8. Describe how staphylococcus aureus becomes resistant to 
Staphylococcus aureus is present on the skin of approximately 30% of normal people, and may be carried by up to 25% of hospital staff. Its ability to rapidly develop resistance to antimicrobial agents, and its propensity to establish a reservoir by colonizing hospital inpatients and staff, contribute to its success as a pathogen. 

There are three known resistance mechanisms in MRSA: 

alteration of penicillin-binding proteins (PBPs), termed intrinsic resistance, is most common. Found in the bacterial cell wall, PBPs are the structures to which beta-lactams bind, thereby inhibiting cell wall synthesis. The PBPs of methicillin-resistant organisms have only a low affinity for methicillin, thereby conferring high level resistance

hyperproduction of penicillinase, first described in 1984, confers borderline methicillin-resistance; and 

organisms which have normal PBPs with a decreased affinity for lactams, are classed as modified resistant strains.

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