Summary
Highlights
Biologically, a species is a group of individuals capable of breeding and producing fertile offspring. This definition, however, doesn't apply universally, especially to organisms like bacteria that reproduce asexually. For example, mules (offspring of donkey and horse) are infertile, making donkeys and horses separate species.
Robert Hooke first observed 'cells' in cork in 1665, leading to the cell theory. Anton van Leeuwenhoek was the first to describe living microorganisms, 'animalcules,' in 1673 using a primitive microscope.
The discovery of microbes sparked a debate: spontaneous generation (life from non-living matter) versus biogenesis (life from pre-existing life). Francesco Redi's 1668 meat and maggot experiment supported biogenesis by showing maggots only appeared when flies had access to meat.
John Needham's 1745 experiment with boiled and then capped broth showed microbial growth, which he interpreted as spontaneous generation. Lazzaro Spallanzani later refuted this in 1765 by boiling broth *after* capping, resulting in no growth, demonstrating that microbes came from the air, not spontaneously from the broth itself.
In 1861, Louis Pasteur conclusively disproved spontaneous generation using S-shaped flasks. He boiled broth in these flasks, which allowed air in but trapped airborne microbes in the bent neck, preventing growth. This showed that microbes from the air, not the air itself, caused contamination. Pasteur also developed pasteurization to prevent spoilage.
In the 1840s, Ignaz Semmelweis advocated handwashing for doctors to prevent childbirth fever, though his ideas were initially rejected. Joseph Lister in the 1860s applied germ theory to surgery by using carbolic acid to disinfect instruments. Robert Koch's postulates (1875) provided a scientific method to link specific microbes to specific diseases, establishing the germ theory of disease.
Edward Jenner developed the first vaccine in 1796 against smallpox, observing that cowpox infection conferred immunity. Pasteur later explained the mechanism of vaccination in the 1880s, noting weakened pathogens could still induce immunity. In 1928, Alexander Fleming accidentally discovered penicillin, the first antibiotic, from a mold (Penicillium), inhibiting bacterial growth.
The 1970s saw the discovery of restriction enzymes by Daniel Nathans, Werner Arber, and Hamilton Smith. These 'molecular scissors' cut DNA at specific sites, enabling genetic engineering and recombinant DNA technology, which revolutionized biological research and applications like human insulin production.
The 1980s brought Polymerase Chain Reaction (PCR), a technique to amplify tiny amounts of DNA, crucial for diagnosing infectious diseases (e.g., COVID-19 tests) and forensic science. The 2000s highlighted the critical role of small non-coding RNAs in regulating cellular processes, opening new avenues for disease treatment.
The Human Microbiome Project (2010s and beyond) revealed the vast diversity and functional importance of microbial communities in and on the human body. It shows that disruptions in the microbiome can lead to various diseases and suggests new interventions, like fecal transplants for C. difficile infections, to restore healthy microbial balance.
Microbiology is the study of living things too small to be seen without magnification. Examples include bacteria, archaea, protozoans, fungi, helminths, viruses, and algae. These microbes vary greatly in size, with bacteria being very small and viruses being ultramicroscopic and acellular.
Microbes have shaped Earth's habitat for billions of years. Life began 3.8 billion years ago as single-celled organisms, leading to three cell types: eukaryotic (true nucleus), archaea, and bacteria (both prokaryotic, lacking a membrane-bound nucleus). Archaea are more closely related to eukaryotes than bacteria.
Bacteria are prokaryotic, lacking a membrane-bound nucleus. Their cell walls are typically made of peptidoglycan (protein and sugar). They are unicellular, reproduce by asexual binary fission, and have circular DNA. Some are photosynthetic (autotrophic), while others are heterotrophic, consuming food for energy.
Archaea are also prokaryotic and lack peptidoglycan in their cell walls, instead using pseudomurin. They are unicellular, reproduce by binary fission, and have circular DNA. Many are extremophiles, thriving in extreme environments like high heat (thermophiles) or high salt (halophiles).
Fungi are eukaryotic with cell walls made of chitin. They are heterotrophic, consuming food, and many are saprobes, feeding on dead matter. Fungi can be unicellular (yeast) or multicellular (molds, mushrooms) and reproduce sexually or asexually. They have linear DNA.
Protozoans are eukaryotic, usually lacking cell walls, and are typically heterotrophic. They are unicellular and can reproduce sexually or asexually. They exhibit various forms of movement, including pseudopods, flagella, or cilia, while some are non-motile. An example is Trypanosoma, which causes African sleeping sickness through antigenic switching.
Algae are eukaryotic with cell walls made of cellulose. They are photosynthetic (autotrophic) and can be unicellular or multicellular. They reproduce sexually or asexually and often contain pigments, making them appear green, red, or brown. Photosynthetic microorganisms are responsible for about 70% of Earth's photosynthesis.
Viruses are acellular and considered non-living, as they are obligate intracellular parasites, requiring a host cell to reproduce. Their genetic information is DNA or RNA, but generally not both. They consist of genetic material surrounded by a protein coat (capsid) and sometimes an envelope, allowing them to evade the immune system.
Helminths are worms classified as multicellular animal parasites. They lack cell walls, are heterotrophic, and can reproduce sexually or asexually. Despite their macroscopic size, they have microscopic stages that can cause disease, making them relevant to microbiology.
Microbes are ubiquitous and essential for life, found in diverse environments from the Earth's crust to within plant and animal bodies. They drive environmental cycles, contribute significantly to photosynthesis and oxygen production, and assist plants in nutrient acquisition and disease protection.
Humans have historically used microbes for producing food (bread, cheese), beverages (alcohol), and treating wounds. In biotechnology, microbes are used in genetic engineering (manipulating genes for new products), recombinant DNA technology (transferring genetic material, such as for human insulin production), and bioremediation (cleaning up pollutants like oil spills).
While often associated with disease, most human-associated microorganisms are harmless or beneficial. Disease-causing microbes are called pathogens. Infectious diseases are a leading cause of death worldwide, and new emerging or re-emerging diseases continue to pose global health challenges. Non-infectious diseases, like gastric ulcers, are also being linked to microbial involvement.
There's an increasing number of patients with weakened defenses, making them susceptible to opportunistic pathogens. Also, there's a rise in drug-resistant microbes, making once-treatable conditions like gonorrhea much harder to manage.
Taxonomy is the science of classifying living things. Nomenclature involves assigning scientific names, established by Carolus Linnaeus in 1735 using a binomial (two-name) system: genus (capitalized) and specific epithet/species (lowercase), both underlined or italicized. Names can be descriptive or honor discoverers.
When first mentioning an organism, its full genus and species name must be written (e.g., Escherichia coli). Subsequent mentions can abbreviate the genus (e.g., E. coli). This prevents ambiguity as different organisms can share a species name.
The current three-domain system (Bacteria, Archaea, Eukarya) was proposed by Carl Woese in 1978, based on ribosomal RNA sequences. Domain is the broadest classification, followed by kingdom. Eukarya includes kingdoms Protista, Fungi, Plantae, and Animalia. The Protista kingdom is constantly being reclassified due to its diversity.
A phylogenetic tree illustrates evolutionary relationships. Bacteria diverged early, while Archaea are more closely related to Eukarya. Classification follows a hierarchical system: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. This system moves from broad to specific categories, with species being the most specific.