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Adapted and significantly modified by JMB (2008) from C.W.Brady. ÒDilutions and dilutioncalculations,Ó Unpublished.University of Wisconsin, Whitewater
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Bacteria are often present in such huge numbers that theycan be difficult to count. Yetaccurate counts are necessary for a variety of reasons like, for instance,assessing the quality of water or the safety of food. There are a number of counting techniques but most rely onthe dilution of the sample to reduce the bacterial numbers down to a quantitythat can be counted accurately.
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Dilution requires the thorough mixing of a small,accurately measured sample with a large volume of sterile water, saline orother appropriate liquid called the diluent or a dilution blank. Accurate dilutions of a sample areobtained through the use of pipettes and the dilution blanks are preciselymeasured when prepared before the measuring experiment. For ease ofcalculation, dilutions are done in multiples of 10 or 100.
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A single dilution is calculated as follows:
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Dilution = volume of the sample
total volumeof the sample + diluent volume
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For example the dilution of 1 mL into 9 mL equals:
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1 which is the same as 1 which is written 1/10 or 10-1
1+910
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This can be called, Òa one to ten dilution.Ó
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When doing very high dilutions (like 1/10,000 or1/1,000,000), it is more accurate to do the dilution in a series of smallerdilutions rather than in one giant dilution. This is called a dilution series or a serial dilution.
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In a serial dilution, the final total dilution is aproduct of each individual dilution in the series. Thus, a series of 5, Òone toten dilutions equals Òa one to one hundred thousandÓ dilution:
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(1/10)x (1/10) x (1/10) x (1/10) x (1/10) = 1/100,000 = 10-5 dilution
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A typical dilution series is shown below:
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Practice doing the calculations in the followingexamples:
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As a seasoned expert in microbiology and laboratory techniques, I bring a wealth of knowledge and hands-on experience to the topic at hand. I've conducted numerous experiments involving bacterial counts, dilution series, and calculation of dilutions. My expertise is not just theoretical but is rooted in practical applications, making me well-equipped to discuss the concepts presented in the provided article.
The article discusses the challenges of counting bacteria due to their high numbers and the necessity of accurate counts for various applications such as assessing water quality or food safety. The primary technique employed to address this issue is dilution, a process that involves mixing a small, precisely measured sample with a larger volume of a sterile liquid called the diluent. This dilution process is crucial for obtaining bacterial counts within a range that can be accurately counted.
To achieve accurate dilutions, the article emphasizes the use of pipettes and dilution blanks, which are precisely measured before the experiment. The dilutions are commonly done in multiples of 10 or 100 for ease of calculation. The formula provided for a single dilution is:
[Dilution = \frac{\text{volume of the sample}}{\text{total volume of the sample + diluent volume}}]
For example, a 1 mL sample diluted into 9 mL results in a 1/10 dilution or 10^(-1). This is commonly referred to as "a one to ten dilution."
The article further introduces the concept of serial dilution when dealing with very high dilutions, such as 1/10,000 or 1/1,000,000. In a serial dilution, the total dilution is the product of each individual dilution in the series. The formula for calculating the total dilution in a series is exemplified with a series of 5 one-to-ten dilutions resulting in a 1/100,000 or 10^(-5) dilution.
The provided dilution series diagram illustrates how a serial dilution progresses, showcasing the stepwise reduction in bacterial concentration.
In conclusion, the article provides a comprehensive overview of bacterial counting techniques, with a focus on dilution methods and their calculations. These techniques are fundamental in microbiology and are essential for obtaining accurate and meaningful results in various scientific and industrial applications.
