Microbiology - Bacteriology Lecture (1) PDF

MICROBIOLOGY BACTERIOLOGY LECTURE (1) Infection Humanity has but three great enemies: Fever, famine and war; of these by far the greatest, by far the most terrible, is fever. Sir William Osler, 1896* Fever Infection was indeed the scourge of the world. Tuberculosis...

MICROBIOLOGY BACTERIOLOGY LECTURE (1) Infection Humanity has but three great enemies: Fever, famine and war; of these by far the greatest, by far the most terrible, is fever. Sir William Osler, 1896* Fever Infection was indeed the scourge of the world. Tuberculosis and other forms of pulmonary infection were the leading causes of premature death among the well to do and the less fortunate. The science of medical microbiology dates back to the pioneering studies of Pasteur and Koch, who isolated specific agents and proved that they could cause disease by introducing the experimental method. Lecture 1 Microbiology- An Introduction INFECTIOUS AGENTS: THE MICROBIAL WORLD Microbiology is a science defined by smallness The study of living organisms too small to be clearly seen by the unaided eye (i.e., microorganisms); these include viruses, bacteria, archaea, protozoa, algae, and fungi Others include macroscopic forms in their life cycle (eg, fungal molds, algae, parasitic worms), are large enough to be visible, but are still included in the field of microbiology Most play benign roles in the environment Products of microbes contribute to the atmosphere Knowledge of Microorganisms Allows humans to: -Prevent food spoilage -Prevent disease occurrence -Led to aseptic techniques to prevent contamination in medicine and in microbiology laboratories. Microbes in Our Lives -Microorganisms are important in maintain earth’s ecological balance. -Some microorganisms live in humans and other animals and are needed to maintain good health. -Produce industrial chemicals such as ethanol and acetone. -Produce fermented foods such as vinegar, cheese, and bread. -Produce products used in manufacturing (e.g., cellulose) and treatment (e.g., insulin). -A few are pathogenic, disease-causing. The four classes of infectious agents The major classes of microorganisms in terms of ascending size and complexity are: Viruses, bacteria, fungi, and parasites. Parasites exist as single or multicellular structures with the same compartmentalized eukaryotic cell plan of our own cells including a nucleus and cytoplasmic organelles like mitochondria. Fungi are also eukaryotic, but have a rigid external wall that makes them seem more like plants than animals. Bacteria also have a cell wall, but with a cell plan called “prokaryotic” that lacks the organelles of eukaryotic cells. Viruses are not cells at all. They have a genome and some structural elements, but must take over the machinery of another living cell (eukaryotic or prokaryotic) to replicate. Features of Infectious Agents CLASSIFICATION OF MICROORGANISMS DOMAIN: EUKARYA---contains 4 kingdoms KINGDOM: PROTISTA---unicellular eukaryocytes(protozoa, slim molds, algae) except yeasts & molds KINGDOM: FUNGI---unicellular yeasts, multicellular molds, macroscopic fungi--these all absorb organic matter through their plasma membranes KINGDOM: PLANTAE- (MULTICELLULAR), plants-- macroscopic algae, mosses, ferns, conifers, flowering plants--all are multicellular, all carry on photosynthesis (photoautotrophs) KINGDOM: ANIMALIA- ((MULTICELLULAR), animals-- sponges, worms, insects, vertebrates--all ingest nutrients (heterotrophs).  Bacteria Bacteria Are the smallest (0.1–10 μm) independently living cells. Bacteria are unicellular organisms They have a cytoplasmic membrane surrounded by a cell wall; a unique interwoven polymer called peptidoglycan makes the wall rigid. The simple prokaryotic cell plan includes no mitochondria, lysosomes, endoplasmic reticulum, or other organelles. In fact, most bacteria are approximately the size of mitochondria. Their cytoplasm contains only ribosomes and a single, d-stranded DNA chromosome. Bacteria have no nucleus, but all the chemical elements of nucleic acid and protein synthesis are present. Although their nutritional requirements vary greatly, most bacteria are free living if given an appropriate energy source. Tiny metabolic factories, they divide by binary fission and can be grown in artificial culture, often in less than 1 day. The Archaea are similar to bacteria but evolutionarily distinct. They are prokaryotic, but differ in the chemical structure of their cell walls and other features. The Archaea can live in environments humans consider hostile (eg, hot springs, high salt areas) but are not associated with disease. Methanogens, halophiles, and extreme thermophiles.  Naming and Classifying Microorganisms Nomenclature: Each living organism is assigned two names The two names consist of genus & specific epithet (species), both of which are underlined or italicized. Examples: Escherichia coli or Escherichia coli, Staphylococcus aureus or Staphylococcus aureus. (first name always capitalized, second not) Staphylococcus aureus and Escherichia coli are found in the human body. S. aureus is on skin and E. coli in the large intestine Type of Microorganisms Bacteria Type of Microorganisms Bacteria: The three major basic shapes of bacteria are -1- Bacillus 2-Coccus 3-Spiral. Type of Microorganisms Fungi (mushroom, molds, and yeasts): Have eukaryotic cells (with a true nucleus). Fungi exist in either yeast or mold forms. The smallest of yeasts are similar in size to bacteria, but most are larger (2–12 μm) and multiply by budding. Molds form tubular extensions called hyphae, which, when linked together in a branched network, form the fuzzy structure seen on neglected bread slices????????? Fungi are eukaryotic, and both yeasts and molds have a rigid external cell wall composed of their own unique polymers, called glucan, mannan, and chitin. Their genome may exist in a diploid or haploid state and replicate by meiosis or simple mitosis. Most fungi are free living and widely distributed in nature. Generally, fungi grow more slowly than bacteria Fungi obtain nutrients by absorbing organic material from their environment. (chemoheterotrophs) Molds Filamentous Fungi In the laboratory, fungi are usually grown from fragments obtained from a fungal thallus. Within the hyphae of a mold, their are two distinct parts: 1. Vegetative hyphae (obtain nutrients), embedded in the media (root). 2. Reproductive (aerial) hyphae, above the surface of the media, and often bear reproductive spores. When environmental conditions are suitable, the hyphae grow to form a filamentous mass called mycelium, which is visible to the unaided eye. 16 Molds Yeast Nonfilamentous, Unicellular fungi. The yeast body takes a globular (oval)or round structure rather than the ribbon-like body of a mold. It is yeast that take the fuzzy white appearance often seen on fruits. Some are capsulated, Cryptococcus neoformans. Where as molds reproduce via spore formation. 1- Budding yeast , the original parent cell forms a bud on its outer surface, this bud will eventually: elongate, the parent cell’s nucleus divides, and one nucleus migrate into the bud, build a cell wall to separate itself, and break off. Saccharomyces, divide unevenly. An individual yeast cell has the capacity to form up to 24 daughter cells by budding before it loses this capacity. In some cases, like that of the yeast Candida albicans, the last step of bud formation does not occur, (detached). This results in a long chain of yeast cells called pseudohyphae. For Candida albicans, this allows for the fungi to invade deeper into tissue and form thrush. Electron Micrograph of Saccharomyces Cryptococcus neoformans. Capsules –Indian ink stain Dimorphic fungi. At 37°C, these fungi take a yeast-like morphology. This represents the body temperature of humans. The advantage here is that spores can be sexual in nature, requiring two different molds to fuse and reproduce. Budding is asexual. Therefore, in the human body, these pathogenic fungi can reproduce in isolation. Also, yeast demonstrate great metabolic diversity which may aid in energy production in the host. Therefore, the ability of some fungi to fluctuate between the yeast-like and mold-like state have provided an evolutionary advantage for pathogenic fungi. Tinea corporis Mycetoma (Madura foot) Type of Microorganisms Viruses: Viruses are strict intracellular parasites of other living cells, not only of mammalian and plant cells, but also of simple unicellular organisms, including bacteria (the bacteriophages). Viruses are simple forms of replicating, biologically active particles that carry genetic information in either DNA or RNA molecules. Most mature viruses have a protein coat over their nucleic acid and, sometimes, a lipid surface membrane derived from the cell they infect. Because viruses lack the protein-synthesizing enzymes and structural apparatus necessary for their own replication, they bear essentially no resemblance to a true eukaryotic or prokaryotic cell (acellular) Viruses replicate by using their own genes to direct the metabolic activities of the cell they infect to bring about the synthesis and reassembly of their component parts. A cell infected with a single viral particle may, thus, yield thousands of viral particles, which can be assembled almost simultaneously under the direction of the viral nucleic acid. Infection of other cells by the newly formed viruses occurs either by seeding from or lysis of the infected cells. Sometimes, viral and cell reproduction proceed simultaneously without cell death, although cell physiology may be affected. The close association of the virus with the cell sometimes results in the integration of viral nucleic acid into the functional nucleic acid of the cell, producing a latent infection that can be transmitted intact to the progeny of the cell. Viral Diseases Edward Jenner  Viral Diseases where do “new” HA and NA come from? 27  2500 Viral Diseases 4000 5000 BIOFAFTY LEVEL 4 REQUIRED (river in Zaire) Marburg(Germany) Viral Diseases AIDS CD4 count drops below 200 person is considered to have advanced HIV disease If preventative medications not started the HIV infected person is now at risk for: Pneumocystis carinii pneumonia (PCP) cryptococcal meningitis toxoplasmosis If CD4 count drops below 50: Mycobacterium avium Cytomegalovirus infections lymphoma dementia Most deaths occur with CD4 counts below 50. Viral Diseases Basic virus structure DNA Capsid Naked or + Nucleocapsid = protein capsid virus RNA Lipid membrane, Nucleocapsid + Enveloped virus glycoproteins Viral structure  Type of Microorganisms Parasites Are the most diverse of all microorganisms. They range from unicellular amoebas of 10 to 12 μm to multicellular tapeworms 1 m long. The individual cell plan is eukaryotic, but organisms such as worms are highly differentiated and have their own organ systems. Most worms have a microscopic egg or larval stage, and part of their life cycle may involve multiple vertebrate and invertebrate hosts. Most parasites are free living, but some depend on combinations of animal, arthropod, or crustacean hosts for their survival Protozoa: Are unicellular eukaryotes. Protozoa obtain nourishment by absorption or ingestion through specialized structures. Malaria, giardiasis, African sleeping sickness, and amebic dysentery are examples of human diseases caused by parasitic protozoa Algae: Are unicellular or multicellular eukaryotes that obtain nourishment by photosynthesis. Together with protozoa, are classified in the kingdom (Protista)  The Human Microbiota WE harbor 10 times the number of microbial cells as we do human cells. This population formerly called the normal flora is now referred to as our microbiota. These microorganisms, which are overwhelmingly bacteria, are frequently found colonizing various body sites in, healthy individuals. The constituents and numbers of the microbiota vary in different areas of the body and sometimes, at different ages and physiologic states. They comprise microorganisms whose morphologic, physiologic, and genetic properties allow them to colonize and multiply under the conditions that exist in particular sites, to coexist with other colonizing organisms, and to inhibit competing intruders. Thus, each accessible area of the body presents a particular ecologic niche, colonization of which requires a particular set of properties of the colonizing microbe. Organisms of the microbiota may have a symbiotic relationship that benefits the host or may simply live as commensals with a neutral relationship to the host. A parasitic relationship that injures the host would not be considered “normal Residents are strains that have an established niche at one of the many body sites, which they occupy indefinitely. Transients are acquired from the environment and establish themselves briefly, but tend to be excluded by competition from residents or by the host’s innate or immune defense mechanisms. The term carrier state is used when potentially pathogenic organisms are involved, although its implication of risk is not always justified. For example, Streptococcus pneumoniae, a cause of pneumonia, and Neisseria meningitidis, a cause of meningitis, may be isolated from the throat of 5% to 40% of healthy people. The Human Microbiota Initial flora is acquired during and immediately after birth Physiologic conditions such as local pH influence colonization Adherence factors counteract mechanical flushing Ability to compete for nutrients is an advantage Blood, Body Fluids, and Tissues Tissues and body fluids such as blood are sterile in health. Transient bacteremia can result from trauma Skin Propionibacteria and staphylococci are dominant bacteria Skin flora is not easily removed Organisms of the skin flora are resistant to the bactericidal effects of skin lipids and fatty acids, which inhibit or kill many extraneous bacteria. The conjunctivae have a very scanty flora derived from the skin flora. The low bacterial count is maintained by ?????????????? Intestinal Tract The mouth and pharynx contain large numbers of facultative and anaerobic bacteria Oropharynx has streptococci and Neisseria Saliva usually contains a mixed flora of about 108 organisms per milliliter The Human Intestinal Tract Microbiota Stomach and small bowel have few residents if any, resident organisms in health Small intestinal flora is scanty but increases toward lower ileum where it begins to resemble that of the colon Adult colonic flora is abundant and predominantly anaerobic The colon carries the most abundant and diverse microbiota in the body. In the adult, feces are 25% or more bacteria by weight (about 1010 organisms per gram). More than 90% are anaerobes, predominantly members of the genera Bacteroides, Fusobacterium, Eubacterium, and Clostridium. The remainder of the flora is composed of facultative organisms such as Escherichia coli, enterococci, yeasts, and numerous other species. There are considerable differences in adult flora depending on the diet of the host. Those whose diets include substantial amounts of meat have more Bacteroides and other anaerobic Gram-negative rods in their stools than those on a predominantly vegetable or fish diet. Recent studies have suggested the composition of the colonic microbiota could play a role in obesity. The Human Microbiota Respiratory Tract S. aureus is carried in anterior nares Approximately 25% to 30% of healthy people carry this organism as either resident or transient flora at any given time. The nasopharynx has a flora similar to that of the mouth; however, it is often the site of carriage of potentially pathogenic organisms such as pneumococci, meningococci, and Haemophilus species. Lower tract is protected by mucociliary action Genitourinary Tract Bladder and upper urinary tract are sterile Hormonal changes affect the vaginal flora Use of epithelial glycogen by lactobacilli produces low pH  The Human Microbiota ROLES IN HEALTH AND DISEASE 1- Opportunistic Infection I-Flora that reach sterile sites may cause disease A- B- II-Compromised defense systems increase the opportunity for invasion III-Mouth flora plays a major role in dental caries 2- Exclusionary Effect I-Competing with pathogens has a protective effect II-Antibiotic therapy may provide a competitive advantage for pathogens (Clostridium difficile) 3-Priming of Immune System I-Sterile animals have little immunity to microbial infection II-Low exposure correlates with asthma risk PROMOTING A GOOD MICROBIOTA In some clinical studies, administration of preparations containing a particular strain of Lactobacillus (L. rhamnosus) has been shown to reduce the duration of rotavirus diarrhea in children. The use of similar preparations to prevent relapses of antibiotic-associated diarrhea caused by C. difficile has shown little success. History of Microbiology The First Observations Before 17th century, study of microbiology was hampered by the lack of appropriate tools to observe microbes. Robert Hooke: In 1665 built a compound light microscope and used it to observe thin slices of cork, and he reported that the smallest structural units were cells, the beginning of the cell theory.  “All living things are composed of cells” Anton van Leeuwenhoek: In 1673 was the first person to observe live microorganisms which he called “animalcules” (bacteria, protozoa), using single-lens microscopes that he designed. The Debate Over Spontaneous Generation vs. Theory of Biogenesis Before 1860s many scientists believed in; Spontaneous generation, i.e.: claimed that living organisms could develop from nonliving or decomposing matter (spontaneously).  Mice come from rags in a basket.  Maggots come from rotting meat.  Ants come from honey.  Microbes come from spoiled broth. Theory of Biogenesis Belief that living cells can only arise from other living cells. Francesco Redi: In 1668 proved that maggots do not arise spontaneously from decaying meat but from eggs laid on the meat by flies. Spontaneous Generation vs Biogenesis Debate was finally settled by Pasteur. Louis Pasteur: In 1861 finally disproved spontaneous generation when he demonstrated that microorganisms in the environment were responsible for microbial growth in nutrient broth. Designed swan neck flasks that allowed air in, but trapped microbes in neck. Developed aseptic technique: Practices that prevent contamination by unwanted microorganisms.  The Golden Age of Microbiology 1857- 1914 Rapid advances led to the development of microbiology as a science. Pasteur’s Contributions to Microbiology: Pasteur showed that microbes are responsible for fermentation. Fermentation is a metabolic process that consumes sugar in the absence of oxygen. The products are organic acids, gases, or alcohol. It occurs in yeast and bacteria, and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is the conversion of sugar to alcohol to make beer and wine. Microbial growth is also responsible for spoilage of food. Bacteria that use alcohol and produce acetic acid spoil wine by turning it to vinegar (acetic acid). Pasteur demonstrated that these spoilage bacteria could be killed by heat that was not hot enough to evaporate the alcohol in wine. The Golden Age of Microbiology 1857-1914 Pasteur’s Contributions: Pasteurization: Developed a process in which liquids are heated (at 65oC) to kill most bacteria responsible for spoilage. Disease Causes: Identified the microbe (protozoan) that caused silkworm disease (1965). Vaccine: Developed a vaccine for rabies from dried spinal cords of infected rabbits.  Directed Pasteur Institute until his death in 1895. The Golden Age of Microbiology 1857-1914 Germ Theory of Disease: Belief that microbes might cause disease ( Pasteur). Before, most people believed diseases were caused by divine punishment, poisonous vapors, curses, witchcraft, etc. Joseph Lister (1860): Used disinfectant to treat surgical wounds, greatly reducing infection rates. Considered the father of antiseptic surgery. Robert Koch (1876): First person to conclusively prove that a specific bacterium caused a disease. Germ Theory: One microbe causes one specific disease.  Proved that Bacillus anthracis causes anthrax in cattle.  Later identified bacterium that causes tuberculosis. The Golden Age of Microbiology 1857-1914 Vaccination In a vaccination, immunity (resistance to a particular disease)is conferred by inoculation with a vaccine. 1796: Edward Jenner inoculated a person with cowpox virus. The person was then protected from smallpox. Vaccination is derived from vacca for cow. Modern vaccines are prepared from: living avirulent microorganisms or killed pathogens, from isolated components of pathogens, and by recombinant DNA techniques. The Birth of Modern Chemotherapy Chemotherapy: Treatment of a disease by using a chemical substance. Chemical must be more poisonous to microbe than host. Quinine: First known chemical to treat a disease (malaria). Used by the Spanish. Salversan against syphilis. Synthetic Drugs: Made in the laboratory. Antibiotics: Produced naturally by fungi and bacteria to inhibit the growth of other microorganisms. The Birth of Modern Chemotherapy Paul Ehrlich (1910): Search for “magic bullet”. Discovered Salversan, an arsenic derivative, was effective against syphilis. Alexander Fleming (1928): Discovered that penicillin produced by the mold Penicillium notatum was able to prevent microbial growth. Penicillin was not mass produced until the 1940s. Researchers are tackling the problem of drug- resistant microbes. MODERN DEVELOPMENTS IN MICROBIOLOGY As we learned more, various branches of micro developed: Bacteriology---study of bacteria Mycology---study of fungi Parasitology--- study of protozoa and parasitic worms We have not yet discovered all pathogens. Immunology- began with Jenner 1796. Another big development was the discovery of phagocytosis by white blood cells, for which Elie Metchnikoff received a Nobel Prize. Study in this area has advanced rapidly in recent years, mainly due to AIDS virus. Virology—as study of bacteria continued, it began to be recognized that some pathogens could pass through a filter small enough to trap all known bacteria. The name given to this unknown agent was virus. Scientists were only able to actually see them after the electron microscope was developed in the 1940’s. MICROBES AND DISEASE Although most microbes are harmless and some are even beneficial, a small number do cause disease. These are pathogens (disease-causing) microbes. Some of these pathogens can be traced as far back as human history. With the development of antibiotics, vaccines and other modern treatments, there was a time that we believed infectious disease could be eradicated. Development of resistant bacteria and appearance of new diseases has ended that belief. Some emerging infectious diseases newly recognized or found in locations where previously unknown in recent years include: Swine flu—(H1N1), Avian influenza A (H5N1)—also called bird flu, SARS, West Nile encephalitis, Bovine spongiform encephalopathy (mad cow disease), Escherichia coli O157:H7, Invasive Group A strep (flesh-eating bacteria) Ebola and AIDS MICROBIOLOGY BACTERIOLOGY LECTURE (2) Lecture 2 Observing Microorganisms Through a Microscope Microscopy Units of Measurement 1. The standard unit of length is the meter (m). 2. Microorganisms are measured in micrometers. µ (10-6m ), and in nanometers, nm (10-9m ). Centimeter (cm) = 0.01 m = 10-2m Millimeter (mm) = 0.001 m = 10-3m Micrometer (µm) = 0.000001 m = 10-6m Nanometer (nm) = 0.000000001 m = 10-9m Microscopy: The Instruments Microscopy refers to the use of light or electrons to magnify objects. Light microscope 1. A simple microscope consists of one lens; a compound microscope consists of multiple lenses. In order to successfully observe a specimen, three principles must be achieved: 1. Magnification 2. Resolution 3. Contrast Light Microscopy Compound Light Microscopy 1. The most common microscope used in microbiology is the compound light microscope (LM). 2. The total magnification of an object is calculated by multiplying the magnification of the objective lens by the magnification of the ocular lens. 3. The compound light microscope uses visible light.  Compound Light Microscopy 1. Total magnification: 1. Light originates from an illuminator and passes through condenser lenses, which direct light onto the specimen. 2. Light then enters the objective lenses, which magnify the image. These are the closest lenses to the specimen: Lens Magnification Ocular Mag. Total Mag. Scanning 4X 10 X = 40 X Low power 10 X 10 X = 100 X High power 40 X 10 X = 400 X Oil immersion 100X 10 X = 1000 X Highest possible magnification with CL microscope is about 2000 X. Compound Light Microscopy 2. Resolution (also called resolving power) is the ability to distinguish between objects (points) that are close together. The better the resolution, the better the ability to distinguish two objects that are close to one another. Modern microscopes can distinguish between objects as close together as 0.2 µm. Compound Light Microscopy A principle of microscopy is that resolution distance is dependent on: (1) the wavelength (2) the numerical aperture of the lens, which is its ability to gather light. White light has a relatively long wavelength (550 nm), and cannot resolve structures less than 220 nm apart.  Ultraviolet (UV) light has a shorter wavelength (100 to 400 nm), and can resolve distances as small as 110 nm Compound Light Microscopy The smaller the distance between objects at which they can be distinguished as separate, the greater the resolving power.  Light must pass between two objects in order for them to be seen as separate.  Depends on light wavelength. If wavelength is too long to pass between objects, they will appear as one. Compound Light Microscopy Immersion oil Has the same index of refraction as glass slide, preventing light loss from refraction Is used to fill the space between the specimen and a lens to reduce light refraction (reduce light loss between the slide and lens). Thus increase the numerical aperture and resolution. Compound Light Microscopy Refraction: Bending of light as it passes from one medium to another of different density.  Refractive index: A measure of the light-bending ability of a medium.  Can be changed by staining, which increases contrast between specimen and surrounding medium.  When two substances have a different index of refraction, the light will bend as it passes from one material to another.  As light passes through a glass slide, air, and the objective lens, it bends each time, causing loss of light and a blurred image.  Immersion oil has the same index of refraction as glass slide, preventing light loss from refraction Microscopy: The Instruments Refractive index is the light-bending ability of a medium. The light may bend in air so much that it misses the small high- magnification lens. Immersion oil is used to keep light from bending. Figure 3.3 Compound Light Microscopy 3. Contrast refers to differences in intensity between two objects, or between an object and its background. Since most microorganisms are colorless, they are stained to increase contrast. Specimens are stained to increase the differences between the refractive indexes of the specimen and medium. Under usual operating conditions, the field of vision in a compound light microscope is brightly illuminated. By focusing the light, the condenser produce a brightfield illumination. Limitations of light microscopy: Magnification: Up to 2000 X.  Resolving Power: Up to 0.2 um. Because of the limits of magnification and resolving power, viruses and most internal structures of cells cannot be seen with a light microscope. Darkfield Microscopy Useful to examine live microorganismsthat is either invisiblein the ordinary light microscope or can not be stained by ordinary methods (difficult to stain) Spirochetes which cause syphilis.  Darkfield condenser with opaque disc blocks light that would enter objective lens directly:  Only light reflected off the specimen enters objective lens.  Because NO direct background light, the specimen appears Light against dark background. Phase Contrast Microscopy Useful to examine live specimens:  Doesn’t require fixing or staining, which usually kill and/or distort microorganisms.  Permits detailed examination of internal structures. Fluorescence Microscopy Fluorescence Microscopy Fluorescence: Ability of substances to absorb short wavelengths of light (ultraviolet light) and emit them at a longer wavelength. Natural Fluorescence: Some microorganisms fluoresce naturally under UV light (Pseudomonas). Image: Luminescent bright object against a dark background. Electron Microscopy Electron microscopes were first developed in 1932, and became widely available in 1940s.  Use a beam of electrons instead of a beam of light.  Wavelength of electron beam is about 100,000 times smaller than visible light.  Used to examine structures too small to be resolved with a light microscope.  Two types of electron microscope: A. Transmission Electron Microscope (TEM) B. Scanning Electron Microscope (SEM) (SEM) & (TEM)