I am reading the WHO Global Antimicrobial Resistance Surveillance System (GLASS) - Manual for Early Implementation. it mentioned some of the drug-bug combination that will be put under surveillance in page 8 of the following link, I have screen-captured the page below that I have some questions.
For S. aureus, Cefoxitin is selected for surveillance, but the remarks mentioned that Cefoxitin is a surrogate for testing susceptibility to oxacillin (methicillin, nafcillin). I know S. aureus showing resistance to methicillin is MRSA, but why test for Cefoxitin but not test methicillin directly?
The same question goes for S. pneumoniae for Oxacillin as a surrogate for testing reduced susceptibility or resistance to penicillin. Why not directly test for penicillin?
Sorry that I am not a microbiologist, just a data analyst recently working on microbiological data, hope that I won't be asking something too stupid.