The cefazolin inoculum effect (CzIE) has been associated with poor clinical outcomes in patients with MSSA infections. We aimed to investigate the point prevalence of the CzIE among nasal colonizing MSSA isolates from ICU patients in a multicenter study in Colombia (2019-2023). Patients underwent nasal swabs to assess for S. aureus colonization on admission to the ICU and some individuals had follow-up swabs. We performed cefazolin MIC by broth-microdilution using standard and high-inoculum and developed a modified nitrocefin-based rapid test to detect the CzIE. Whole genome sequencing was carried out to characterize BlaZ types and allotypes, phylogenomics and Agr-typing. All swabs were subjected to 16S-rRNA metabarcoding sequencing to evaluate microbiome characteristics associated with the CzIE. A total of 352 patients were included; 46/352 (13%) patients were colonized with S. aureus; 22% (10/46) and 78% (36/46) with MRSA and MSSA, respectively. Among 36 patients that contributed with 43 MSSA colonizing isolates, 21/36 (58%) had MSSA exhibiting the CzIE. BlaZ type A and BlaZ-2 were the predominant type and allotype in 56% and 52%, respectively. MSSA belonging to CC30 were highly associated with the CzIE and SNP analyses supported transmission of MSSA exhibiting the CzIE among some patients of the same unit. The modified nitrocefin rapid test had 100%, 94.4% and 97.7% sensitivity, specificity and accuracy, respectively. We found a high prevalence point prevalence of the CzIE in MSSA colonizing the nares of critically-ill patients in Colombia. A modified rapid test was highly accurate in detecting the CzIE in this patient population.

The authors have declared no competing interest.

This work was supported by Ministerio de Ciencia, Tecnologia e Innovacion, grant 130880764152, CT 776 2018 to J.R. and Universidad El Bosque, Convocatoria Interna grant PCI 2023 0002 to J.R. CAA is supported by National Institutes of Health (NIH)/National Institute of Allergy and Infectious Diseases (NIAID) grants K24AI121296, R01AI134637, R01AI148342 01, and P01AI152999.

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Ethics committee/IRB of Universidad El Bosque gave the ethical approval for this work under code of the protocol 476-2018.

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The genomes of the Methicillin-Susceptible Staphylococcus aureus isolates, and the nasal microbiome sequencing were submitted to the NCBI GenBank database under the BioProject numbers PRJNA1107600 and PRJNA1108236, respectively.