netting the elusive MRSA

Micro Chat microchat at aol.com
Sat Nov 2 12:04:41 EST 1996

MRSA, PBP2', clinical reporting, NCCLS......

Greetings....your assistance and comments on these topics is greatly

The elusive MRSA s have outdistanced hospital laboratory technology.  In
the past year our lab has seen a 30-fold increase in multi-resistant Staph
aureus strains which test as sensitive to oxacillin by agar diffusion and
MIC by ALL methods COMMONLY available to hospital labs. YES, we have
looked for micro-colonies, incubated up to 48 hours, used 4% NaCl, etc.

USA clinical labs use NCCLS (nat. committee for clin. lab stds.)
guidelines to legally substantiate test result reporting.  For OXACILLIN
this is limited to Kirby Bauer and oxacillin agar results.  

Question 1:  Are multi-resistant Staph aureus strains more likely to be
resistant to oxacillin than strains  which appears to be resistant to Amp
and Pen only?

Question 2: How often are agar diffusion oxacillin "sensitive" strains
proved to be true MRSA by sandwich immunoradiometric (IRMA) for PBP2'  ?

Question 3: How common are autolysin-defective mutants of altered PBP
organisms ?

Question 4:  IMPORTANT. Is it correct that a standard broth dilution MIC
for oxacillin on Staph aureus will also fail to detect true resistance due
to altered PBP sites ?

Question 5: Why do some organisms test IN VITRO as sensitive when in fact
they are known to be in vivo resistant ? Example: penicillin resistant,
beta lactamase negative enterococcus will test as sensitive to imipenem
when in fact imipenem will be in vivo ineffective due to PBP mechanism.

Question 6: By what mechanism are these organisms producing a visual
inhibitory effect (Kirby-Bauer or broth)?  I REALLY want to understand

Question 7:  Why, biochemically, is oxacillin resistance to best indicator
of penicillin resistance in STREPTOCOCCUS Pneumoniae?

++Your opinion please++   How are physicians and hospital labs to deal
with this emergence of low-beta -lactam-affinity PBP isolates? Costs and
48 hour turn-around times present limitations.

We fear that we are obliged to report many Staph aureus isolates as
oxacillin sensitive when our gut feeling is that they may be MRSA.  They
don't present as BORSA either.

Thank you for any comments or referrals to reading material.

B.J. Ruis, M.S.               Micro Chat at aol.com    OR
New York                       BJR445566 at aol.com

More information about the Microbio mailing list

Send comments to us at biosci-help [At] net.bio.net