : bacteria , fungi , trauma . but the most important way is bacteria especially s.aureus .

The main mastitis-causing pathogens are Escherichia coli (E. coli), Streptococcus uberis and Staphylococcus aureus, and a wide variety of other organisms have been identified as potential mastitis pathogens. These organisms are termed majorpathogens and are generally regarded as those commonly associated with clinical mastitis in dairy cattle. Staph. aureus is the number one cause of mastitis in the USA. But in other countries different pathogens might be the primary cause of mastitis. The main problem in controlling the mastitis causing pathogens is the false negative results obtained by common tests for diagnosing mastitis like culture and SCC. New biomarkers like milk amyloid A (MAA) with high diagnostic values are good candidates for diagnosis of mastitis. The negative result of a test with high sensitivity rules out the disease. I refer Dr. Erminio Trevisi to our paper on mastitis entitiled: "Acute phase proteins in the diagnosis of bovine subclinical mastitis".

Bacteria that most frequently cause mastitis can be divided into two large groups based on the source of the bacteria: contagious pathogens and environmental pathogens. The primary contagious pathogens are Streptococcus agalactiae, Staphylococcus aureus and Mycoplasma species. Primary environmental pathogens include two types of bacteria: coliform bacteria and species of streptococci other than Strep. agalactiae. These other streptococci are referred to as the "environmental streptococci". 

Staph. aureus is the number one cause of mastitis in the USA. But in other countries different pathogens might be the primary cause of mastitis. The 

Staph aureus is quite serious because it is very difficult to treat definitively. Good luck for your research.

In some part of Argentina subclinical mastitis are still highly prevalent, being Staphylococcus aureus the most important and involved microorganism found, nevertheless since the last years, CNS is raising concern.

E. Coli, staphylococcus aureus, streptococcus agalactiae are the commonest. But mycoplasma and fungal mastitis are uncommon but dangerous.

The most common pathogens are:- E. coli; Staph. aureus and Streptococcus agalactiae. Apart from that Myccoplasmal and mycotic mastitis aree alsso important. These days complications of protothecal mastitis are also common and increasing.

Staphylococcus aureus, Streptococcus agalactiae, E. Coli and Pseudomonals & Mycoplasma are common mastitis causing pathogens.

Staph.aureus , Streptococcus agalactia, E.coli, Mycoplasma

Mastitis is a major production disease of dairy animals caused by a wide variety of pathogens, such as bacteria, fungi, Mycoplasma, Prototheca, and actinomycetes. We have worked on mastitis and published several papers.The treatment of mastitis caused by Nocardia asteroides, Cryptococcus neoformans and Prototheca is very challenging.

Very recently, we have published a comprehensive review on bovine mastitis that is uploaded at Research Gate.