A fluorogenic biotracer for reliable detection of staphylococcal beta-lactamase: towards improved precision therapy in severe MSSA infections.

BACKGROUND AND OBJECTIVES: Accurate detection of staphylococcal beta-lactamase remains a diagnostic challenge, despite its importance for guiding antibiotic therapy, particularly in severe infections, such as endocarditis and bacteraemia. Currently, no available method reliably detects beta-lactamase activity in all staphylococcal species. Here, we evaluated a novel fluorogenic biotracer, 352600-C (MOLSID, Lyon, France), designed to detect beta-lactamase activity in staphylococci. METHODS: The 352600-C biotracer was incorporated into Mueller-Hinton agar plates onto which staphylococcal strains were subcultured at 37°C. Colony fluorescence was assessed at 4, 6 and 24 h using a 365 nm UV lamp. The assay was performed on a large collection of blaZ-positive (n = 124, including BlaZ types A, B, C and D) and blaZ-negative (n = 69) staphylococci from 18 species (Staphylococcus aureus, n = 100; S. non-aureus, n = 93). Molecular detection of the blaZ gene using WGS served as the gold standard. Phenotypic susceptibility to penicillins was also tested using EUCAST recommendations. RESULTS: After 4, 6 and 24 h of incubation, the assay showed sensitivities of 92.2%, 96.9% and 100% for S. aureus and 50.0%, 88.3% and 100% for non-aureus staphylococci, respectively. None of the 69 blaZ-negative strains produced fluorescent colonies (specificity 100%). Notably, the assay detected all BlaZ types. Penicillin disc diffusion showed 98.4% sensitivity and 77.8% specificity for MSSA. CONCLUSIONS: This novel assay for the detection of staphylococcal beta-lactamase demonstrated simplicity, rapidity and excellent accuracy after 24 h incubation (100% sensitivity and specificity) across a genetically diverse and representative collection of clinical isolates, making it a valuable tool for routine diagnostics and antimicrobial stewardship.