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Peer-reviewedCC BY 4.0International Journal of Microbiology

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

  • Amir Reza Abdollahi1
  • Seyed Sadeq Mousavi Ghahfarrokhi1,2,3
  • Fatemeh Aghajafari4
  • Hossein Jafari-Nodoushan2,5
  • Marjan Tootianquds6
  • Nasrin Samadi1,2
6 affiliations
  1. 1Department of Drug and Food Control Faculty of Pharmacy Tehran University of Medical Sciences Tehran Iran tums.ac.ir
  2. 2Research Group for Quality Control and Development of Pharmaceutical Food and Cosmeticeutical Products The Institute of Pharmaceutical Sciences (TIPS) Tehran University of Medical Sciences Tehran Iran tums.ac.ir
  3. 3Universal Scientific Education and Research Network (USERN) Tehran Iran usern.org
  4. 4Department of Drug and Food Control Faculty of Pharmacy and Biotechnology Research Center International Campus Tehran University of Medical Sciences (IC-TUMS) Tehran Iran
  5. 5Department of Pharmaceutics School of Pharmacy Shahid Sadoughi University of Medical Sciences and Health Services Yazd Iran ssu.ac.ir
  6. 6Diamond New Age Sağlık Gıda Turizm Sanayi Ticaret İthalat İhracat Limited Şirketi Ankara Turkey
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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 (Ca2+, Mg2+, Zn2+, Mn2+, Fe2+) 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.

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Cite

BibTeXRIS
BibTeX
@article{abdollahi2026when,
  title = {{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}},
  author = {Amir Reza Abdollahi and Seyed Sadeq Mousavi Ghahfarrokhi and Fatemeh Aghajafari and Hossein Jafari-Nodoushan and Marjan Tootianquds and Nasrin Samadi},
  journal = {International Journal of Microbiology},
  year = {2026},
  month = nov,
  doi = {10.1155/ijm/1638304},
  url = {https://doi.org/10.1155/ijm/1638304},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - 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
AU  - Amir Reza Abdollahi
AU  - Seyed Sadeq Mousavi Ghahfarrokhi
AU  - Fatemeh Aghajafari
AU  - Hossein Jafari-Nodoushan
AU  - Marjan Tootianquds
AU  - Nasrin Samadi
PY  - 2026
DA  - 2026/11/09
JO  - International Journal of Microbiology
DO  - 10.1155/ijm/1638304
UR  - https://doi.org/10.1155/ijm/1638304
AB  - 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 (Ca2+, Mg2+, Zn2+, Mn2+, Fe2+) 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.
SN  - 1687-918X
SN  - 1687-9198
ER  -