Hello everyone. I'm getting confused about my work. please help me. I put a few Antimicrobial disks including Imipenem, Amoxicillin/tazobactam,
piperacillin/ tazobactam and cefuroxime for measuring the Susceptibility (S,R,I) for Staphylococcus aureus. The problem is here, because the CLSI 2013 and older version haven't any guidelines. I don't know what I am suppose to do? Thanks.
Follow EUCAST (European Committee on Antimicrobial Susceptibility Testing)
Breakpoint tables for interpretation of MICs and zone diameters.
Try to understand page 17 of EUCAST guidelines (file attached).
Thank you dear Menezes, I downloaded this file, but unfortunately I can't understand :-(
Check susceptibility against- cefoxitin; oxacillin; amoxicillin-clavulanate; cefotaxime; gentamicin; amikacin; ciprofloxacin; tetracycline.
If it is resistant to both oxacillin and cefoxitin it is MRSA. Only if it is MRSA check for vancomycin susceptibility.
Test for beta-lactamse production.
Isolates resistant to cefoxitin l(zone size less than 22mm) are considered resistant to beta-lactam agents (you have to interpret the antibiotics that you are talking about), except those with approved anti-MRSA activity (Ceftaroline
Ceftobiprole) and clinical breakpoints.
CLSI guidelines 2013 (Read this and consult your Professor, showing this ask him to explain it to you):
Penicillin-susceptible staphylococci are also susceptible to other beta-lactam agents with established clinical efficacy for staphylococcal infections. Penicillin-resistant staphylococci are resistant to penicillinase-labile penicillins. Oxacillin-resistant staphylococci are resistant to all currently available beta-lactam antimicrobial agents, with the exception of the newer cephalosporins with anti-MRSA activity. Thus, susceptibility or resistance to a wide array of beta-lactam antimicrobial agents may be deduced from testing only penicillin and either cefoxitin or oxacillin. Routine testing of other beta-lactam agents, except those with anti-MRSA activity, is not advised.
The results of either cefoxitin disk diffusion or cefoxitin MIC tests can be used to predict the presence of mecA-mediated oxacillin resistance in S. aureus and S. lugdunensis. For coagulase-negative staphylococci (except S. lugdunensis), the cefoxitin disk diffusion test is the preferred method for detection of mecA-mediated oxacillin resistance. Cefoxitin is used as a surrogate for detection of oxacillin resistance; report oxacillin as susceptible or resistant based on cefoxitin results. If a penicillinase-stable penicillin is tested, oxacillin is the preferred agent, and results can be applied to the other penicillinase-stable penicillins, cloxacillin, dicloxacillin, and flucloxacillin.
Prathusha you are talking about MIC by broth micro dilution method.
I'm used to agar dilution method.
If you have detailed procedure for broth micro dilution (specially the antibiotic serial dilution), could you email it to [email protected] with a reference.
The graduate students would find broth micro dilution less cumbersome compared to agar dilution method.
Hi
why do you want to use mentioned antibiotic for s.aureus?
cefoxitin and oxacillin are suitable agents for evaluate the beta- lactam resistance in s.aureus,Cefoxitin is a very sensitive and specific marker of mecA/mecC mediated methicillin resistance and is the agent of choice for disk diffusion. Oxacillin resistant
staphylococci are resistant to all β-lactam agents; other β-lactam agents should be
reported as resistant or should not be reported.
Few changes are there in CLSI 2015
You have to use both Penicillin and oxacillin/ cefoxitin
if Penicillin Sensitive (>/= 29mm ) always do Beta lactamase detection like Nitrocefin test, Clover leaf assay and Penicillin zone edge
Oxacillin / cefoxitin also sensitive means sensitive to all beta lactams, Cephem and Carbapenems
R to P but S to Ox/CX means R to Penicillinase - labile penicillins but susceptible to Penicillinase - stable penicillin
R to both P and OX/CX - that means R to all beta lactams except cephem with anti MRSA activity eg: Ceftaroline
Va - MIC is important
E & CD - also look for inducible resistance
You will find what you want in the updated version of CLSI 2016 document (attached file).
Regards
There is a free version of an updated CLSI M100 available for free online. Look it up.
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