Methicillin is an antibiotic of the narrow-spectrum penicillin
("β-lactamase resistant") group. It is also known as meticillin.
Originally it was hoped that it could be used to control bacteria, especially Staphylococcus aureus
that were resistant to early versions of penicillin, due to its sidechain interfering with the bacterial β-lactamase enzyme which opens up the lactam ring of other penicillins, preventing the antibiotic from stopping the synthesis of bacterial cell walls.
However, some bacteria have developed resistance to methicillin by producing a different version of the enzyme which causes cross linking in their cell walls - a protein known as PBP - which is unaffected by penicillin. This resistance to methicillin is normally found in combination with resistance to other antibiotics, so these organisms, especially methicillin resistant Staphylococcus aureus
) have proved to be difficult to control in clinical situations.
It is no longer manufactured because similar compounds such as flucloxacillin, and dicloxacillin are more stable.