Aside from the streptococci, micrococci have been the next most frequent group of organisms isolated from inflamed udders. They produce various types of disease. Some give rise to only a mild catarrh of the larger milk ducts and cystern, while others produce more or less severe parenchymatous inflammation. On the whole, the prognosis is more favorable with micrococcic infection than with that associated with streptococci. Cases of considerable severity have, however, been attributed to staphylococci.
Micrococci similar in many respects to those associated with mastitis have been found to occur in the normal udder. This has led Savage to question their true etiological significance. In many instances micrococci may gain access to the udder and produce slight disturbances that are entirely overlooked. Even more severe changes may follow infection. After recovery the organisms still remain in the milk. This was observed in the case of Cow 60 infected with staphylococci. One frequently observes the elimination of streptococci from the udder even after apparent recovery from an attack of streptococcic mastitis. Doubtless streptococci and micrococci observed in these udders would be classed as belonging to the normal flora. Even though micrococci do occur in supposedly normal udders Evans has shown that many are pathogenic for rabbits. The introduction of these organisms into the udders of non-resistant individuals might well give rise to more or less intense inflammation. The multiplication would doubtless be rapid until resistance had been established.
In addition to the micrococci two other groups of rod-shaped organisms have been found associated with udder inflammation. In two instances Bacillus coli has been isolated from cases of mastitis and in another Bacillus lactis aerogenes. In four, tiny motile Gramstaining microorganisms have been obtained in pure culture. Two of these strains (Nos. C.79 and M.44) have been identified as Bacillus pyogenes.