In the US, Synercid might be approved by the FDA only
for staph and strep, but once a drug is on the market and approved
for any particular purpose, it can be prescribed for anything at the prescribing provider's discretion. (The only exceptions are for narcotics and other drugs with addiction/abuse potential.) Few docs would agree to it for an infection for which it has never even been studied, like M gen, and whether your insurance would cover it is a another matter.