From the NMC Newsletter "Udder Topics", October 2004

Microorganisms that belong to the genus Corynebacterium are aerobic or facultative anaerobic, irregularly shaped, nonsporeforming, gram-positive rods.

The lipophilic Corynebacterium bovis is the most frequently isolated species in milk from dairy cows with intramammary infections (IMI). Four nonlipophilic Corynebacterium species have also been isolated from cows with clinical and subclinical mastitis: Corynebacterium amycolatum, Corynebacterium minutissimum, Corynebacterium pseudotuberculosis, and Corynebacterium ulcerans. Identification of Corynebacteria to the species level is performed infrequently whenever they grow in pure culture from quarter milk samples or associated with other bacteria in composite milk samples.

Corynebacterium bovis can be found in milk from infected mammary glands and teat canals of cows. It is considered a highly contagious mastitis pathogen that can cause IMI with a slight increase in somatic cell counts and a small decrease in milk production in affected cows.

Corynebacterium bovis are commonly spread from cow to cow at milking via milking machines in those herds that do not practice proper postmilking teat disinfection with efficacious germicidal products. Corynebacterium pseudotuberculosis causes caseous lymphadenitis in several animal species, but rarely in cattle. Corynebacterium pseudotuberculosis are less known as an exclusive agent of bovine mastitis without simultaneously-occurring cutaneous lesions. Corynebacterium amycolatum and C. minutissimum are associated with human skin while C. ulcerans are a domestic animal-associated pathogen. Little is known about the epidemiology of the last four organisms as agents of bovine mastitis.

Source: NMC publication: Microbiological Procedures for the Diagnosis of Bovine Udder Infection and Determination of Milk Quality, pg. 24 (4th Edition, 2004).