Prevalence of the Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus Isolates Collected from Kingston Health Sciences Centre.
Bibliographic record
Abstract
Background: Staphylococcus aureus is a bacterium that causes serious human infections, including bloodstream infections, pneumonia, deep abscesses, and bone and joint infections. Methicillin-susceptible S. aureus (MSSA) are S. aureus bacteria susceptible to the antibiotic methicillin. Cefazolin is the first-line antibiotic to treat MSSA infections. The cefazolin inoculum effect (CzIE) is a lab phenomenon in which S. aureus is susceptible to cefazolin at a standard bacterial inoculum but develops resistance at a higher cell inoculum. CzIE occurs when the bacteria produce beta-lactamases that degrade cefazolin, preventing the antibiotic from inhibiting cell wall synthesis and allowing bacterial growth. In theory, CzIE may lead to cefazolin treatment failure, but currently, there is insufficient evidence in the literature. Objective: We aimed to determine the prevalence of the CzIE in patients with serious MSSA infections at Kingston Health Sciences Centre (KHSC). Methods: Clinical MSSA isolates from patients with serious infections at KHSC were collected. The MSSA isolates were then tested for CzIE using broth microdilution testing at a standard inoculum (5×10^5 CFU/mL) and a high inoculum (5×10^7 CFU/mL). The CzIE was defined as a ≥ 4-fold increase in minimum inhibitory concentration (MIC) from the standard to the high and with a MIC above the resistant breakpoint of 16 µg/mL. Results: Of the 51 MSSA isolates tested to date, 17 (33%) displayed CzIE, with 13 of the 17 (76%) requiring 64 µg/ml of antibiotic to prevent growth. Conclusion: Our initial testing indicates that the CzIE is common in MSSA infections at KHSC. We will complete testing 100 MSSA isolates for a more precise estimate of CzIE prevalence, which will be followed up with a chart review of the patients to determine if there is a correlation between CzIE and cefazolin treatment failure.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".