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When the Medium Skews the Message: Commercial Mueller–Hinton Agar Media and Their Effect on Antibiogram Results of Imipenem, Ciprofloxacin, and Gentamicin Against Standard and Clinical Strains of Pseudomonas aeruginosa

Year: 2026

Type: article

Abstract: Accurate antimicrobial susceptibility testing (AST) is essential for guiding effective therapy and monitoring antibiotic resistance, particularly for Pseudomonas aeruginosa , a pathogen noted for its intrinsic and adaptive resistance mechanisms. Mueller–Hinton agar (MHA) is the standard medium for AST; however, variations in its composition among commercial brands can significantly affect test outcomes. This study evaluated the influence of five different commercial MHAs (HiMedia, Merck, Condalab, Quelab, and Sigma) on the antibiogram results of imipenem, ciprofloxacin, and gentamicin against both standard ( P. aeruginosa ATCC 15442) and 50 clinical isolates. Three inoculation methods, swab, pour plate, and double‐layer, were compared with identify the most reproducible approach. Inductively coupled plasma mass spectrometry (ICP‐MS) was used to quantify divalent cations (Ca 2+ , Mg 2+ , Zn 2+ , Mn 2+ , Fe 2+ ) in each medium, and microscopic examination assessed gel microstructure. The double‐layer method demonstrated the lowest coefficient of variation, indicating superior reproducibility compared with the swab and pour plate techniques. Significant differences were observed among media in inhibition zone diameters, with Quelab and Condalab producing consistently smaller zones ( p < 0.05). Cation analysis revealed wide variability, with calcium and magnesium levels differing up to fourfold across brands and being associated with differences in antibiotic activity, particularly for gentamicin and ciprofloxacin. Microscopic evaluation revealed minor gel‐structure variations without measurable impact on antibiotic diffusion. Susceptibility profiles also differed among MHA brands. These findings highlight the potential importance of medium composition, particularly cation concentration, in the variability of AST results. Standardization and rigorous quality control of commercial MHAs are essential to ensure reliable, reproducible, and clinically meaningful susceptibility testing.

Source: International Journal of Microbiology

DOI: 10.1155/ijm/1638304

Authors: Amir Reza Abdollahi, Seyed Sadeq Mousavi Ghahfarrokhi, Fatemeh Aghajafari, Hossein Jafari-Nodoushan, Marjan Tootianquds, Nasrin Samadi

Institutions: Tehran University of Medical Sciences, Universal Scientific Education and Research Network, Shahid Sadoughi University of Medical Sciences and Health Services

Language: English

Topic: Bacterial Identification and Susceptibility Testing

Subfield: Microbiology (medical)

Field: Medicine

Domain: Health Sciences

SDG: Good health and well-being

Open Access status: gold

Estimated APC paid: $1,000