- What did you do today? Today, we tested different samples from different areas around the campus for bacteria. First, we had to put on our lab coats and gloves so that we won't get contaminated. Next we had to sterilise our tables and hands using 70% ethanol. I collected samples from different canteens using the sterilised inoculating loop, then swabbed them onto the agar in the petri dishes to let them grow for one day. The dishes were divided into two. One of them was the control and nothing was supposed to be swabbed on it, while the other was the actual test. Then, I used a cotton swab and ran it through my air to swab it onto another petri dish.After that, I went out and took samples from the handlebar entrance to the School of Biomedical Sciences using another cotton swab to test.
- What did you learn today? I learnt how to collect samples from different areas, then apply them onto petri dishes for testing. I also learnt some surprising facts about bacteria. For example, the doorknob to the toilet has 200x more bacteria than the toilet seat itself. I also learnt that lab coats and gloves cannot be worn outside of laboratories in case of contamination.
- How do you feel about today’s activities? I understand how to properly test for bacteria now, and that might help me in the future, when I may become a researcher, or even next year, when I might take Biotechnology as my Applied Science. I feel that it was actually quite fun and interesting, and it helped me learn a lot about how scientists discover the different types of bacteria on different surfaces.
What are some characteristics of bacteria?
- They are unicellular
- Some of them can photosynthesise
- They have no nucleus
- They have a cell wall
- They are asexual
How can bacteria be identified?
This technique is named after a pioneering Danish microbiologist. It works as follows.
- The bacteria are stained with a purple dye (Crystal Violet). Most bacteria are then stained red.
- The bacteria are stained again with Potassium Iodide.
- The bacteria are then washed with alcohol. Those bacteria that retain the dye after washing are known as Gram Positive bacteria. Those that lose the dye are known as Gram Negative bacteria.
- The bacteria are further stained with a pink dye (Safranin). Gram Negativebacteria will go pink after this dyeing, whereas Gram Positive bacteria will remain purple, from the original purple dye.
This technique is also referred to as acid-fast staining. This technique is necessary because some bacteria, notably all Mycobacteria, have waxy coats on their cell walls that prevent them taking in the dye from the Gram Staining procedure. As a part of the acid-fast staining process, detergents are applied which remove this waxy coat.
- The bacteria are stained with hot Carbol-Fuchsin, a red dye which contains detergents. All bacteria are then stained red.
- The bacteria are washed with acid alcohol. Those bacteria that retain the red dye are known as acid-fast bacteria.
- The bacteria are then stained with Methylene Blue, a blue dye. Those bacteria that retain the red dye from the original stain are known as acid-fast bacteria, all others go blue.
All Mycobacteria test positive using the Ziehl-Neelsen acid-fast stain test, i.e. all species of Mycobacteria stain red using this procedure. Thus, this procedure cannotbe used to differentiate between species of Mycobacteria.
Staining techniques, as described above, are useful for identifying the genus of a bacterium, but not for identifying the species of a bacterium, within a genus. Differentiation of a mycobacterium from a streptococcus can be done with stain testing, but to differentiate a Mycobacterium tuberculosis from a Mycobacterium leprae requires much more specific testing.
Recent advances in genetic technology enable highly specific testing for bacteria. What is required is that a segment of the DNA structure of the bacterium be identified. These segments are referred to as "insertion sequences", and consist of a sequence of proteins that makes up the DNA of the bacterium. Although the DNA structure of Mycobacterium tuberculosis will have a lot in common with the DNA structure of Mycobacterium paratuberculosis, there will be some parts of the DNA structure of each organism that is unique to that organism.
To identify the bacterium by testing, its DNA is "multiplied", using the Polymerase Chain Reaction. This technique creates large numbers of copies of the DNA to be tested. This delivers enough DNA to be tested for the presence of the target genetic sequence. This DNA presence can be tested by techniques such as "Nucleic Acid Hybridization", "Autoradiography", "Southern Blot Hybridization", etc. I will not attempt to describe these techniques here.
The IS900 insertion sequence.
Recently discovered mycobacterial "insertion sequences" that are important are theIS900 and IS902 sequences. The IS900 sequence is unique to Mycobacterium paratuberculosis. The IS902 sequence is unique to Mycobacterium avium subspecies silvaticum. Even though Mycobacterium paratuberculosis and Mycobacterium avium are very closely related, IS900 will test positive with Mycobacterium paratuberculosis but will test negative with Mycobacterium avium. Similarly, IS902 will test positive with Mycobacterium avium subspecies silvaticum, but will test negative with Mycobacterium paratuberculosis. Insertion sequences have been identified for many mycobacteria, including IS6110 for Mycobacterium tuberculosis and IS901 for Mycobacterium avium type A/I.
Name three common bacteria and places where they are commonly found.
Escherichia coli (E. coli) in human intestine.
Staphlycoccus aureus , which causes a type of food poisoning
Lactobacillus which generally cause milk to spoil
(Vibrio Cholerae ), and are generally found in stagnant water
What are some non-harmful bacteria?
Lactobacillus species. Found in human gut and body surface, used in making cheese, yoghurt and other fermented foods. Also used in probiotic health foods.
Rhizobium species. Found in root nodules of leguminous plants where they fix nitrogen from the atmosphere.
Acetic acid bacteria. Convert alcohol to acetic acid in production of vinegar.
Ruminococcus species. Found in the stomachs of cows where they digest cellulose in grass.