Linezolid MIC creep among clinically significant S. aureus: a retrospective eight-year study

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.

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.

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: r_s  = 0.893, p = 0.001; MRSA: r_s  = 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.

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.