An Overview of Bacteria

by Matt Emery (UK), an A level sciences student, and amateur microbiologist



Essentially, there are 3 general types of bacteria, each being specialised to different environments and having specific advantages in various organisms in relation to other forms. They exist all around us, in the air we breathe, on the surfaces we touch, on our skin, and within us. Many bacteria, contrary to popular misconception do not actually do any harm to mankind, and in fact, can be helpful to us, such as the bacteria responsible for turning milk to yoghurt, breaking down detritus into soil, and those which live symbiotically in our gut.


These bacteria are generally some of the smallest, and simplest, being small and spherical, hence Cocci (berry shaped). There are a number of bacteria in this category which are pathogenic (disease causing) such as Staphlycoccus aureus , which causes a type of food poisoning, and is rapidly becoming known as the hospital superbug, a variant of this species called MRSA (Methicillin Resistant Staphylococcus aureus ), N. meningitidis, which causes the often deadly diseases, meningitis, Staphylococcus epidermis which inhabits the skin and can cause spots and boils, and finally Moraxella catarrhalis, which generally tends to cause infections in the lower respiratory tract in humans.

Fig 1 - Micrograph of a pair of Cocci (Stained with Fuschin, 100x objective (oil)).

Bacillus :-

The second and slightly more complex type of bacteria are the genus Bacillus. These bacteria are rod shaped, and can be very short or very long, approx 5 - 30 microns long, although there are some larger species. Most bacilli are capable of forming spores, which can protect the bacteria from harsh conditions such as dryness etc, and the bacteria within can remain viable for up to 100 years. Bacilli are also capable of causing many diseases, such as Yersinia pestis, which causes bubonic, pneumonic and sceptacaemic plague, and Bacillus anthracis, which causes the, by now probably very famous disease, due to the post 9/11 attacks, anthrax. Common types of bacillus are Lactobacillus spp which generally cause milk to spoil, and a number of various soil dwelling species.

Fig 2 - Micrograph of Bacilli (Stained with Fuschin, 100x objective (oil)).


A variation of the bacillus genus is the vibrio, which is part of the spirochaete family, but looks like a curved rod. This type of bacteria is generally responsible for diseases such as cholera ( Vibrio Cholerae ), and are generally found in stagnant water, and water which has been contaminated with sewage.

The final type of bacteria, which I generally regard to be the most complexly shaped is the spirochetea, which is basically a corkscrew shaped bacterium. It causes diseases such as lyme disease ( Borrelia burgdorferi ).

Fig 3 - Micrograph of a Spirochaete (Stained with Fuschin, 100x objective (oil)).

After all of this incriminating talk about bacteria, I feel that I should also mention the fact that many bacteria do no harm to man, such as those found in probiotic yoghurt, the wide spectrum of bacterial flora naturally found in the mouth, and bacteria found in root nodules of plants such as clover which naturally produce fertiliser for the plant they infect, along with many types of bacteria that exist around us everyday and still do no harm to us.

Microscopic examination of bacteria

The easiest way to study bacteria without any specialised equipment such as nutrient agars and petri dishes is to let a layer of plaque accumulate on the teeth for a day (make sure to brush your teeth at bedtime!) and mix a small amount of the plaque with a single drop of water on a cleaned, grease free slide (wash slide and pass through a bunsen flame twice). Then add a drop of a simple stain to the preparation on the slide such as crystal violet or methylene blue, and lower a coverslip onto this preparation. Don't bother wasting time with the 4x objective, skip to the 10x objective and find a space with a high concentration of small stained specs, then switch to the 40x objective.

You should be able to observe a number of various types of bacteria such as bacilli and cocci, however it is unlikely that there will be any spirochetea in the normal flora of the mouth. These bacteria, provided that the stain concentration is not so great as to kill the cells, should be observed to be moving around in the water, this is accomplished by numerous flagellae around the surface of the bacteria (small "hairs" which enable the bacteria to "swim").

Ideally, bacteria should be observed as a dry, heat fixed, stained smear without a coverslip, under the 100x oil immersion objective to give the best image, however, many bacteria can be seen at 400x for those who are unfortunate enough to not possess a microscope with a 100x objective.

The procedure for this is to prepare a smear as in the example above, and instead of placing a coverslip on the sample, spread it over a portion of the slide and leave to dry, then pass the slide through a flame smear side up twice to heat fix the cells to the slide.

Then stain with a simple stain such as methylene blue. Rinse the slide, then allow it to dry. After this, place a single drop of immersion oil on the slide and lower the 100x objective onto the oil and focus. A fairly high amount of information can be obtained about the bacteria via this method, especially if the specimen has been gram stained (a method of differentiating between types of bacteria, used in medical diagnostics and bacteriology laboratories)

My humble work environment and equipment

Here is my workbench, cultures and all. My microscope, a Bresser Biolam BLK, by all means not the best microscope in the world, but it has excellent build quality, and very good optics for a microscope costing 280. Note the desk tidy to keep inoculating loops, spreaders, pippettes etc in to keep mess to a minimum, along with the grey trays on the side of the bench (just visible on the right). Overall, I find the study of bacteria very fascinating, and once I finish my A level courses, I aim to study microbiology for my degree.

Micrographs were all taken by myself, using a Polaroid PDC 2070 2.1 Mpixel digital camera held to the eyepiece of the microscope.

Safety note :- after working with any kind of bacteria, once finished, immediately place the slides into a disinfectant solution and wash, wipe work surfaces down with an appropriate disinfectant / antibacterial agent such as 70 % ethanol, sodium hypochlorite (aqueous, 10% sol), or a household disinfectant made up to the manufacturer's directions, and wash hands with an antiseptic soap. Avoid hand contact with the eyes or mouth whilst working with bacteria, and always handle cultures in the correct manner, assume everything is a pathogen.

I hope this article has provided some useful information on bacteria.

Any constructive comments welcome to the author, Matt Emery