In this video, we're going to talk about the 4th consideration when selecting a microbial growth control method, and that is environmental factors. Environmental factors such as the temperature, for example, and the pH can influence the effectiveness of a microbial growth control method. For example, if the temperature or pH ranges are not correct within the appropriate range, then the microbial growth control method may be ineffective, meaning it may not work as intended. On top of environmental factors such as temperature and pH, other factors such as substances including grease, dirt, and body fluids can interfere with some microbial growth control methods. Therefore, these items should be cleaned out appropriately prior to implementing any disinfection or sterilization control methods.

If we take a look at our image below, we can get a better understanding of how environmental factors affect microbial growth control methods. We're showing you a microbial death curve, with the use of sodium hypochlorite as a chemical microbial growth control method. Notice on the y-axis we have the number of living microbial cells, and on the x-axis we have the amount of time that these cells have been exposed to the microbial growth control method. Notice that we have two lines here; one in orange and one in blue. The orange line represents a hot temperature of 55 degrees Celsius, and the blue line represents a colder temperature of 50 degrees Celsius. Notice that there is only a 5 degrees Celsius difference, which you might think might not make a significant difference in the control method. However, in this particular example, it does make a significant difference.

For the hotter temperatures, the d value is much smaller. The d value, the decimal reduction time, is the amount of time it takes to kill 90% of the population. The d value here for the orange curve is 10 minutes to kill 90% of the population, starting from an initial population of 10,000 and going down to 1,000. The hotter temperature is more effective than the colder temperature because the d value for the colder temperature is double and shows that these microbes are more resistant to colder temperatures. It takes a lot longer to kill all of the microbes when the temperature is colder and much less time when it is hotter.

This emphasizes the fact that household bleach is more effective at killing microbes at hot temperatures, specifically temperatures of 55 degrees Celsius. This shows that even small temperature changes, like 5 degrees Celsius, can have a major impact on the microbial growth control method. Environmental factors are an important consideration when selecting a microbial growth control method. This concludes our lesson on this idea, and we'll be able to get some more practice and talk about the 5th and final consideration of selecting a microbial growth control method in our next video. See you all there.