Are medical terms such as Staphylococcus aureus and their acronyms italicized only at first instance?
Medical terms such as Staphylococcus aureus are italicized at each instance, but acronyms for these terms (in this case, MRSA), are always set in roman type.
In the passage below, the term Staphylococcus aureus is only used once. After its first mention, the acronym, MRSA, is used in its place. This approach is not mandatory. The author could also have chosen to use the full term throughout the passage, italicized at each instance.
According to the Centers for Disease Control and Prevention, staph infections caused by Methicillin-resistant Staphylococcus aureus (MRSA) are hard to treat because they are resistant to some antibiotics. Infections caused by MRSA spread not just in hospitals but also “in the community where you live, work, and go to school” (“Methicillin-resistant Staphylococcus aureus“).
“Methicillin-resistant Staphylococcus aureus (MRSA).” Centers for Disease Control and Prevention, U.S. Department of Health and Human Services, 5 Feb. 2019, www.cdc.gov/mrsa/index.html.