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Linezolid MIC creep among clinically significant S. aureus: a retrospective eight-year study
Chinchana Shylaja Eshwarappa1, Veerabhadra Swamy Gs1, Supreetha R Shettar1
1Department of Microbiology, JSS Medical College and Hospital, JSS AHER, Mysuru, India.
Introduction:
The rising threat of antimicrobial resistance, particularly the phenomenon of MIC creep, poses a significant challenge to the clinical effectiveness of key antibiotics. Linezolid is a crucial last-resort agent for treating severe infections caused by Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). This study aimed to evaluate the temporal trends of linezolid minimum inhibitory concentrations (MICs) in clinical S. aureus isolates at a tertiary care hospital over an eight-year period.
Methods:
A retrospective study was conducted on clinical S. aureus isolates over eight years from 2017-2024 in a tertiary care hospital. MICs were determined using the VITEK-2 system and interpreted according to CLSI guidelines. Data were analyzed using IBM SPSS Statistics version 20.0.
Results:
From 2017 to 2024, A total of 4,325 S. aureus isolates were analyzed, comprising 2,682 (62.01%) MSSA and 1,643 (37.98%) MRSA. The mean linezolid MIC for MSSA increased from 1.41 ± 0.53 µg/mL (2017) to 1.72 ± 0.45 µg/mL (2024), and for MRSA from 1.34 ± 0.47 µg/mL (2017) to 1.78 ± 0.42 µg/mL (2024). A progressive rightward shift in MIC distribution from 1 µg/mL to 2 µg/mL was observed in both groups. Spearman's rank correlation demonstrated a significant positive trend (MSSA: rs = 0.893, p = 0.001; MRSA: rs = 0.857, p = 0.001), consistent with the "MIC creep." All isolates remained within the CLSI susceptibility breakpoint (≤ 4 µg/mL), and no high-level resistance was detected.
Conclusion:
Linezolid continues to be highly effective; however, the progressive MIC creep from 1 µg/mL to 2 µg/mL indicates emerging reduced susceptibility. These findings stress the need for ongoing MIC surveillance and strict antibiotic stewardship to preserve linezolid efficacy.
Insights
Linezolid minimum inhibitory concentrations (MICs) for Staphylococcus aureus showed a concerning upward trend, indicating emerging resistance. Continuous monitoring and antibiotic stewardship are crucial to maintain linezolid
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Antimicrobial resistance (AMR) is a growing global health threat.
- Linezolid is a critical last-resort antibiotic for Gram-positive infections, including MRSA.
- MIC creep, a gradual increase in MICs, can reduce antibiotic effectiveness.
Purpose of the Study:
- To analyze temporal trends in linezolid MICs for Staphylococcus aureus isolates.
- To assess the prevalence of MIC creep over an eight-year period.
- To evaluate the impact on linezolid's clinical utility.
Main Methods:
- Retrospective analysis of 4,325 S. aureus isolates from 2017-2024.
- MICs determined using VITEK-2 system and interpreted per CLSI guidelines.
- Statistical analysis using Spearman's rank correlation.
Main Results:
- Mean linezolid MIC increased for both MSSA and MRSA isolates.
- A shift in MIC distribution towards higher values (1-2 µg/mL) was observed.
- Significant positive trends in MICs confirmed MIC creep (p < 0.001).
Conclusions:
- Linezolid remains effective, but emerging reduced susceptibility is evident.
- The observed MIC creep necessitates ongoing surveillance.
- Strict antibiotic stewardship is vital to preserve linezolid efficacy.
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