What is microbiology and why is it important?

Microbiology is the scientific study of microorganisms (or “microbes”), microscopic living things. The three most common types of microbe are bacteria, fungi and viruses. Viruses, although widely regarded as microorganisms, are generally thought of as non-living. Microbes can be found everywhere on the planet from the most extreme environments, such as hydrothermal vents deep beneath the oceans, to common household surfaces and even inside our bodies. It is estimated that there are approximately ten times the amount of bacterial cells living in our gastrointestinal (GI) tract alone (stomach and intestines) than there are cells which make up our bodies (Conlon & Bird, 2014). That’s a lot of bacteria! But this is not a bad thing. Bacteria (and other microorganisms), contrary to popular belief, offer many advantages to us. One of the best-known bacteria, Escherichia coli (E. coli), lives in our intestines. We share a symbiotic relationship with this species, meaning we offer each other advantages - E. coli get a warm, moist home while they provide us with vitamins, such as vitamin B, and aid in food digestion. It is also believed that the “good” bacteria living inside us prevent the colonization of other microbes which would do us harm by competing for living space (Tlaskalová-Hogenová et al, 2004). See figure 1 for a summary of the different species of bacteria living in the human GI tract.

As figure 1 illustrates, there are approximately two hundred different bacterial

species living in the human mouth. Experiments have shown that tooth decay,

one of the most prevalent chronic diseases of people worldwide, is caused by

bacteria. A number of bacteria living on the crowns or roots of teeth ferment

carbohydrates (sugars) that may remain after meals and produce acids. This

complex reaction leads to dental caries over time (Selwitz et al, 2007). One

such bacteria involved in this process is the lactobacillus species, which

ferments the sugar lactose, found in milk, and produces lactic acid. This

bacteria is also commonly used to produce cheese and yogurt. Another

common human process which can be attributed to bacteria is body odour.

The armpits, or “axilla”, offer a great living environment for certain bacteria

who thrive in the warm and moist conditions and also make use of molecules

contained in human sweat. Experiments have also shown that an

“acidic odour”, caused by the bacteria’s interaction with the sweat, is what

causes the common human body odour rather than the sweat itself

(Leyden et al, 1981).

The study of these microscopic creatures has provided insight into many

important processes, including ones involving human health, which would be

impossible without microbiology. Its importance to society and to our wellbeing

is still being investigated and our understanding continues to grow.

Bacteria

At a structural level, bacteria are single-celled (unicellular) organisms. They possess a single chromosome, on which they carry their DNA – the information required by all cells to carry out their functions. They also have a cell wall – a thick outer layer which protects the cell and holds it together. See figure 2 for an overview of a basic bacterial cell. Similarly to normal human somatic (body) cells, bacteria have a cell membrane surrounding the cell. Different proteins make up a considerable portion of bacterial structure. The proteins and sugars expressed by a bacteria are what determine the strain. For example, E. coli normally lives inside us but one strain in particular causes severe food poisoning. This strain is characterised by specific proteins which make up its structure. One such protein is a flagellar protein, which is involved in the structure of the flagella, a thread-like “tail” which some bacteria possess in order to help them move.

When thought about with respect to size, approximately one thousand bacterial cells would fit into the full stop at the end of this sentence. Bacteria replicate through a process known as binary fission, by which the bacterial

cell simply splits to divide into two. For further information on bacterial

structure, specifically in relation to the mechanism of antibiotics and how

bacteria can become resistant, see the second and third blogs of this series.

Fungi

Fungi can be both unicellular and multicellular. Examples of multicellular fungi

include mushrooms. These are not considered microorganisms. Yeast is a

well-known example of a fungus. . It is a unicellular microbe and the yeast

Saccharomyces cerevisiae (S. cerevisiae) is commonly used to make alcohol

and baked goods, such as bread. Fungi are the largest type

of microorganism. S. cerevisiae cells are typically 10-15 times larger than

E. coli cells, for example. Fungi, when thought about in terms of human

interaction, are mostly found growing on the skin and can sometimes cause

skin infections. Common areas where various species of fungus grow include

the scalp and feet. Aspergillus niger (A. niger) is one of a few species of fungus which causes the fungal skin infection known as “athlete’s foot”. Most fungi reproduce through spores, which mature fungal cells release into their environment. A good example of this is seen in the species Penicillium notatum (P. notatum), most commonly seen in bread mould. Spores which may be floating in the air can land on bread and lead to the growth of the mould. Most fungal cells have a tubular structure, such as A. niger and P. notatum. However, S. cerevisiae cells are basic round cells.

Viruses

Viruses are smaller than bacteria and fungi. Their general structure consists of a protein coat (“capsid”) surrounding a single piece of DNA or RNA, which codes for proteins essential to their survival/replication. Viruses cannot survive without a host. This is a major difference between them and other microorganisms. It is also one of the reasons why viruses are not considered as “living”. Without viable hosts, viruses could not replicate and would die. The flu, for example, is caused by the influenza virus and is spread from person to person through saliva or droplets from sneezing, coughing, etc. These can carry the virus into a new host by entering the eyes, nose or mouth. The virus, like all other viruses, invades host cells, injecting its own DNA (or, in the case of influenza, RNA) into host cells and using them as sites of replication. The host cell is used like a machine, to make everything required by the invading virus. This is done until the cell fills up with newly-replicated viral cells, which are released into the surrounding environment, killing the host cell, ready to invade more cells.

As previously mentioned, differences in structure of some microorganisms lead to different strains. There are different strains of influenza virus also, characterized by surface proteins on the viral capsid, namely hemagglutinin and neuraminidase. Mutations can come about which cause the constant changing of virus ultrastructure. This is the fundamental reason why we get the flu more than once. If there was just one strain of influenza virus, which never changed/mutated, we would become immune to that virus after getting the flu once, or after receiving a vaccination. The next time we came in contact with it, our immune system would be able to detect and deal with it quickly and effectively.

Viruses offer a number of advantages (although significantly less than other microbes!). One such advantage is offered by the bacteriophage, a particular type of virus which infects bacteria. Using bacterial cells as hosts, bacteriophage can lead to the rapid death of certain bacteria. Bacteriophage can now be used to treat poultry, in order to kill the specific strain of E. coli which would cause food poisoning (Monk et al, 2010). Figure 3 shows how some of the microorganisms mentioned above look when viewed using a microscope. These images are included in Microbe Match, a game created for fifth and sixth class students as part of an overall project to promote microbiology and raise awareness about antibiotic resistance (see further blogs). All images online, open-access, through Creative Commons

Bibliography

Berg, R.D. (1996). The indigenous gastrointestinal microflora. Trends in

Microbiology 4, 430–435.

Conlon, M.A., Bird, A.R. (2014). The Impact of Diet and Lifestyle on Gut Microbiota

and Human Health. Nutrients 7, 17.

Flotte, T., Carter, B. (1995). Adeno-associated virus vectors for gene therapy. Gene therapy 2, 357–362.

Leyden, J.J., McGinley, K.J., Holzle, E., Labows, J.N., Kligman, A.M. (1981). The Microbiology of the Human Axilla and Its Relationship to Axillary Odor. J Investig Dermatol 77, 413–416.

Monk, A.B., Rees, C.D., Barrow, P., Hagens, S., Harper, D.R., 2010. Bacteriophage applications: where are we now? Bacteriophage applications. Letters in Applied Microbiology 51, 363–369.

Selwitz, R.H., Ismail, A.I., Pitts, N.B. (2007). Dental caries. The Lancet 369, 51–59.

Tlaskalová-Hogenová, H., Štěpánková, R., Hudcovic, T., Tučková, L., Cukrowska, B., Lodinová-Žádnı́ková, R., Kozáková, H., Rossmann, P., Bártová, J., Sokol, D., Funda, D.P., Borovská, D., Řeháková, Z., Šinkora, J., Hofman, J., Drastich, P., Kokešová, A. (2004). Commensal bacteria (normal microflora), mucosal immunity and chronic inflammatory and autoimmune diseases. Immunology Letters 93, 97–108.