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From this perspective, D-values can be understood as roughly analogous to half lives of radioactive substances, however a half life involves a reduction of 50% rather than 90%. The half life is actually roughly 30% (log 10 2 ≈ 30.103%) of the D-value, so if D = 10 minutes, the number of living microorganisms will be halved in about 3 minutes.
What is often called a "log reduction" (technically a reduction by one order of magnitude) represents a 90% reduction in microbial population. Thus a process that achieves a "6-log reduction" (10 −6 ) will theoretically reduce an initial population of one million organisms to very close to zero.
Log reduction is a measure of how thoroughly a decontamination process reduces the concentration of a contaminant.It is defined as the common logarithm of the ratio of the levels of contamination before and after the process, so an increment of 1 corresponds to a reduction in concentration by a factor of 10.
In water treatment practice, tables of the product C×t are used to calculate disinfection dosages. The calculated CT value is the product of the disinfectant residual (in mg/L) and the detention time (in minutes), through the section at peak hourly flow. [ 5 ]
Heat is applied by baking in a dry heat oven that is designed specifically for the depyrogenation process. Although endotoxins are relatively thermally stable, sufficient heating (250 °C for 30 min) results in a 3-log reduction of endotoxin levels. Due to the high temperature levels, this method is also not suitable when purifying proteins.
Specialist companies will often advertise a certain log reduction, e.g., 6-log reduction or 99.9999% effective, instead of sterilization. This takes into consideration a phenomenon known as light and dark repair ( photoreactivation and base excision repair , respectively), in which a cell can repair DNA that has been damaged by UV light.
The inoculum / suspension is serially diluted by adding 1x of suspension to 9x of diluent. When the quantity of bacteria is unknown, dilutions should be made to at least 10 −8. Three plates are needed for each dilution series, for statistical reasons an average of at least 3 counts are needed.
[6] [7] [8] The purpose of measuring MICs and grading microbes is to enable physicians to prescribe the most appropriate antimicrobial treatment. The first step in drug discovery is often measurement of the MICs of biological extracts , isolated compounds or large chemical libraries against bacteria and fungi of interest.