The Preparation of Spread and Streak Plates
Bacteria are found just about everywhere, and most of them are nonpathogenic. Others are just plain harmful, pathogenic forms. Still others are harmless as long as they maintain their “personal space”, but become a threat when they get into areas other than their natural habitat.
E. coli, for example, is a natural resident of the large (colon) intestine. There it causes no harm and actually helps by assisting with waste processing, vitamin K production and food absorption. When E. coli or some of the other type of microorganism (microbe) leaves its normal habitat and enters areas where it is not normally found, it can cause disease.
Contamination of foods by E. coli or some other microbe is a serious threat to health. How can we test for organisms such as E. coli that might cause microbial contamination? What if we find that an organism is present in some substances - how can we determine the degree of contamination of the material?
The rate of microbial spoilage depends upon the chemical composition of the affected substance(s) and the types of microbes causing the infection. Freezing, boiling and secure packaging help prevent contamination.
Improper handling, such as employees returning to processing areas from the bathroom without washing their hands, can cause serious contamination. Improper slaughter and packaging procedures can also cause contamination. Careless beef processing has apparently caused recent outbreaks of a lethal form of E. coli. Animal feces containing E. coli were included in beef processing along with the beef body tissues.
Refer to your lab manual for the spread and streak plate procedures.