# Clinical Impact of the Cefazolin Inoculum Effect

> **NIH NIH R01** · METHODIST HOSPITAL RESEARCH INSTITUTE · 2024 · $641,584

## Abstract

ABSTRACT
Staphylococcus aureus is a major human pathogen responsible for a wide range of life-threatening infections.
Many of these infections are caused by methicillin-susceptible S. aureus (MSSA). MSSA represent a major
burden among S. aureus infections and are important contributors to mortality. For decades, the first line of
therapy for severe MSSA infections have been the isoxazolyl-penicillins (ISP, e.g., nafcillin). However, recent
data suggest that clinical outcomes in MSSA bacteremia are similar in patients treated with cefazolin (vs
nafcillin), a cephalosporin with activity against MSSA that appears to be less toxic. Indeed, treatment with nafcillin
seems to be associated with increased costs, more drug reactions (including hepatotoxicity, interstitial nephritis
and neutropenia) and, possibly, higher mortality. Due to these concerns, an important shift in the treatment of
MSSA is occurring whereby clinicians are now using cefazolin as first line of therapy for severe MSSA
infections. An important concern of using cefazolin and other cephalosporins as primary therapy for these serious
infections is the cefazolin inoculum effect (CzIE), defined as a cefazolin minimal inhibitory concentration of >
16 µg/ml when a high inoculum (107 CFU/ml) is used. The CzIE has been associated with failures in the treatment
of deep-seated MSSA infections and with the production of certain isotypes of the staphylococcal β-lactamase.
However, the characterization of the clinical impact of this phenomenon in deep-seated MSSA infections
is limited. In addition, it is currently not possible to detect the CzIE in a standard clinical microbiology laboratory
given the cumbersome and expensive nature of the gold standard test for its detection. Our published and
preliminary clinical data suggest that the CzIE is an important contributor to worse clinical outcomes of severe
MSSA infections. Furthermore, we have developed and published a novel colorimetric nitrocefin-based rapid test
(~3 h) that detects the CzIE with high sensitivity and specificity that can be incorporated in the routine clinical
microbiology laboratory. We postulate that, i) the CzIE negatively impacts clinical outcomes in MSSA bacteremia
treated with cefazolin and, ii) a rapid test can be readily implemented for the identification of the CzIE in S.
aureus bacteremia and can detect patients at higher risk of poor outcomes. In order to address these hypotheses,
we will take advantage of the Staphylococcus aureus Network Adaptive Platform (SNAP) trial, a multicenter,
pragmatic, multi-arm, open-label adaptive platform trial addressing multiple therapeutic questions in patients with
S. aureus bacteremia. We will focus in the MSSA “domain” that evaluates the effectiveness and safety of
cefazolin vs ISP in a randomized fashion, currently enrolling in Australia, Singapore, Canada, Israel, New
Zealand, United Kingdom, United States, Colombia and Chile. The specific aims of our proposal are: i) to define
t...

## Key facts

- **NIH application ID:** 10857198
- **Project number:** 5R01AI173138-02
- **Recipient organization:** METHODIST HOSPITAL RESEARCH INSTITUTE
- **Principal Investigator:** Cesar Augusto Arias
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $641,584
- **Award type:** 5
- **Project period:** 2023-06-05 → 2028-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10857198

## Citation

> US National Institutes of Health, RePORTER application 10857198, Clinical Impact of the Cefazolin Inoculum Effect (5R01AI173138-02). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10857198. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*