Rapid differentiation and enumeration of the total, viable vegetative cell and spore content of thermophilic bacilli in milk powders with reference to Anoxybacillus flavithermus

• Published in: Journal of applied microbiology Vol. 99; no. 5; pp. 1246 - 1255
• Main Authors: Rueckert, A, Ronimus, R.S, Morgan, H.W
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.01.2005; Blackwell Science; Oxford University Press
• Subjects: Anoxybacillus flavithermus; Azides - pharmacology; Bacillaceae - drug effects; Bacillaceae - growth & development; Bacillaceae - isolation & purification; bacterial contamination; bacterial spores; Biological and medical sciences; cell enumeration; Colony Count, Microbial; Cultured Milk Products - microbiology; Deoxyribonuclease I - pharmacology; DNA, Bacterial - analysis; DNA, Bacterial - drug effects; DNase I; dried milk; ethidium monoazide bromide; food contamination; Food Contamination - analysis; Food Microbiology; food pathogens; Fundamental and applied biological sciences. Psychology; Geobacillus stearothermophilus - isolation & purification; Intercalating Agents - pharmacology; live/dead cell differentiation; Microbiology; Microscopy, Fluorescence - methods; Microscopy, Phase-Contrast - methods; milk powder; Polymerase Chain Reaction - methods; quantitative PCR; spores; Spores, Bacterial - drug effects; Spores, Bacterial - growth & development; Spores, Bacterial - isolation & purification; thermophilic bacilli; live/dead cell differentiation; Enumeration; Enzyme; Applied microbiology; quantitative PCR; Dried milk; Dead cell; cell enumeration; milk powder; Thermophily; Esterases; Cell differentiation; Spores; Enumeration(counting); Polymerase chain reaction; Deoxyribonuclease I; ethidium monoazide bromide; Hydrolases; Differentiation; DNase I; Viability; thermophilic bacilli; Quantitative analysis
• ISSN: 1364-5072; 1365-2672
• DOI: 10.1111/j.1365-2672.2005.02728.x

Summary Aims:  The development of a rapid method for the selective detection and enumeration of the total and viable vegetative cell and spore content of thermophilic bacilli in milk powder by PCR. Methods and Results:  Quantitative PCR and microscopy indicate the presence of up to 2·9 log units more cells in milk powder than accounted for by plate counting due to the majority of cells being killed during milk processing. Two approaches for viable and dead cell differentiation of thermophilic bacilli by quantitative PCR were evaluated, these being the nucleic binding dye ethidium monoazide (EMA) and DNase I digestion. The former agent exposed to a viable culture of Anoxybacillus flavithermus caused considerable cell inactivation. In contrast, DNase I treatment had no effect on cell viability and was utilized to develop DNA extraction methods for the differential enumeration of total, viable vegetative cells and spores in milk powder. Moreover, the methods were further applied and evaluated to 41 factory powder samples taken throughout eight process runs to assess changes in numbers of vegetative cells and spores with time. DNase I treatment reduced vegetative cell numbers enumerated with PCR by up to 2·6 log units. The quantification of spores in the factory milk powders investigated indicates on average the presence of 1·2 log units more spores than determined by plate counting. Conclusions:  The method presented in this study provides the ability to selectively enumerate the total and viable cell and spore content of reconstituted milk. Significance and Impact of the Study:  The current study provides a tool to monitor the extent of thermophilic contamination during milk powder manufacturing 60–90 min after sampling.