What is microbiology?
To keep things simple, studying microbiology means to investigate microorganisms like bacteria, viruses, archaea, algae, fungi, and protozoa. But for this article, we will be mainly placing a focus on the study of bacteria (termed bacteriology).
Some key findings in the past about microbiology
You will most likely come across these two names at some point during your studies in microbiology - Louis Pasteur (1822-1895) and Robert Koch (1843-1910).
Though, what were they famous for?
Essentially, you could say that Mr Pasteur was the father of medical microbiology. He was the guy who showcased the concept of sterilisation and thereby disproved the existence of spontaneous generation. This eventually led to the idea of pasteurization. Besides that, he was also the guy who first introduced the principles of vaccination, whereby he eventually developed the novel vaccines against rabies, anthrax, and fowl cholera.
Meanwhile, Mr Koch was the 1905 Nobel laureate in Physiology or Medicine who came up with the Koch’s postulates, which are basically hypotheses to prove that certain microorganisms cause a particular disease(s). These four postulates can be summarised as below:
1. The suspected pathogenic organism should be present in all cases of the disease, and absent from healthy animals
2. The suspected organism should be grown in pure culture
3. Cells from a pure culture of the suspected organism should cause disease in a healthy animal
4. The organism should be re-isolated and shown to be the same as the original
Some basic details about bacteriology
The nomenclature for prokaryotes
Figure 1: The hierarchical model used in the taxonomic classification system to categorise living organisms into highly specific groups.
There are three domains of life which can be broadly categorised into eukaryotes, bacteria, and archaea, in which bacteria and archaea are considered to be prokaryotic species (meaning organisms that do not possess a membrane-bound nucleus, mitochondria, or any other organelle that is unique to eukaryotes).
The bacteria domain has more than 40 phyla (such as Bacteroidetes and Proteobacteria), with each of them containing various genera like Escherichia and Pseudomonas. Furthermore, each of these genus constitutes several species (e.g. Staphylococcus aureus and Escherichia coli).
The many shapes of bacteria
When observing bacteria under the microscope, it is extremely important for microbiologists to take note of their shape. Some of the typical shapes that one may observe include rod (commonly found amongst Eschericihia coli and Bacillus subtilis), coccus (meaning spherical; the regular shape for Staphlyococcus aureus and Streptococcus pyogenes), and spirilla. Though of course, unusual shapes do exist too, such as spirochetes, tightly coiled bacteria, and filamentous bacteria.
Interestingly, some bacteria can form characteristic arrangements. For instance, bacteria with a cocci shape may form chains, whilst others might remain in clusters after they divide. On the other hand, the size of bacterial species can range between 0.25mm x 1.2mm (in the case of Haemophilus influenza) to 600mm in length (a great example, Epulospicum fishelsoni).
Bacterial growth characteristics
Carbon, hydrogen, oxygen, and nitrogen are known as macronutrients, which are major elements that are needed in large quantities within a bacterium. Besides that, there are also some minor elements that are required in smaller amounts by bacteria, including sulphur, phosphorus, potassium, calcium, magnesium, and sodium. In contrast, there are micronutrients such as iron and other variants of metal that are only essential in minute proportions. In general, the majority of bacterial species make use of organic carbon as their source of energy. These types of bacteria are known as heterotrophs. On the other hand, other species of bacteria utilise nitrogen (e.g. nitrifying bacteria), sulphur, iron (by iron-oxidising bacteria) as an energy source, whereas others like cyanobacteria may have photosynthetic machinery in their cytoplasmic membrane.
Most bacteria reproduce by binary fission. During the batch culturing of bacteria, the typical growth curve can be divided into four main phases, namely the lag phase, exponential phase (sometimes called the log phase), stationary phase, and the death phase.
Figure 2: Graphical illustration of a typical bacterial growth curve in a batch culture medium.
The lag phase takes place when the bacteria species is adapting to the growth conditions of their new environment. This is a period where the bacteria are not dormant as they are actively maturing. However, they are still unable to reproduce as they are still in the middle of synthesising the necessary nucleic acids, molecules, and enzymes required for growth. The length of this phase differs greatly depending on the bacterial species. Moving forward, the exponential phase involves having a rising number of bacteria within the culture. This growth rate is often dependent on various factors such as the incubation temperature, availability of nutrients, pH level, and genetic factors. The doubling time (meaning the time taken for a bacterial population to double in size) can range from approximately 20 minutes to several hours, based on the bacterial species.
Next in line, the stationary phase occurs when the exponential growth phase becomes unsustainable due to the limited amount of nutrients within the culture or an accumulation of toxic by-products (such as reactive oxygen species) to an inhibitory level. Bear in mind that these bacteria are still alive; it is just that there is no net increase or decrease in the bacteria count. And last but not least, the death phase includes dying (i.e. non-viable) bacteria, which is usually caused by a depletion of a specific nutrient or a fatal change in certain environmental condition(s).
The general structure of a bacterium
The many types of bacterial species can also be broadly classified as Gram-positive or Gram-negative bacteria. The key difference between these two is that Gram-positive bacteria have cell walls made out of thick layers of peptidoglycan (murein). As a matter of fact, 90% of certain Gram-positive bacteria’s cell walls may even be made up of peptidoglycan. Conversely, Gram-negative bacteria have a thin layer of peptidoglycan in their cell walls. However, their cell wall also constitutes an outer membrane that is composed of lipopolysaccharide molecules.
In general, the peptidoglycan layer serves as a rigid layer that wraps around the cytoplasmic membrane of both Gram-positive and Gram-negative bacteria, providing the bacteria its shape and structural integrity.
The fundamental peptidoglycan structure comprises glycan chains formed by connecting two sugar derivatives, N-acetyl glucosamine (GlcNac) and N-acetyl muramic acid (MurNac) in an alternating order, whereby they are held together by strong glycosidic bonds. These glycan chains are then linked together by peptide cross links between amino acids. Although peptidoglycan layers share a similar composition across the majority of bacterial species, more than 100 variations in its structure have been discovered, thus revealing its large diversity.
Why should we care?
“The role of the infinitely small in nature is infinitely large”
Louis Pasteur
One way to think about the value of learning all these is for the beauty of knowledge, where we can gain a much clearer understanding of a bacterium’s biochemistry, genetics, cell growth characteristics, and molecular mechanisms of action for survival.
Meanwhile, the more practical aspect of this field is highlighted by its great positive impact on our daily lives. Its versatile industrial applications include (but are definitely not limited to) medicine and healthcare, food and beverage, agriculture, and biotechnology. Evidently, a huge plummet in infectious diseases over the previous decades was the result of antibiotics, vaccinations, having a greater promotion of public health (including the implementation of improved hygiene measures, water treatment, disease monitoring and epidemiology, as well as general public education). All of these were made possible due to our increased knowledge in microbiology.
Though of course, microbiological research is nowhere nearing an end as there are still many types of infectious diseases out there in our world today (such as AIDS, malaria, tuberculosis, bird and swine flu). Not to mention, new modern problems such as the emergence of antibiotic-resistant bacteria remain a serious health hazard to both developing as well as developed countries.
Author: Bianca Khor, BSc Biochemistry