# How to calculate log kill

Learn how to calculate log kill with writing patterns using this step-by-step guide.

## How to Calculate Log Kill with Writing Patterns Using

Log kill is a measure of disinfection that indicates the reduction in the number of microorganisms after a certain treatment. It is expressed in logarithmic units, which makes it easier to compare different disinfection methods. In this article, we will show you how to calculate log kill with writing patterns using a step-by-step approach.

To calculate log kill, you need to know the initial concentration of microorganisms, the concentration after disinfection, and the time of exposure. The initial concentration is usually expressed in colony forming units (CFU) per milliliter, while the concentration after disinfection can be expressed in CFU/mL or as a percentage of the initial concentration.

The first step is to determine the initial concentration of microorganisms. This can be done by taking a sample from the surface or substance you want to disinfect, and plating it on a nutrient agar plate. After incubation, count the number of colonies that have grown and calculate the CFU/mL.

The second step is to apply the disinfectant according to the instructions provided by the manufacturer. This can involve spraying, wiping, or soaking the surface or substance with the disinfectant. Make sure to follow the recommended concentration and exposure time.

After the exposure time has elapsed, take another sample and repeat the plating and counting procedure to determine the concentration of microorganisms after disinfection. This value can be expressed in CFU/mL or as a percentage of the initial concentration.

To calculate log kill, you need to use the following formula: Log kill = log10 (N1/N0), where N1 is the concentration of microorganisms after disinfection, and N0 is the initial concentration.

For example, if the initial concentration is 10^6 CFU/mL and the concentration after disinfection is 10^3 CFU/mL, the log kill would be: Log kill = log10 (10^3/10^6) = log10 (0.001) = -3.

Note that a log kill of -3 means that 99.9% of the microorganisms have been killed. This is because each log unit represents a tenfold reduction in the number of microorganisms. Therefore, a log kill of -1 would mean a 90% reduction, while a log kill of -2 would mean a 99% reduction.

In some cases, the concentration after disinfection may be below the limit of detection of the plating method. In this case, you can assume a concentration of zero and use the following formula: Log kill = log10 (N0), where N0 is the initial concentration.

Another way to express log kill is in terms of the decimal reduction time (D value). This is the time it takes for the number of microorganisms to be reduced by one log unit. It can be calculated using the following formula: D value = t/log10 (N1/N0), where t is the exposure time.

For example, if the initial concentration is 10^6 CFU/mL, the concentration after disinfection is 10^3 CFU/mL, and the exposure time is 10 minutes, the D value would be: D value = 10/log10 (10^3/10^6) = 10/3 = 3.33 minutes.

Writing patterns can be used to visualize the log kill and the D value. One common pattern is the linear pattern, where the log kill or D value is plotted on the y-axis and the time of exposure is plotted on the x-axis. Another pattern is the semi-logarithmic pattern, where the log kill or D value is plotted on the y-axis and the logarithm of the time of exposure is plotted on the x-axis.

To create a linear pattern, you can use graph paper or a spreadsheet program. Plot the log kill or D value on the y-axis and the time of exposure on the x-axis. Connect the points with a line to visualize the trend. You can also add labels and a legend to make the graph easier to interpret.

To create a semi-logarithmic pattern, you need to use logarithmic scales for the x-axis. This can be done by selecting the logarithmic scale option in the graph settings of your spreadsheet program. Plot the log kill or D value on the y-axis and the logarithm of the time of exposure on the x-axis. Connect the points with a line to visualize the trend. Again, you can add labels and a legend to make the graph easier to interpret.

Other writing patterns that can be used include the exponential pattern, where the log kill or D value is plotted on the y-axis and the inverse of the time of exposure is plotted on the x-axis, and the sigmoidal pattern, where the log kill or D value is plotted on the y-axis and the logistic function of the time of exposure is plotted on the x-axis.

In conclusion, calculating log kill with writing patterns using a step-by-step approach involves determining the initial concentration of microorganisms, applying the disinfectant, determining the concentration after disinfection, and using the log kill formula. Writing patterns such as the linear and semi-logarithmic patterns can be used to visualize the log kill or D value over time. This information can be useful for evaluating the effectiveness of disinfection methods and optimizing disinfection protocols.

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