TLR9 and bacterial infectious disease: Using TLR9 knockout mice, several investigators discovered that TLR9 exhibits a protective role against select bacterial infections, including Mycobacterium tuberculosis, Mycobacterium avium, Klebsiella pneumoniae, Legionella pneumophila, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA)8–13.