PROS1 and infection: The other PCR primer was designed for three target genes, ddl (which encodes a D-alanine∶D-alanine ligase that is essential for bacterial cell wall biosynthesis [25]), esp (which is non-essential and encodes a large surface protein involved in biofilm formation and infection [26]–[28]) and nox (which is a non-essential gene encoding a predicted NADH oxidase).