1.1 What Our Ancestors Knew

Microorganisms (or microbes) are living organisms that are generally too small to be seen without a microscope.
Throughout history, humans have used microbes to make fermented foods such as beer, bread, cheese, and wine.
Long before the invention of the microscope, some people theorized that infection and disease were spread by living things that were too small to be seen. They also correctly intuited certain principles regarding the spread of disease and immunity.
Antonie van Leeuwenhoek, using a microscope, was the first to actually describe observations of bacteria, in 1675.
During the Golden Age of Microbiology (1857–1914), microbiologists, including Louis Pasteur and Robert Koch, discovered many new connections between the fields of microbiology and medicine.

1.2 A Systematic Approach

Carolus Linnaeus developed a taxonomic system for categorizing organisms into related groups.
Binomial nomenclature assigns organisms Latinized scientific names with a genus and species designation.
A phylogenetic tree is a way of showing how different organisms are thought to be related to one another from an evolutionary standpoint.
The first phylogenetic tree contained kingdoms for plants and animals; Ernst Haeckel proposed adding kingdom for protists.
Robert Whittaker’s tree contained five kingdoms: Animalia, Plantae, Protista, Fungi, and Monera.
Carl Woese used small subunit ribosomal RNA to create a phylogenetic tree that groups organisms into three domains based on their genetic similarity.
Bergey’s manuals of determinative and systemic bacteriology are the standard references for identifying and classifying bacteria, respectively.
Bacteria can be identified through biochemical tests, DNA/RNA analysis, and serological testing methods.

Microorganisms are very diverse and are found in all three domains of life: Archaea, Bacteria, and Eukarya.
Archaea and bacteria are classified as prokaryotes because they lack a cellular nucleus. Archaea differ from bacteria in evolutionary history, genetics, metabolic pathways, and cell wall and membrane composition.
Archaea inhabit nearly every environment on earth, but no archaea have been identified as human pathogens.
Eukaryotes studied in microbiology include algae, protozoa, fungi, and helminths.
Algae are plant-like organisms that can be either unicellular or multicellular, and derive energy via photosynthesis.
Protozoa are unicellular organisms with complex cell structures; most are motile.
Microscopic fungi include molds and yeasts.
Helminths are multicellular parasitic worms. They are included in the field of microbiology because their eggs and larvae are often microscopic.
Viruses are acellular microorganisms that require a host to reproduce.
The field of microbiology is extremely broad. Microbiologists typically specialize in one of many subfields, but all health professionals need a solid foundation in clinical microbiology.