It is sometimes said that you can get Streptococcus causing mastitis, and
this is more common in the neonatal period, but essentially mastitis is
caused by Staphyloccocus aureus.  I have heard people say that a case here
or there has been caused by E. coli, but I doubt it somehow.  I would use
cephalexin at any time, as it would cover strep as well as staph.
Streptococcus can be relatively resistant to cloxacillin, for example.

Jack Newman, MD, FRCPC