The bacteriology of the organisms causing Malta fever and infectious abortion of cattle has been extensively studied, especially by Evans,1 who has classified them into three main groups. Using forty strains, she showed that the majority of human, bovine, porcine, equine and caprine strains of organisms of the genus Brucella could be separated into three main groups.
The majority of bovine and porcine strains fall into a group designated as Brucella melitensis, variety abortus. This includes the organism of infectious abortion of cattle, which was known as B. abortus (Bang).
A second large group includes strains from human, caprine, bovine and equine origins and is designated as variety melitensis A.
A third group, characterized by the occurrence of coccoid forms, is probably the variety that Bruce was working with when he discovered his Micrococcus melitensis. It is designated variety melitensis B.
Simple agglutination tests do not differentiate these varieties, and it is necessary to do absorption of the agglutinins to differentiate the organism known as B. abortus (Bang) from that causing true Malta fever.
Gage EE, Gregory DA. HUMAN INFECTION WITH BRUCELLA MELITENSIS, VARIETY ABORTUS, TREATED WITH MERCUROCHROME. JAMA. 1926;87(11):848-849. doi:10.1001/jama.1926.92680110001015a