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 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.

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 (1939 genes) of 43 MSSA and one S. argenteus isolate (outgroup). Support bootstrap values are showed 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 (CFZ MIC)

Fig. 4.. Count matrix of the core SNPs among colonizing MSSA isolates.

Core SNPs of the 43 MSSA analyzed (30927 SNPs). MSSA pairs are indicated by white blocks and MSSA isolates with CzIE in red. Cells are colored according to SNPs count between two MSSA: Green, MSSA genomes with < 20 SNPs difference; yellow to orange scale, MSSA genomes with ≥ 20 SNPs likely representing a different strain. Day 0 correspond to admission sampling.