Microbiology

Microbiology Study Guide

©2018 of 132 decimal reduction time, is also computed and denotes the time needed to destroy about 90 percent of the microbes. Heat methods can further be divided into dry and moist heat. The treatment with dry heat is done in an incineration device, like a hot air oven. It has been found that sterilization in a hot air oven is achieved in two hours at a temperature of 170° Celsius. In contrast, using boiling water as a treatment source is a kind of moist heat method. The time taken to kill organisms with moist heat depends on the nature of organism. For instance, it takes only about a few minutes to kill the vegetative organisms, while microbial spores are killed in no less than two to three hours. In the case of moist heat, the process of denaturation takes place wherein the microbial proteins break down from three dimensions to two dimensions and the microbial proteins finally break down. Moist heat is also used in an autoclave where organisms are killed using super-heated steam. Another method is pasteurization and is a common practice for treating milk and other dairy products. However, pasteurization does not lead to sterilization. Pasteurization, therefore, is a successful treatment method to eliminate bacteria species like tuberculosis bacillus and rickettsia (both are capable of causing diseases like tuberculosis and Q fever). Pasteurization can be done using three methods: the holding method, the flash method, and the ultra-pasteurization method. In the holding method, milk is boiled for 30 minutes at a temperature of about 62° Celsius, whereas in the flash method, the boiling is done for 15 seconds at a temperature as high as 72° Celsius. When pasteurization is done at a temperature of 138ºC (280ºF) for about two seconds this method is known as ultra-pasteurization. Another physical method used for microbial control is tyndallization. This is generally done for liquids and similar items. Tyndallization is a three-day process using free flow steam for about half an hour each day in order to kill organisms. The outcome at the end of the first day is the elimination of vegetating organisms, except bacterial spores, which are germinated at the first night. These bacterial spores are killed by the steam the next day. The remaining bacterial spores get an opportunity to germinate on the night of the second day and are finally killed on the third day. The other category of physical methods is non-heat. The filtration process helps eliminate organisms from liquid or gaseous items. It is the process in which the liquid or gas is allowed to pass through pores small enough so that microorganisms are retained and separated from the item. The liquid or gas substance can be pulled from the filter using a vacuum. This process is mostly used in the case of heat sensitive substances, like vaccines, which need to be sterilized. Another method is drying, which upon removing water and moisture from the cells, the survival of the microorganisms is threatened. Salt or sugar is used as the agent to achieve osmosis in the process of drying. For instance, preservation of meat is done by treating it with salt while fruits are treated with sugar to increase shelf life. Lyophilization is another process that facilitates the killing of organisms through drying. In this process, liquids are treated by quick freezing followed by evacuation, which dries the substance and kills the microorganisms. Achieve Page 44