Staphylococcus aureus causes various diseases against humans, including skin infection, pneumonia, food poisoning, and meningitis. Methicillin-resistant S. aureus (MRSA) is resistant to a broad range of antibiotics, causing serious clinical problems. In this review, I summarize our studies to evaluate S. aureus virulence and identify novel virulence regulators. First, we utilized silkworms as an infection model of S. aureus and identified novel virulence factors of S. aureus. Some of the virulence factors interact with RNA in bacterial cells and regulate the expression of virulence factors. Second, we found that S. aureus cells spread on soft agar plates and form a giant colony. We call this phenomenon colony-spreading. High virulence community-acquired MRSA exhibits higher colony-spreading activity than hospital-associated MRSA. The difference in colony spreading is attributed to a specific gene in the mobile genetic element SCCmec carried by hospital-associated MRSA. The gene transcription product inhibits translation of a master regulator against S. aureus virulence genes, resulting in the attenuation of colony-spreading, exotoxin production, and animal killing ability.