Development of the PlyC endolysin as a bovine mastitis therapeutic for lactating dairy cows

Bovine mastitis, defined as an inflammation of the cow’s mammary gland, is the most common and economically significant disease affecting dairy cattle and the leading cause of antimicrobial use on dairy farms. Streptococcus uberis is currently the most prevalent Gram-positive pathogen causing this infection. Recent growing concerns among consumers regarding the potential for antimicrobial resistance have led to the examination of alternative strategies for controlling mastitis. The streptococcal C1 bacteriophage endolysin, PlyC, is a cell wall hydrolase that rapidly lyses S. uberis and other susceptible streptococci on contact, and as such, represents an alternative to this conventional antibiotic therapy. Therefore, the objective of this study investigates PlyC as a novel antimicrobial enzyme against S. uberis mastitis. The activity of PlyC was determined by dose response and standard microbiological assays. Binding of PlyC in raw milk was visualized by fluorescent microscopy. Toxicity was evaluated on mammalian cells and in various in vivo models. Our results show that PlyC possesses potent lytic activity against all S. uberis strains tested. Despite the ability of other endolysins that are known to lyse S. uberis, none have yet successfully functioned in raw cow’s milk, presumably due to inactivation by native proteins and lipids. In contrast to the latter, PlyC attained three logs of killing at a dose of only two times the minimal inhibitory concentration when administered to raw, mastitic milk derived from clinically affected cows. Due to the absence of neutralizing antibodies that specifically target PlyC, the potential of this enzyme as a novel antimicrobial treatment is further bolstered. PlyC was found to be non-toxic as observed on a bovine mammary cell line and non-irritating as observed on rabbit epidermis and mucous membrane models. Taken together these in vitro and in vivo findings, PlyC is now ready to advance to S. uberis-associated bovine mastitis clinical trials, which will commence Fall 2019.

Jaar van publicatie:2019