Abstract
Background:
Although milk is nutritionally valuable, it also serves as a significant medium for the transmission of pathogens and their toxins.
Aim:
This study aimed to investigate the role of enterotoxin gene A (SEA) in the development of bovine mastitis. We accomplished this by examining milk through PCR testing, amino acid substitution analysis, and phylogenetic analysis.
Methods:
A total of fifty milk samples collected from locally bred dairy cows in Al-Diwaniyah, located in southern Iraq. We employed the VITEK-2 platform to validate the diagnosis of S. aureus and confirming the results of routine tests (culturing and biochemical tests). Subsequently, the genetic mutation and phylogeny analysis were achieved utilizing DNA sequencing to 16S rRNA and enterotoxin A genes.
Results:
66% (33/50) of the milk samples found to be contain S. aureus by the VITEK-2 system. Furthermore, 25/33 of the samples were positive by the PCR test. While 60% (15 out of 25) tested positive for the SEA gene. After genomic analysis, we identified amino acid substitutions of serine, glutamine with arginine, tyrosine with cysteine, and aspartic acid with glycine at positions 9, 101, 119, 187, and 191. The phylogenetic investigation demonstrated a genetic relationship between our isolates (Iraqi isolates) and isolates from Indian and the United States.
Conclusion:
Our study indicated the widespread distribution of the enterotoxin gene A (SEA) of S. aureus among dairy cows. The molecular study revealed significant changes in key amino acids that could play an important role in the bacterium's pathogenesis. The phylogenetic similarities among S. aureus samples from various countries suggest that the bacteria has spread globally.

Key words: S. aureus, 16S rRNA gene, Amino acid substitutions, Phylogenetic analysis, Mastitis