Optimization and improvement of the disc diffusion method for mucoid Pseudomonas aeruginosa

OBJECTIVES: Antibiotic susceptibility test for mucoid Pseudomonas aeruginosa is challenging to conduct using the disc diffusion method based on Mueller-Hinton agar (MHA) due to its slow growth. The aim of this study is to compare the suitability of blood MHA versus MHA for disc diffusion antimicrobial susceptibility testing of mucoid P. aeruginosa and to determine the optimal reporting time. METHODS: The study employed blood MHA for the disc diffusion method, using MHA as a control, and broth microdilution as a reference method. We tested 60 mucoid P. aeruginosa strains on both media types, with results assessed at 16, 18, 20, 24 and 48 hours. RESULTS: The findings indicated that the interpretive categories determined by using blood MHA were comparable to those obtained with standard MHA. Notably, compared with the reference method, the categorical agreement for the blood MHA at multiple time points was significantly better than that for the standard MHA. For most tested antibiotics, results could be reliably interpreted at 20-24 hours by using blood MHA, with 24 hours being the optimal interpretation time. Results for quinolones particularly levofloxacin, were unacceptable when compared with reference methods. Additionally, the study found that blood agar and chocolate agar could help reduce the expression of mucus and accelerate its growth rate. CONCLUSIONS: In conclusion, the blood MHA can serve as a substitute for the MHA, with an optimal interpretation time of 24 hours.