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

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.


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.


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.


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


A. Sabin, None

B. Ford, None

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