Abstract:
Staphylococcus aureus, a major cause of bovine mastitis, produces a wide range of
immune-evasion molecules. The bi-component leukocidin LukMF’ is a potent killer of bovine
neutrophils in vitro. Since the role of LukMF’ in development of bovine mastitis has not been studied
in natural infections, we aimed to clarify whether presence of the lukM-lukF’ genes and production
levels of LukMF’ are associated with clinical severity of the disease. Staphylococcus aureus was
isolated from mastitis milk samples (38 clinical and 17 subclinical cases) from 33 different farms.
The lukM-lukF’ genes were present in 96% of the isolates. Remarkably, 22% of the lukM-lukF’-positive
S. aureus isolates displayed a 10-fold higher in vitro LukMF’ production than the average of the
lower-producing ones. These high producing isolates were cultured significantly more frequently
from clinical than subclinical mastitis cases. Also, the detection of LukM protein in milk samples
was significantly associated with clinical mastitis and high production in vitro. The high producing
LukMF’ strains all belonged to the same genetic lineage, spa-type t543. Analysis of their global
toxin gene regulators revealed a point mutation in the Repressor of toxins (rot) gene which results
in a non-functional start codon, preventing translation of rot. This mutation was only identified in
high LukMF’ producing isolates and not in low LukMF’ producing isolates. Since rot suppresses
the expression of various toxins including leukocidins, this mutation is a possible explanation for
increased LukMF’ production. Identification of high LukMF’ producing strains is of clinical relevance
and can potentially be used as a prognostic marker for severity of mastitis.
Description:
Supplementary Materials: The following are available online at http://www.mdpi.com/2072-6651/10/5/200/s1,
Figure S1: (a) Alignment of lukM of seven S. aureus isolates obtained from cases of bovine mastitis; (b) Alignment
of lukF’ of seven S. aureus isolates obtained from cases of bovine mastitis; (c) Alignment of the putative promotor
region of lukM-lukF’ operon of seven S. aureus isolates obtained from cases of bovine mastitis; (d) Alignment of
the first 20 nucleotides of rot in 12 S. aureus isolates obtained from cases of bovine mastitis. Table S1: Number
of S. aureus of different lineages found among bovine isolates, with percentage of lukM-lukF’ positive S. aureus
among these lineages.