Assessment of Omitting MacConkey Agar as a Primary Inoculating Medium for MALDI-TOF MS-Based Bacterial Identification from Urine, Blood, and Respiratory Samples.

OBJECTIVE: MacConkey agar (MAC) is commonly used as a primary medium for conventional bacterial identification in clinical microbiology laboratories. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) has revolutionized microbial identification and is considered a reliable identification tool. While conventional identification methods rely on colony characteristics, MALDI-TOF MS requires a pure isolate on a solid medium.

METHODS: This study investigated whether MAC can be omitted as a routine inoculation medium for urine, lower respiratory tract (LRT), and positive blood culture samples. The study included 462 clinical samples. Among these, 221 were urine samples, 141 were positive blood cultures, and the remaining 100 were LRT samples. The samples were inoculated on blood agar (BA) and MAC for the control group and on BA only for the experimental group, followed by incubation and identification with MALDI-TOF MS.

RESULTS: The BA only group showed the same microbial identification using MALDI-TOF MS as the control BA and MAC groups for blood and LRT specimens. For urine samples, 99.1% (219/221) of the samples produced the same identification results for the two groups. The cause of discrepant results for two urine specimens was due to Proteus species overgrowth on BA, which hindered non- Proteus spp. identification for the BA-only group.

CONCLUSION: Our results may indicate that omitting MAC has little or no impact on the recovery of organisms present in culture. However, due to possible Proteus spp. overgrowth, caution should be exercised in the decision to omit MAC from the primary inoculating medium, which necessitates further studies in other centers with a larger sample size.