Disc Test for Detecting Staphylococcus aureus Strains Producing Type A and Type C β-Lactamases.

Area of Science:

• Clinical Microbiology
• Antimicrobial Resistance
• Diagnostic Test Development

Background:

• Staphylococcus aureus produces beta-lactamases that hydrolyze penicillins and cephalosporins.
• Type A and C beta-lactamase-producing S. aureus (TAPSA and TCPSA) can exhibit a cefazolin inoculum effect (CIE), leading to treatment failure.
• Current laboratory methods often fail to detect strains with CIE.

Purpose of the Study:

• To develop a straightforward, high-performing beta-lactamase disc test for routine laboratories.
• To identify and differentiate TAPSA and TCPSA strains.
• To assess the test's potential in predicting cefazolin treatment failure risk.

Main Methods:

• Clinical isolates of penicillin-resistant S. aureus were analyzed.
• blaZ genes were sequenced, and Minimum Inhibitory Concentrations (MICs) were determined at low and high inocula.
• A semimechanistic model and receiver operating characteristic (ROC) curve analysis were used to develop differentiation models.

Main Results:

• Genetic analysis identified 26 TAPSA and 45 TCPSA isolates among 99 penicillin-resistant S. aureus.
• A cefazolin-to-cephalothin ratio model differentiated TAPSA with 96.2% sensitivity and 98.6% specificity.
• A model using cefazolin, cephalothin, and oxacillin differentiated TCPSA with 88.6% sensitivity and 96.6% specificity.

Conclusions:

• A simple three-disc agar plate test can differentiate TAPSA and TCPSA.
• This test can identify S. aureus isolates associated with CIE and potential cefazolin treatment failure.
• The method is suitable for routine diagnostic laboratory workflows.