TAILIEUCHUNG - Chapter 129. Staphylococcal Infections (Part 12)

As a result of the widespread dissemination of plasmids containing the enzyme penicillinase, few strains of staphylococci ( | Chapter 129. Staphylococcal Infections Part 12 As a result of the widespread dissemination of plasmids containing the enzyme penicillinase few strains of staphylococci 5 remain susceptible to penicillin. However against susceptible strains penicillin remains the drug of choice. Penicillin-resistant isolates are treated with semisynthetic penicillinaseresistant penicillins SPRPs such as oxacillin or nafcillin. Methicillin the first of the SPRPs is now used infrequently. Cephalosporins are alternative therapeutic agents for these infections. Second- and third-generation cephalosporins do not have a therapeutic advantage over first-generation cephalosporins for the treatment of staphylococcal infections. The carbapenem imipenem has excellent activity against methicillin-sensitive S. aureus MSSA but not MRSA. The isolation of MRSA was reported within 1 year of the introduction of methicillin. The prevalence of MRSA has since increased steadily. In many hospitals 40-50 of S. aureus isolates are now resistant to methicillin. Resistance to methicillin indicates resistance to all SPRPs as well as all cephalosporins. Many MRSA isolates are also resistant to other antimicrobial families including aminoglycosides quinolones and macrolides. Production of a novel penicillin-binding protein PBP 2a or 2 is responsible for methicillin resistance. This protein is synthesized by the mecA gene which as stated above is part of a large mobile genetic element a pathogenicity or genomic island called SCCmec. It is hypothesized that acquisition of this genetic material resulted from horizontal transfer from a related staphylococcal species such as S. sciuri. Phenotypic expression of methicillin resistance may be constitutive . expressed in all organisms in a population or heterogeneous . displayed by only a proportion of the total organism population . Detection of methicillin resistance in the clinical microbiology laboratory can be difficult if the strain expresses heterogeneous .

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