Prevalence of the cefazolin inoculum effect (CzIE) in nasal colonizing methicillin-susceptible Staphylococcus aureus in patients from intensive care units in Colombia and use of a modified rapid nitrocefin test for detection

Abstract

The cefazolin inoculum effect (CzIE) has been associated with poor clinical outcomes in patients with methicillin-susceptible Staphylococcus aureus (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. A total of 352 patients were included; 46/352 (13%) patients were colonized with S. aureus and 22% (10/46) and 78% (36/46) with MRSA and MSSA, respectively. Among 36 patients who 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 single-nucleotide polymorphism (SNP) analyses supported possible 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 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.

Keywords: Colombia; MSSA; cefazolin; inoculum effect; intensive care unit.

Fig 1

Modified protocol of the nitrocefin rapid test to identify MSSA with the CzIE.

Fig 2

Timeline of sampling and isolate recovery in ICU patients colonized by S. aureus (MSSA or MRSA). Sampling is denoted by circles; filled circles represent nasal swabs positive for MSSA (red) or MRSA (blue); empty circles indicate samples negative for S. aureus. Circles with lines across indicate isolates positive for the CzIE. Patient B74-046 and E76-005 had two isolates recovered in the same swab, one displaying the CzIE and the other lacking the CzIE.

Fig 3

Phylogenetic tree of colonizing methicillin-susceptible S. aureus (MSSA) isolates recovered from ICU patients in Colombian hospitals. Maximum likelihood phylogenetic tree from the core genome (1,939 genes) of 43 MSSA and one S. argenteus isolate (outgroup). Support bootstrap values are shown for each clade. Colored shadows over the tree branches show the clonal complexes (CC) within the sample, followed by sequence type (ST) and agr type. The CzHI was determined by gold standard (cefazolin MIC determination)