Written by:Musfirah Rizwan
Introduction
The term microbiology derives its name from three Greek words: mikros (small), bios (life), and logos (study). Microbiology is the study of microscopic organisms; living organisms that are too small to be visible to the naked eye. They may include bacteria, archaea, algae, fungi, protozoa, and viruses.
Microbiology encompasses all aspects of these microbes or microorganisms such as their behavior, evolution, ecology, biochemistry, and physiology, along with the pathology of diseases that they cause.
Figure 1. Bacteria culture in petri dish
The Golden Era of Microbiology
The existence of microorganisms was speculated for many centuries before the actual discovery in the 17th century. Athanasius Kircher, a Jesuit priest, was likely the first person to observe microbes in milk and putrid material in 1658. Kircher was among the first to design magic lanterns for projection purposes, and so he was well acquainted with the properties of lenses. He also wrote 'Concerning the Wonderful Structure of Things in Nature, Investigated by Microscope' in 1646 and published his 'Scrutinium Pestis' (Examination of Plague) in 1658.
The English naturalist, Robert Hooke (1635-1703), was an early microscopist and published the first book devoted entirely to microscopic observations of microorganisms. He made his first recorded microscopic observation of the floating bodies of molds (fungi) in 1665.
In 1676, the Dutch amateur microscopist, Antonio Van Leeuwenhoek observed bacteria and other microorganisms through a single-lens microscope that he designed. Through a lens with a magnification of 300×, he observed bacteria and some other microorganisms. In 1684, his discoveries were published in the Royal Society of London's journal ‘Philosophical Transactions.’ He is considered the 'Father of Microbiology'. Leeuwenhoek's discoveries boosted the long held belief that infectious diseases were caused by some sort of invisible agent.
Later in the 19th Century, scientific microbiology advanced when the French Chemist, Louis Pasteur (1822-1895), disproved the theory of spontaneous generation through his series of microscopic observations and rigorous experiments; the theory proposed that living organisms originated spontaneously, particularly from decaying organic matter. Pasteur showed that microorganisms are responsible for fermentation and spoilage and developed the method of 'Pasteurization'. He also claimed that many diseases are caused by microbes (the germ theory) and developed the vaccines for rabies and anthrax.
The claim of Pasteur was supported by Robert Koch (1843-1910), a German Physician, who is best known for his contributions to the germ theory of diseases, alongside Pasteur. Koch developed a series of criteria for linking a specific microbe to a specific infectious disease, now known as Koch's postulates. He isolated the suspected pathogens in pure cultures in the laboratory and used Postulates to link the pathogen to the disease; for instance, linking mycobacterium tuberculosis to the disease tuberculosis. He was awarded a Nobel Prize in 1905 for this monumental achievement.
As microbiology entered the 20th century, the true breadth of microbiology was revealed. Notable microbiologists in this era were Martinus Beijerinck and Sergei Winogradsky. Beijerinck made two major contributions to microbiology: the discovery of viruses and the development of enrichment culture techniques. On the other hand, Winogradsky was the first to propose the concept of chemolithotrophy, the oxidation of inorganic compounds for the purpose of obtaining the energy necessary for growth. He was also responsible for the first isolation and description of both nitrifying and nitrogen-fixing bacteria.
Alongside, other achievements of the 20th century included the development of first vaccines and antibiotics and discovery of DNA as the genetic material of the cell introducing the field of genetics.
The Modern Era of Microbiology
Figure 2. Important landmarks in microbiology in the past 65 years
Microbiology has never been more exciting or significant than it is today. Powerful new technologies, including novel imaging techniques, genomics, proteomics, nanotechnology, rapid DNA sequencing and massive computational capabilities have converged to make it possible for scientists to delve into many unapproachable inquiries.
Classification of Microorganisms
Figure 3. Classification of Microorganisms
Microorganisms encompass an enormous diversity of microscopic life forms, each with distinct characteristics. The science of classifying organisms is called taxonomy which consists of classifying new organisms or reclassifying existing ones. Microorganisms are scientifically recognized using a binomial nomenclature using two words that refer to the genus and the species.
The classification of microorganisms into various taxonomic groups is rapidly undergoing revision and modification as novel species continue to be discovered. With the progressing modifications, methods of classifications are constantly changing, however, the most widely used methods for classifying microbes include morphological characteristics, different staining, biochemical testing, DNA fingerprinting or DNA base composition, polymerase chain reaction, and DNA chips.
On the basis of their genotypic (genetic) and phenotypic (observed) properties all organisms are classified into three domains – the bacteria, archaea, and eukarya. The classification of these domains is based on identifying similarities in ribosomal RNA sequences of microorganisms, emphasizing whether an organism is a prokaryote or a eukaryote. The prokaryotes (Bacteria and Archaea), simple cell microbes, do not contain membrane bound organelles and lack true nucleus. As opposed to that, the presence of membrane bound organelles, including a defined nucleus, is the hallmark of eukaryotic cells, also described as complex cells. Microbial Eukarya include fungi, protozoa, algae and certain helminths, especially the parasitic worms.
Bacteria and Archaea:
The Bacteria and Archaea are vast groups of microorganisms consisting of potentially hundreds of thousands to millions of species, most of which remain uncharacterized. These microbes are ubiquitous, inhabiting and subsisting in nearly every imaginable environment on Earth.
Three major morphological forms of cells of bacteria and archaea are spherical (coccus), rod-shaped (bacillus) and spiral-shaped (spirillum). Other forms include tightly-coiled (spirochete), appendaged and filamentous. Many of these cells are motile, having flagella, and have a cell wall that confers structural integrity and shape to the cell.
Most species divide by binary fission; their DNA plasmids often carry genes for antibiotic resistance, toxin production or specialized metabolisms. Although certain bacteria are notorious for the devastating human diseases they cause such as cholera and tuberculosis, many are beneficial. The human gut has many bacteria that aid in digestion and some species are used in food production processes such as fermentation. Archaea differ from bacteria in certain aspects such as different cell wall compositions (they lack peptidoglycan), RNA sequences and coenzymes. Also many species are unculturable unlike bacteria species.
Eukaryotes
Like bacteria and archaea, eukaryotic microorganisms are remarkably diverse, consisting of hundreds of thousands of species of fungi, protozoa and algae, as well as hundreds of trees of parasitic worms.
Fungi
These microorganisms resemble plants in that they have rigid cell walls and are not mobile, but unlike plants they do not have chloroplasts and do not photosynthesise. Fungi exist in two basic forms: molds that consist of hyphae or mycelium and yeast which are unicellular and oval-shaped. Fungi can reproduce through asexual and sexual reproduction both. They play a key role in the decomposition and recycling of nutrients. Fungi are also important in food processing and medicine, used to make bread, beer and some cheeses.
Figure 5. Mushroom fungi
Protozoa
Protozoa are unicellular microorganisms which are heterophobic. They do not have a cell wall, their membrane is often surrounded by pellicles. Some of the important protists are: Malaria pathogen, Plasmodium and another is vector transmitted Trypanosoma Brucei. Protozoa play a significant role in the food chain. They recycle nutrients, decompose organic matter and increase biomass in aquatic habitats. They also form symbiotic relationships and contribute to the production of the planet's oxygen through photosynthesis.
Algae
Figure 6. Algae bloom
Algae are plant-like protists, their ability to perform photosynthesis distinguish them from fungi and most protozoa. Algae may be unicellular or multicellular, some of them being non-motile welcome move through flagella. Some algae are important sources of food or food additives for humans; for example, the red alga, Porphyra is popular in sushi preparation and other red algae are a source of agar. Aquatic algae provide food and oxygen for other organisms in natural waters and may play a significant role in water treatment.
Parasitic Worms
Parasitic worms including roundworms and flatworms are multicellular organisms that can be viewed with the naked eye. They are often the cause of multiple infectious diseases and require multiple hosts for their life cycles including humans, other mammals, insects (eg. flies, mosquitoes), fish species and certain aquatic invertebrates, like squids, snails and crustaceans.
Virus
Figure 7. Colorized transmission electron micrograph of Influenza A virus
Viruses are acellular microorganisms that require living host cells to reproduce and replicate. They infect all forms of life, from humans, animals and plants to microorganisms like bacteria or algae. Viral infections lead to death of the host cell. Many severe diseases, such as AIDS, influenza, measles, poliomyelitis, rabies and haemorrhagic fevers are brought on by viruses.
With a broad overview of microbial diversity, we are able to explore the crucial role that microorganisms play in the environment around us.
Branches of Microbiology
As a result of diverse microbial discoveries and developments in modern times, Microbiology is claimed to have expanded into numerous widely accepted areas.
The branches of Microbiology are as follows:
Pure Microbiology
Bacteriology
Virology
Mycology
Parasitology
Immunology
Protozoology
Applied Microbiology
Medical Microbiology
Pharmaceutical microbiology
Industrial microbiology
Food microbiology
Agricultural microbiology
Environmental microbiology
Water microbiology
Aero microbiology
Microbial biotechnology
The microbial world lays the foundation of the biosphere, and thus, the science of microbiology.
Bibliography
[1] Wainwright , M. “An Alternative View of the Early History of Microbiology.” Advances in Applied Microbiology’ , vol. 52, 2003, pp. 333–355, https://doi.org/10.1016/S0065-2164(03)01013-X.
[2] Sattley, W Matthew, and Michael T Madigan. “Microbiology.” ELS, 19 Aug. 2015, pp. 1–10, https://doi.org/10.1002/9780470015902.a0000459.pub2. Accessed 5 Sept. 2023.
[3] Madigan, M, and J Martinko. Brock Biology of Microorganisms . 13th ed., Pearson Education , 2006, p. 1096.
[4] Boundless. “1.2B: Classification of Microorganisms.” Biology LibreTexts, 6 May 2017, bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/01%3A_Introduction_to_Microbiology/1.02%3A_Microbes_and_the_World/1.2B%3A_Classification_of_Microorganisms.
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