As stated in the historical background section, the research of these scientists demonstrated the germ theory of sickness and the germ theory of fermentation. In their laboratories, they created techniques for microscopic material examination, laboratory culturing (growing) of microbes, distinguishing pure cultures from mixed-culture populations, and a variety of other laboratory manipulations. These techniques, which were initially established to study bacteria, have since been refined to study all microorganisms, resulting in the transition from bacteriology to microbiology.
Microorganisms:
The microbial world is made up of two categories of creatures: prokaryotes and eukaryotes. All bacteria are prokaryotes, which means they are single-celled animals without a membrane-bound nucleus. Their DNA (cell genetic material) exists as a long, folded thread with no definite position within the cell, rather than being housed in the nucleus.
After this discovery, the eukarya (bacteria that diverged from other bacteria at an early stage of evolution and are distinct from the eubacteria), archaea (bacteria that diverged from other bacteria at an early stage of evolution and are distinct from the eubacteria), and eubacteria (bacteria that diverged from other bacteria at an early stage of evolution and are distinct from the eubacteria) were all named (the eukaryotes). Eubacteria are now referred to as “actual bacteria” (or “bacteria”) and are members of the Bacteria domain. However, examinations of the DNA sequences of various microbes have revealed many surprising commonalities, casting doubt on the evolutionary links between different members of these three categories. As a result, tracing the ancestry of today’s germs is extremely difficult.
Bacteria:
Even characteristics thought to be unique to specific taxonomic groups have been found in other microbes. For example, in 1999, an anaerobic ammonia-oxidizer was isolated for the first time, completing the global nitrogen cycle’s “missing link.” This bacterium (an abnormal member of the Planctomycetes order) was revealed to contain internal structures similar to eukaryotes, an archaean cell wall, and a budding form of reproduction similar to yeast cells.
Bacteria appear in a variety of shapes, including spheres, rods, and spirals. Individual cells range in size from 0.5 to 5 micrometres (m; millionths of a metre). Bacteria, despite being unicellular, frequently appear as pairs, chains, tetrads (groups of four), or clusters. Some contain flagella, which are external whiplike structures that propel the creature through liquid media; others have a capsule, which is an outside cell shell; and still others produce spores, which are reproductive organisms that function in the same way as seeds do in plants. One of the most defining characteristics of bacteria is their response to the Gram stain.
Depending on the chemical and structural makeup of the cell wall, certain bacteria are gram-positive and take on the purple colour of the stain, while others are gram-negative.
Although archaea look similar to bacteria under a microscope, they differ significantly in their chemical composition, metabolic activities, and habitats. The chemical substance peptidoglycan is found in all true bacteria’s cell walls but not in archaeans’ cell walls.
Archaebacteria:
Many archaeans are notable for their ability to thrive in severe settings, such as high salt or acid levels or great heat. Extremophiles are microorganisms that thrive in harsh settings such as salt flats, thermal pools, and deep sea vents. Some are capable of producing methane gas from carbon dioxide and hydrogen, which is a chemical activity that is unique to them. Archaea that create methane can only live in oxygen-free environments, such as swamp mud or the intestines of ruminants such as cattle and sheep. This group of bacteria has a wide range of variety in the chemical changes they induce in their surroundings.
The nuclei of eukaryotic microorganisms are similar to those of plant and animal cells in that their DNA is surrounded by a nuclear membrane. Eukaryotic microorganisms include algae, protozoa, and fungus. Protists (order Protista, also known as Protoctista) are creatures that include algae, protozoa, and some lower fungi. Others are multicellular, while some are unicellular.
Algae:
Algae, unlike bacteria, are eukaryotes, which means they have rigid cell walls, the green pigment chlorophyll, and can photosynthesis. They are most typically observed in damp soil and watery environments. These eukaryotes can be unicellular and small, or multicellular and large, reaching lengths of up to 120 metres (almost 400 feet). Algae come in a variety of forms in general. Single-celled species might be spherical, rod-shaped, club-shaped, or spindle-shaped. Some of them can move about. Multicellular algae come in a variety of forms and sizes, each with a different level of complexity. Some are structured as end-to-end strands of cells that intertwine to generate macroscopic plantlike structures in some species. Algae colonies occur; some are simple aggregations of single cells, while others have a range of cell types with distinct roles.
Fungi:
Fungi are unicellular or multicellular eukaryotic animals with rigid cell walls comparable to algae. Some are microscopic, but others, such as mushrooms and bracket fungus, which grow in soil or on moist wood, develop much larger structures. Fungi, unlike algae, do not have chlorophyll and hence cannot undertake photosynthesis. Fungi do not eat and instead rely on the environment to give dissolved nutrients. Moulds are multicellular fungi that generate filamentous, microscopic structures, whereas yeasts are unicellular fungi that generate filamentous, microscopic structures.
Moulds have cylindrical cells that link end to end to form threadlike filaments (hyphae) that can transport spores. When a large number of hyphae clump together, such as on a piece of bread or a tablespoon of fruit jelly, they form a fuzzy mass called a mycelium, which is visible to the naked eye.
The unicellular yeasts range in shape and size from spherical to egg-shaped to filamentous. Yeasts are well-known for fermenting carbohydrates and producing alcohol and carbon dioxide in products such as wine and bread.
Conclusion:
Until the late 1970s, it was widely considered that all bacteria evolved in the same way. Carl R. Woese and colleagues at the University of Illinois called this theory into doubt in 1977, when they revealed that two groups of bacteria evolved from a similar and ancient ancestral form via separate pathways employing ribosomal RNA from a diverse spectrum of living species.