Estimation method for serial dilution experiments

• Published in: Journal of microbiological methods Vol. 107; pp. 214 - 221
• Main Authors: Ben-David, Avishai, Davidson, Charles E.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2014
• Subjects: Agar plates; Algorithms; Colony Count, Microbial - methods; Colony Count, Microbial - standards; Density (concentration) of microorganisms; Environmental Microbiology; Models, Statistical; Reproducibility of Results; Serial dilution; Titration; Too numerous to count (TNTC); Viable bacterial counts; TE; Agar plates; RE; CFU; Too numerous to count (TNTC); VBNC; Viable bacterial counts; MPN; Titration; TNTC; Serial dilution; Density (concentration) of microorganisms
• ISSN: 0167-7012; 1872-8359
• DOI: 10.1016/j.mimet.2014.08.023

Titration of microorganisms in infectious or environmental samples is a corner stone of quantitative microbiology. A simple method is presented to estimate the microbial counts obtained with the serial dilution technique for microorganisms that can grow on bacteriological media and develop into a colony. The number (concentration) of viable microbial organisms is estimated from a single dilution plate (assay) without a need for replicate plates. Our method selects the best agar plate with which to estimate the microbial counts, and takes into account the colony size and plate area that both contribute to the likelihood of miscounting the number of colonies on a plate. The estimate of the optimal count given by our method can be used to narrow the search for the best (optimal) dilution plate and saves time. The required inputs are the plate size, the microbial colony size, and the serial dilution factors. The proposed approach shows relative accuracy well within ±0.1log10 from data produced by computer simulations. The method maintains this accuracy even in the presence of dilution errors of up to 10% (for both the aliquot and diluent volumes), microbial counts between 104 and 1012 colony-forming units, dilution ratios from 2 to 100, and plate size to colony size ratios between 6.25 to 200. [Display omitted] •We estimate microbial concentration from measured counts on a single agar plate.•Estimation is a function of dilution factor and ratio of plate size to colony size.•We give a procedure to select optimal dilution plate for serial dilution process.•We predict optimal number of colonies on agar plate that minimizes the total error.•Total error includes the competing effects of sampling and counting errors.