New MRSA concerns -Reply

Steven Projan PROJANS at
Mon Aug 5 07:07:08 EST 1996

A recent posted asked the following:

"We recently have encountered strains of Staph aureus which have
multiple drug resistance but test in vitro as sensitive to oxacillin
MICROSCAN, KIRBY-BAUER, and MRSA testing agar.  These strains seem to
exhibit tremendous BETA LACTAMSE PRODUCTION and produce clinical
treatment failures if cephalosporins or oxacillin are used.  HAVE YOU

There have been several reports of BORSA "borderline oxacillin
resistant S. aureus"  due to hyper production of beta-lactamase.  The
S. aureus beta-lactamase is inhibited by sulbactam, tazobactam and
clavulonic acid so it may possible to use these in the therapy of
patients infected with BORSA.  This would not be true for strains
carrying the mecA gene, the product of which, PBP2a, has a low
affinity for most all beta-lactams in clinical use.

It is also possible that you have a heterotypic strain of S. aureus
(a strain carrying the mecA gene but which, in vitro, has only a
small subpopulation of bacteria which appear to express resistance).
This strain may also is  a hyper beta-lactamase producer (many, if
not most, mecA strains produce beta-lactamase).  Therefore it would
be necessary to probe for the presence of the mecA gene to determine
if this is indeed the case.

There is a lot work published in this area:  you check out the work
of the Archer, Berger-Bachi, Hiramatsu and Tomasz laboratories on
this subject.

Steve Projan
Wyeth-Ayerst Research

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