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.

http://apps.who.int/iris/bitstream/10665/188783/1/9789241549400_eng.pdf

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 testing methicillin directly?

The same question goes for S. pneumoniae for Oxacillin is 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.

Thanks!

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.

http://apps.who.int/iris/bitstream/10665/188783/1/9789241549400_eng.pdf

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.

Thanks!