Chapter Summary


  • Microbes affected human civilization for centuries before humans guessed at their existence through their contributions to our environment, food and drink production, and infectious diseases.
  • Robert Hooke and Antoni van Leeuwenhoek were the first to record observations of microbes through simple microscopes.
  • Spontaneous generation is the theory that microbes arise spontaneously, without parental organisms.
  • Lazzaro Spallanzani showed that microbes arise from preexisting microbes and demonstrated that heat sterilization can prevent microbial growth.
  • Louis Pasteur discovered the microbial basis of fermentation. He also showed that providing oxygen does not enable spontaneous generation.
  • John Tyndall showed that repeated cycles of heat were necessary to eliminate spores formed by certain kinds of bacteria.
  • Florence Nightingale statistically quantified the impact of infectious disease on human populations.


  • Robert Koch devised techniques of pure culture to study a single species of microbe in isolation. A key technique is culture on solid medium using agar, as developed by Angelina and Walther Hesse, in a double-dish container devised by Julius Petri.
  • Koch’s postulates provide a set of criteria to establish a causative link between an infectious agent and a disease.
  • Edward Jenner established the practice of vaccination, inoculation of cowpox to prevent smallpox. Jenner’s discovery was based on earlier observations by Lady Mary Montagu and others that a mild case of smallpox could prevent future cases.
  • Louis Pasteur developed the first vaccines based on attenuated strains, such as the rabies vaccine.
  • Ignaz Semmelweis and Joseph Lister showed that antiseptics could prevent transmission of pathogens from doctor to patient.
  • Alexander Fleming discovered that the Penicillium mold generated a substance that kills bacteria.
  • Howard Florey and Ernst Chain purified the substance penicillin, the first commercial antibiotic to save human lives.
  • Martinus Beijerinck discovered viruses as filterable infective particles.


  • Sergei Winogradsky first developed a system of enrichment culture, the Winogradsky column, to grow microbes from natural environments.
  • Chemolithotrophs (or lithotrophs) metabolize inorganic minerals, such as ammonia, instead of the organic nutrients used by the microbes isolated by Koch.
  • Geochemical cycling depends on bacteria and archaea that cycle nitrogen, phosphorus, and other minerals throughout the biosphere.
  • Endosymbionts are microbes that live within multicellular organisms and provide essential functions for their hosts, such as nitrogen fixation for legume plants.
  • Martinus Beijerinck first demonstrated that nitrogen-fixing rhizobia grow as endosymbionts within leguminous plants.
  • Beijerinck showed that a disease could be transferred by a noncellular particle called a virus (tobacco mosaic virus).


  • Classifying microbes was a challenge historically because of the difficulties in observing distinguishing characteristics of different categories.
  • Ernst Haeckel recognized that microbes constitute a form of life distinct from animals and plants.
  • Herbert Copeland and Robert Whittaker classified prokaryotes as a form of microbial life distinct from eukaryotic microbes such as protists.
  • Lynn Margulis proposed that eukaryotic organelles such as mitochondria and chloroplasts evolved by endosymbiosis from prokaryotic cells engulfed by proto-eukaryotes.
  • Carl Woese discovered a domain of prokaryotes, Archaea, whose genetic sequences diverge equally from those of bacteria and those of eukaryotes. Many, though not all, archaea grow in extreme environments.  


  • Genetics of bacteria, bacteriophages, and fungi in the early twentieth century revealed fundamental insights about gene transmission that apply to all organisms.
  • Structure and function of the genetic material, DNA, were revealed by a series of experiments in the twentieth century.
  • Molecular microbiology generated key advances, such as the cloning of the first recombinant molecules and the invention of DNA sequencing technology.
  • Genome sequence determination and bioinformatic analysis became the tools that shape the study of biology in the twenty-first century.
  • Microbial discoveries transformed medicine and industry. Biotechnology enables the production of new kinds of pharmaceuticals and industrial products.