The microenvironment in antibiotic susceptibility testing.

Area of Science:

• Microbiology
• Clinical Pharmacy
• Infectious Diseases

Background:

• Standard antibiotic susceptibility testing (AST) methods, such as agar diffusion, are widely used to guide antibiotic treatment.
• Mueller-Hinton agar (MHA) is the recommended medium for AST, but its composition differs significantly from human interstitial fluid.
• Human cells do not thrive in MHA, raising questions about its suitability for accurately reflecting in vivo conditions.

Purpose of the Study:

• To evaluate the impact of different media on AST results for Pseudomonas aeruginosa.
• To compare the efficacy of RPMI 1640 medium versus MHA for azithromycin susceptibility testing.
• To investigate the potential for alternative media to detect resistance mechanisms not identified by standard AST.

Main Methods:

• Antibiotic susceptibility testing of Pseudomonas aeruginosa against azithromycin was performed using both Mueller-Hinton agar (MHA) and RPMI 1640 medium.
• Minimal inhibitory concentrations (MICs) were determined for each medium.
• The study considered the known mechanisms of azithromycin resistance in cystic fibrosis patients.

Main Results:

• Use of RPMI 1640 medium resulted in lower minimal inhibitory concentrations (MICs) for azithromycin compared to MHA.
• RPMI 1640 medium enhanced azithromycin uptake and reduced its efflux by Pseudomonas aeruginosa.
• Standard MHA-based AST may fail to detect mutational resistance developing during azithromycin treatment in cystic fibrosis patients.

Conclusions:

• The composition of the growth medium significantly impacts AST results, potentially affecting clinical predictions.
• RPMI 1640 medium offers a more physiologically relevant environment for AST, improving accuracy for certain antibiotic-bacteria combinations.
• Further research is crucial to explore alternative AST media and their ability to detect emerging resistance during antibiotic therapy across various pathogens and drugs.