Abstract:
The early diagnosis of mastitis is an essential factor for the prompt detection of the
animal for further actions. In fact, if not culled, infected cows must be segregated from the milking
herd and milked last, or milked with separate milking units. Besides microbiological analysis, the
somatic cell count (SCC) commonly used as predictor of intramammary infection, frequently lead
to a misclassification of milk samples. To overcome these limitations, more specific biomarkers
are continuously evaluated. The total amino acid content increases significantly in mastitic milk
compared to normal milk. S. aureus requires branched-chain amino acids (BCAAs—isoleucine,
leucine, and valine) for protein synthesis, branched-chain fatty acids synthesis, and environmental
adaptation by responding to their availability via transcriptional regulators. The increase of BCAAs
in composite milk has been postulated to be linked to mammary infection by S. aureus. The aim
of this work is to demonstrate, by a direct ion-pairing reversed-phase method, based on the use of
the evaporative light-scattering detector (IP-RP-HPLC-ELSD), applied to 65 composite cow milk
samples, a correlation between the concentration of isoleucine and leucine, and S. aureus load. The
correlation coe cient, r, was found to be 0.102 for SCC (p = 0.096), 0.622 for isoleucine (p < 0.0001),
0.586 for leucine (p < 0.0001), 0.013 for valine (p = 0.381), and 0.07 for tyrosine (p = 0.034), standing for
a positive correlation between S. aureus and isoleucine and leucine concentration. The link between
the content of BCAAs, isoleucine and leucine, and udder infection by S. aureus demonstrated with
our study has an important clinical value for the rapid diagnosis of S. aureus mastitis in cows.
