QMPS Cornell lab has a nice brochure out to help guide treatment of this type of mastitis - quite informative and when you have cows cultured with this organism, very worthwhile to follow the advice. I've seen a number of these over the past year, typically presenting as chronic mastitis cases that don't respond to most antibiotic treatments. Prognosis is guarded for them and be careful to milk them last to limit spread to other animals.
here's the brochure:
