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.