PSVIII-31 Sources of variations, repeatability and reproducibility of microbial count data

Abstract

Abstract The current study consists of two separate experiments to evaluate the relative contribution of systematic factors to the total variability of count data using same silage inoculant. In experiment-1, two lots of a raw material were used to prepare three batches of media. Each of three technicians weighed five independent samples per day, over 10 days and worked on their samples through dilution, plating, and counting. Each sample was grown on three plates. In experiment-2 technician effect was further evaluated at weighing-diluting, plating and counting subprocesses. Each technician weighed and diluted three independent samples per day, over three days. Samples were then shared between technicians during plating and counting. Count data were converted to log10 values and data in experiment-1 were analyzed using a random effects model that included lot, batch, technician, day, and sample within lot-batch-day-technician. Model for experiment-2 included similar effects, except technician effect was partitioned into weighing-diluting technician, plating technician, and counting technician, and there were no batch and lot effects. In experiment-1 sampling variance accounted for a relatively large proportion of the total variance (35%) followed by 33% due to technician effects. The overall repeatability standard deviation of count data was 0.08. Reproducibility standard deviation of measurements made by different technicians on the same day was 0.12. Reproducibility standard deviation of microbial count by the same technician at different days was 0.10. Contribution of sampling variance remained the highest in experiment-2 (39%). The total technician variance was mainly due to weighing-diluting technician, contributing to 31% of the total variance. The relatively large contribution of sample-to-sample variance suggests a major improvement in the precision of count data may be obtained through additional samples rather than increasing number of plates per sample. Technician training can further help reduce variation and improve consistency in sample weighing and dilution.