Thermonuclease test accuracy is preserved in methicillin-resistant Staphylococcus aureus isolates

Area of Science:

• Clinical Microbiology
• Bacteriology
• Diagnostic Testing

Background:

• The thermonuclease test targets the nuc gene, present in Staphylococcus aureus but absent in coagulase-negative staphylococci (CoNS).
• Previous studies noted nuc gene variations affecting PCR, but the impact of methicillin resistance on thermonuclease enzyme activity was unclear.

Purpose of the Study:

• To evaluate the sensitivity and specificity of the thermonuclease test for differentiating S. aureus from CoNS in blood cultures.
• To assess the thermonuclease test's performance differences between methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) isolates.

Main Methods:

• Retrospective analysis of 1404 bacterial isolates from positive blood cultures.
• Initial identification via microscopy and the thermonuclease test, followed by VITEK systems and cefoxitin disc diffusion for confirmation.

Main Results:

• The thermonuclease test demonstrated a sensitivity of 93.1% and specificity of 98.9% for S. aureus identification.
• For MRSA isolates, the test showed 88.9% sensitivity.
• Overall accuracy for MSSA and MRSA isolates was high, at 98.1% and 98.8%, respectively.

Conclusions:

• The thermonuclease test is a reliable, cost-effective, and simple method for the presumptive identification of S. aureus from blood cultures.
• Its diagnostic value persists even in the context of increasing molecular diagnostic methods and irrespective of methicillin resistance status.