Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo

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.

1.3 Types of Microorganisms

  • 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.
Order a print copy

As an Amazon Associate we earn from qualifying purchases.

Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/microbiology/pages/1-introduction
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/microbiology/pages/1-introduction
Citation information

© Jan 10, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.