A Test for the Rapid Detection of the Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus

Abstract

The cefazolin inoculum effect (CzIE) has been associated with therapeutic failures and mortality in invasive methicillin-susceptible Staphylococcus aureus (MSSA) infections. A diagnostic test to detect the CzIE is not currently available. We developed a rapid (∼3 h) CzIE colorimetric test to detect staphylococcal-β-lactamase (BlaZ) activity in supernatants after ampicillin induction. The test was validated using 689 bloodstream MSSA isolates recovered from Latin-America and US. Cefazolin MIC determination at high inoculum (107 CFU/mL) was used as reference standard (cut-off of ≥16 μg/mL). All isolates underwent genome sequencing. A total of 257 (37.3%) MSSA exhibited the CzIE by the reference standard method. The overall sensitivity and specificity of the colorimetric test was 82.5% and 88.9%, respectively. Sensitivity in MSSA isolates harboring type A BlaZ (most efficient enzyme against cefazolin) was 92.7% with a specificity of 87.8%. The performance of the test was lower against type B and C enzymes (sensitivities of 53.3% and 72.3%, respectively). When the reference value was set to ≥32 μg/mL the sensitivity for isolates carrying type A enzymes was 98.2%. Specificity was 100% for MSSA lacking blaZ. The overall negative predictive value ranged from 81.4% to 95.6% in Latin-American countries using published prevalence rates of the CzIE. MSSA from US were genetically diverse, with no distinguishing genomic differences from Latin-American MSSA, distributed among 18 sequence types. A novel test can readily identify most MSSA isolates exhibiting the CzIE, particularly those carrying type A BlaZ. In contrast to the MIC determination using high-inoculum, the rapid test is inexpensive, feasible and easy to perform. After minor validation steps, it could be incorporated into the routine clinical laboratory workflow.

Genomic epidemiology ofS. aureusisolated from bloodstream infections in South America during 2019 supports regional surveillance

“…Although cefazolin is a common treatment option for MSSA bacteraemia, older South American bloodstream MSSA isolates carrying BlaZ types A and C have previously exhibited a high frequency of the cefazolin inoculum effect, which is associated with treatment failure and increased mortality [83,87]. Almost half of the MSSA genomes analysed here, including CC398, harboured BlaZ type B (113/239, 47.3%) (Suppl Table 5), which does not lead this phenotype [87,88]. However, a high proportion of genomes from other CCs carried BlaZ type A or C (90/239, 37.7%) (Suppl Table 5), especially so in Bolivia (18/31, 58.1%) where CC398 is not as prevalent.…”

Clinical significance of cefazolin inoculum effect in serious MSSA infections: a systematic review

“…Diagnostic properties are summarized in Table 3 . The rapid colorimetric test specifically designed for detection of CzIE 28 had the best combination of sensitivity and specificity based on the point estimate and CI.…”

Genomic epidemiology of Staphylococcus aureus isolated from bloodstream infections in South America during 2019 supports regional surveillance

“…Although cefazolin is a common treatment option for MSSA bacteraemia, older South American bloodstream MSSA isolates carrying BlaZ types A and C have previously exhibited a high frequency of the cefazolin inoculum effect, which is associated with treatment failure and increased mortality [83, 87 ]. Almost half of the MSSA genomes analysed here, including CC398, harboured BlaZ type B (113/239, 47.3 %) (Table S6), which does not lead to this phenotype [87, 88]. However, a high proportion of genomes from other CCs carried BlaZ type A or C (90/239, 37.7 %) (Tale S6), especially in Bolivia (18/31, 58.1 %) where CC398 is not as prevalent.…”