Bovine mastitis and ecology of Streptococcus uberis

Abstract

Bovine mastitis caused by Streptococcus uberis is a common problem in pasture-based dairying systems. This study examines both the ecology of S. uberis and infection of the bovine mammary gland on a New Zealand dairy farm. Initially, the REP-PCR strain typing method was developed and the potential of MALDI-TOF mass spectrometry evaluated as a strain typing method. While strain-specific mass spectra were obtained with MALDI-TOF mass spectrometry, the irreproducibility of spectra was its major downfall. With further work, this rapid method could be very useful for strain typing S. uberis on a large scale. Using optimised REP-PCR and anchored typing methods, multiple S. uberis strains were isolated and strain typed from the dairy environment, including farm races and paddocks, faeces, teat skin, the cow body and from intramammary infections. High strain diversity was observed in all sampled locations; however, some strains were found at more than one site, suggesting transmission may occur between the environment and cows. The most likely means of S. uberis distribution throughout the dairy farm was via excretion with faeces and, although not all cow faeces contained this pathogen, the gastrointestinal tract of some cows appeared to be colonised by specific strains, resulting in persistent shedding of this bacteria in the faeces. Infection of the mammary gland is likely to occur through contamination of the teat skin with highly diverse environmental strains of S. uberis. However, only one or two strains are generally found in milk from mastitis cases, suggesting that, although infection may arise from a random or opportunistic event, a strain selection process may take place. Intramammary challenge with multiple strains of S. uberis revealed that selection of a single infective strain can occur within the mammary gland. The predominance of one strain over others may be related to production of virulence factors allowing enhanced ability to establish in the gland and evade the immune response, or due to direct competition between strains through the production of antimicrobial factors such as bacteriocins. In addition to strain-specific factors, the individual cow and quarter response may play an important role in the development of infection and selection of the infective strain. Using results from this study, a model of S. uberis strain transmission has been proposed, which includes potential mechanisms of infection and persistence of S. uberis within the mammary gland.