Commonly referred to as E. coli, Escherichia coli is a bacterium that is typically found in a number of environments including various foods, soil and animal intestines. E. coli is very diverse and belongs to the genus Escherichia. While most of the strains are harmless (and also important in the human intestinal tract), others are harmful and can cause very serious health implications. A good example of this is E coli 0157:H7.
Apart from food poisoning that can cause diarrhea and other problems in the digestive system, harmful strains of E coli can cause the following health problems;
In the intestinal tract, E coli plays an important role in that it aids in digestion and supports the absorption of vital vitamins from food in the body. In addition, it has been shown to be beneficial in that it prevents the growth and proliferation of other harmful species of bacteria that would otherwise cause health problems.
E. coli is described as a Gram-negative bacterium. This is because they stain negative using the Gram stain.
The Gram stain is a differential technique that is commonly used for the purposes of classifying bacteria. The staining technique distinguishes between two main types of bacteria (gram positive and gram negative) by imparting color on the cells.
Being Gram-negative bacteria, E. coli have an additional outer membrane that is composed of phospholipids and lipopolysaccharides. The presence lipopolysaccharides on the outer membrane of bacteria gives it an overall negative charge to the cell wall. Because of these properties, E. coli does not retain crystal violet during the Gram staining process.
See page explaining Gram positive and Gram negative bacteria
To determine whether a strain(s) is present in a sample, it's necessary to stain the sample. Here, Gram stain is used as it helps distinguish between the gram positive and gram negative bacteria in a sample.
Being a differential stain,
Gram stain is more complex compared to more simple stains like methylene blue.
As such, more than one stain is used for the purposes of differentiating the its cellular components.
For this technique, 3 different stains are used.
Here, heat is used for fixing. Heat fixing procedure involves the following steps:
* During heat fixing, avoid overheating the slide.
When viewed under the microscope, Gram-negative E. Coli will appear pink in color. The absence of this (of purple color) is indicative of Gram-positive bacteria and the absence of Gram-negative E. Coli.
The hanging drop method refers to a technique that helps examine small, living and unstained organisms. It can be used for such motile bacteria as Ps. fluorescens and E. coli.
Using the hanging drop
technique is also very important given that it helps distinguish between Brownian
movement (the movement of particles that are suspended in a fluid) and motility
Some of the requirements include:
To determine whether the bacteria is really moving, it's important to ensure that it's moving in different directions and changing position. This differentiates motility of the organism from mere Brownian motion.
See Also: Bacteria under the Microscope, See Eubacteria page, Learn more about Coliforms, Microscopy Culture and Sensitivity
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Chris Hayhurst (2004) E. Coli.
Dr Kaiser, G. E. (February 2009). Gram Stain of Escherichia Coli.
MiSAC's Practical Microbiology for Secondary Schools, pp 22-23
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