General Microbiology (MICR300) Lecture 1 Introduction, History and Microscopy (Text Chapters: ; )

Introduction Definitions: Microbes (or microorganisms) are a large and diverse group of organisms that are too small to be seen with the eye (microscopic) and that exist as single cells or cell clusters, and the viruses, which are not cellular. Microbiology is the study of microorganisms

Introduction (continued) Microorganisms are excellent models for understanding cell function in higher organisms, including humans. Microorganisms are central to the very functioning of the biosphere. The science of microbiology is the foundation of all the biological sciences.

Microorganisms

History: First Description of Microorganisms Robert Hooke was the first to describe microorganisms (Figure 1.8) in mold

History: First Description of Bacteria Antoni van Leeuwenhoek was the first to describe bacteria in 1676 (Figure 1.9). The field of microbiology was unable to develop until Leeuwenhoek constructed microscopes that allowed scientists to see organisms too small to be seen with the naked eye.

cocci rods

Human blood smear – showing red blood cells

History: Spontaneous Generation Louis Pasteur's work on spontaneous generation led to the development of methods for controlling the growth of microorganisms.

History (continued) Spontaneous generation was the hypothesis that living organisms can originate from nonliving matter. Pasteur disproved this idea through a famous experiment (Fig 1.11) in which he compared the growth of microorganisms in one flask containing sterile broth that was exposed to the air and one containing sterile broth that was not exposed to the air.

History (continued) Microorganisms grew only in the flask exposed to the air, thereby refuting the idea that cells can arise spontaneously from nonliving matter.

History: Koch’s Postulates Robert Koch developed a set of postulates (Figure 1.12) to prove that a specific microorganism causes a specific disease:

History: Microbial Diversity Beijerinck and Winogradsky studied bacteria in soil and water and developed the enrichment culture technique for the isolation of representatives of various physiological groups.

History: Discovery of Penicillin Sir Alexander Fleming

History: Modern Era In the middle to latter part of the twentieth century, basic and applied microbiology worked hand in hand to usher in the current era of molecular microbiology. Figure 1.17 depicts landmarks in microbiology.

Landmarks in Microbiology

Microscopy Microscopes are essential for microbiological studies. Various types of light microscopes exist, including: bright-field dark-field phase contrast fluorescence microscopes.

Microscopy: Resolution All compound light microscopes (Figure 4.1) optimize image resolution by using lenses with high light-gathering characteristics (numerical aperture). The limit of resolution for a light microscope is about 0.2  m.

Microscopy: Staining Simple and/or differential cell staining (Figures 4.3, 4.4) are used to increase contrast in bright-field microscopy.

E. coli – pink S. aureus - purple

One step fluorescence stain Pseudomonas aeruginosa – green Bacillus cereus - orange

Microscopy: Special Light Microscopes A phase-contrast microscope may be used to visualize live samples and avoid distortion from cell stains; image contrast is derived from the differential refractive index of cell structures. Greater resolution can be obtained using dark-field microscopy, in which only the specimen itself is illuminated.

Microscopy: Fluorescence Microscopes Fluorescent light microscopy allows for the visualization of autofluorescent cell structures (e.g., chlorophyll) or fluorescent stains and can greatly increase the resolution of cells and cell structures.

Microscopy: Electron Microscopes Electron microscopes have far greater resolving power than light microscopes, with limits of resolution of about 0.2 nm. Two major types of electron microscopes: Transmission electron microscopy, for observing internal cell structure down to the molecular level, and Scanning electron microscopy, for three- dimensional imaging and examining surfaces.