1003. Characterization of Beta-Lactamase Activity in Staphylococcus aureus Using MALDI-TOF Mass Spectrometry
Session: Poster Abstract Session: Diagnostic Microbiology: Staphylococci
Friday, October 9, 2015
Room: Poster Hall
Background:

Staphylococcus aureus infections remain a serious threat to public health and are complicated by frequent resistance to beta-lactam antibiotics. Rapid and accurate detection of beta-lactamase activity allows treatment of methicillin-susceptible S. aureus infections with beta-lactamase-labile cefazolin. MALDI-TOF mass spectrometry has emerged as a viable technology for identification of pathogenic organisms, as well as a direct means of observing beta-lactamase activity. Most work to date in characterizing beta-lactamase activity via MALDI-TOF has focused on Gram negative organisms. We evaluated a MALDI-TOF method for detecting beta-lactamase activity in Staphylococcus aureus clinical isolates.

Methods:

We analyzed 110 clinical isolates of Staphylococcus aureus for beta-lactamase activity (blaZ genes) using MALDI-TOF MS to detect hydrolysis of beta lactam substrates.  Strains were designated as either blaZ(+) or blaZ(-) via gold-standard PCR for four S. aureus beta lactamases. Standardized S. aureus inocula were incubated for two hours with ampicillin, penicillin, or cefazolin prior to MALDI-TOF analysis of their supernatants to detect products of beta lactamase activity. A ratio of peak heights comparing the hydrolysis products to parent molecules was constructed to permit differentiation by blaZ status.  Performance of MALDI-TOF was further compared to penicillin MIC testing, which is a common clinical test for beta lactamase activity in S. aureus.

Results:

Of 108 isolates analyzed with each of three antibiotics, determination of blaZ status was confirmed in 99% of all isolates by MALDI-TOF MS. Errors in detection were attributed to inadequate ratios of hydrolysis products to parent molecules. Ampicillin provided the best discrimination between blaZ(-) and blaZ(+) as compared to PCR results.  Penicillin MIC testing was comparable to MALDI-TOF analysis with sensitivity/specificity of 80% and 95% respectively.

Conclusion:

MALDI-TOF mass spectrometry is a simple, cost-effective, and rapid method for identification of beta-lactamase activity in clinical isolates of Staphylococcus aureus, using clinical-standard MALDI-TOF equipment and inexpensive laboratory reagents. This method may serve as a simple and efficient alternative to existing PCR-based and phenotypic methods.

Arick Sabin, DO, MPH, Internal Medicine, University of Minnesota, Minneapolis, MN and Bradley Ford, MD, PhD, Pathology, University of Iowa, Iowa City, IA

Disclosures:

A. Sabin, None

B. Ford, None

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