Purpose: The purpose of the study was to compare plate-to-plate count variation between four different aerobic count media manufacturers.
Methods: One lot of Standard Methods Agar (SMA) from three global dehydrated media manufacturers and a rehydratable film media, 3M Petrifilm Aerobic Count (AC) Plate, were compared. A multi-organism certified reference material containing four microorganisms was used to prepare the inoculum in three diluents. Plates were incubated at 30°C, 32°C or 35°C for inoculum prepared in Peptone Saline Buffer (PS), Butterfield’s Phosphate-Buffered Dilution Water (BPB) or Buffered Peptone Water (BPW), respectively. Following incubation, plates were counted and results were recorded. The experiment was repeated three times with 10 replicates for each agar media brand.
Results: Plate count variance, based on the standard deviation between replicates (n=30), of each agar media brand were compared to determine any significant differences (P-value < 0.05) between them. The rehydratable film media had the lowest standard deviation for plates incubated at 30°C and 32°C. Agar media A had the lowest standard deviation for plates incubated at 35°C. However, the standard deviation value was not significantly different than the standard deviation for the rehydratable film media. All agar media brands incubated at 32°C had significantly higher standard deviations than that of the rehydratable film media. Agar media B, when incubated at 30°C, had a significantly different standard deviation than the rehydratable film media.
Significance: Repeatability from microbiological media is an important component when conducting proficiency test and testing routine samples. The rehydratable film media had statistically equivalent or better plate-to-plate repeatability than three global manufacturers of SMA dehydrated media.