Staphylococcus aureus Strains With a Negative Coagulase Tube Test are Associated With Staphylocoagulase Genotypes

Area of Science:

• Microbiology
• Clinical Diagnostics
• Genomics

Background:

• Staphylococcus aureus is a major human pathogen requiring accurate identification.
• Differentiating S. aureus from coagulase-negative staphylococci is crucial in diagnostics.
• The coagulase tube test is a preliminary identification method, but can yield false negatives.

Purpose of the Study:

• To investigate why some S. aureus isolates test negative in the coagulase tube test.
• To determine if specific coagulase (coa) genotypes are associated with false-negative results.
• To assess the clinical impact of false-negative coagulase tube test results.

Main Methods:

• Whole genome sequencing was used to determine coa genotypes of 122 clinical S. aureus bloodstream isolates.
• Analysis of genomic regions flanking the coa gene for regulatory differences.
• Reverse transcription quantitative real-time PCR (RT-qPCR) assessed coa gene expression in a subset of isolates.

Main Results:

• All 122 isolates possessed the coa gene; 18.9% tested negative in the coagulase tube test.
• No association was found between negative tube test results and complicated bloodstream infections.
• Staphylocoagulase types X and XI isolates were more likely to yield negative tube test results despite having functional coa genes.

Conclusions:

• Specific S. aureus lineages, particularly ST15 and ST3911 (staphylocoagulase genotype X), may test negative in the coagulase tube test.
• The observed negativity is attributed to the inability of certain coagulase types to clot the commercial test substrate.
• This has significant implications for the reliability of the coagulase tube test in routine diagnostics.