Positive in vitro interactions with β-lactam linked to lower 14-day mortality in MRSA bacteraemia

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Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases Published May 1, 2026 Medically reviewed May 8, 2026 Study authors: Soo Joel Z Y, Lim Tze-Peng, Ho Jayden J Y, Tong Steven Y C, Lye David C, Davis Joshua S, Kwa Andrea … PubMed ↗ DOI ↗ By Dr. Amelia Tan, PhD · Internal Medicine & Chronic Disease

Key Takeaway

Consider that positive in vitro interactions may predict early benefit from combination therapy in MRSA bacteraemia, but findings are hypothesis generating.

This post hoc analysis of a multicentre randomized trial evaluated the association between in vitro interaction results and clinical outcomes in 150 adults with MRSA bacteraemia who received combination therapy with vancomycin or daptomycin plus an antistaphylococcal β-lactam. Of 174 combination-therapy patients, 24 were excluded. Patients were categorized based on positive (synergy or additivity) or negative (antagonism or indifference) in vitro interactions.

The primary outcome, 90-day mortality, did not differ significantly between groups: 34% (16 of 47) in the positive interaction group vs. 32% (33 of 103) in the negative interaction group (p=0.81). However, 14-day all-cause mortality was significantly lower in the positive interaction group: 2.9% (3 of 103) vs. 12.8% (6 of 47) (p=0.03). Persistent bacteraemia at day 2 was numerically higher in the positive interaction group (32.0% vs. 19.1%, p=0.10).

Safety data were not reported. The authors note that these findings should be interpreted with caution and are considered hypothesis generating. Combination therapy may be beneficial when positive interactions are present but is not universally effective. Synergy testing may help optimize combination therapy, but confirmation in a randomized trial is warranted.

Limitations include the post hoc design and small sample size. The association between positive in vitro interactions and lower 14-day mortality does not imply causation. Clinicians should consider these results as preliminary and await further evidence before changing practice.

Study Details

Study typeRct

Sample sizen = 150

EvidenceLevel 2

PublishedMay 2026

PMID41679464

View Original Abstract ↓

OBJECTIVE: Combination antibiotic therapy may improve treatment success for bacterial infections, but bacterial strain-to-strain variability in synergy and its clinical impact remains unclear. This study evaluated whether positive in vitro interactions (synergy or additivity) between vancomycin or daptomycin and a β-lactam in methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infections are associated with improved clinical outcomes in the Combination Antibiotic for Methicillin Resistant Staphylococcus aureus (CAMERA2) trial. METHODS: In this post hoc analysis of the multicentre randomized CAMERA2 trial, adults with MRSA bacteraemia were randomly assigned to standard care (vancomycin or daptomycin) or combination therapy with an antistaphylococcal β-lactam. Of 174 combination-therapy patients, 24 were excluded for unavailable isolates or missing outcome data, leaving 150 for analysis. Synergy testing was performed centrally, post hoc, using a microdilution checkerboard assay, stratifying patients into positive interaction (synergy/additivity) and negative interaction (antagonism/indifference) groups. Clinical outcomes-including primary composite end point of 90-day mortality-were compared between groups, using statistical tests and multivariate regression adjusting for confounders. RESULTS: Among 150 patients, 103 were in the positive interaction group and 47 in the negative interaction group. Patient characteristics were similar. The primary composite endpoint of 90-day mortality (34% [16 of 47] vs. 32% [33 of 103], p 0.81) did not differ significantly between groups. Persistent bacteraemia rate at day 2 was higher (32.0% [33 of 103] vs. 19.1% [9 of 47], p 0.10) in the positive interaction group. However, 14-day all-cause mortality was significantly lower in the positive interaction group (2.9% [3 of 103] vs. 12.8% [6 of 47], p 0.03). CONCLUSIONS: Positive interactions between vancomycin or daptomycin and a β-lactam were associated with lower 14-day mortality in MRSA bacteraemia. However, these findings should be interpreted with caution and considered hypothesis generating. Combination therapy may be beneficial when positive interactions are present, not universally effective. Synergy testing may help optimize combination therapy, warranting confirmation in a randomized trial.