has long been recognised as an udder pathogen. It was first described by Diernhofer (1932) and subsequently classified as a group-C streptococcus by Lancefield (1933). It can be found in various extramammary sites, including the vagina (Francis, 1941) and tonsils (Daleel and Frost, 1967) of adult cattle, but the significance of these sites as reservoirs of infection in a herd is not known. Udder infection with is generally less frequent than with either or (Slanetz and Naghski, 1940). Results from 32 herds in the United Kingdom showed that 2.9% of cows were infected with compared with 4.0% and 4.4% for and respectively (Wilson and Kingwill, 1975). McDonald and McDonald (1976) found that 41 of 455 (9%) streptococcal isolates from infected udders were compared with 7 (1.5%) for and 257 (56%) for

Experimental infections of goats and cattle with have been successful (Edwards, 1932; Holman, Pattison and Gordon, 1952; Pattison and Smith, 1953) but we are not aware of published results comparing the infectivity of strains. It was partly for this reason and partly to identify an infective strain for milking-machine studies that this work was done.

The ability of streptococci to form chains means that the number of colony-forming units per unit volume could be a misleading indication of the number of viable organisms in a challenge volume, particularly if one of the strains tended to produce long chains whilst others produced short chains in the growth conditions used. To investigate the possible effects of this factor, strains for infection studies were compared before and after treatment that would disrupt chains.


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