Microbiology Chapter 2 Summary Notes

2.1           microorganisms, you are confronted by some unique problems o So it is often necessary to separate the species from one another o To maintain and keep track of such small research subjects, microbiologists usually have to grow them under artificial conditions o they are not visible to the eye o microbes are everywhere, and undesirable ones can be introduced into your experiment, causing misleading result Microbiologists use five basic techniques to manipulate, grow, examine, and characterize microorganisms in the laboratory: inoculation, incubation, isolation, inspection, and identification To grow, or culture, microorganisms, one introduces a tiny sample (the inoculum) into a container of nutrient medium (plural, media). The medium provides an environment in which they multiply. This process is called inoculation. The observable growth that appears in or on the medium after incubation is known as a culture. Once a container of medium has been inoculated, it is incubated, which means it is placed in a temperature-controlled chamber (incubator) to encourage multiplication o Incubators can also control the content of atmospheric gases such as oxygen and carbon dioxide that may be required for the growth of certain microbes o During the incubation period (ranging from a day to several weeks), the microbe multiplies and produces growth that is observable macroscopically Various conditions of cultures: o pure culture is a growth medium that contains only a single known species or type of microorganism. This type of culture is most frequently used for laboratory study, because it allows the systematic examination and control of one microorganism by itself. o A mixed culture is a container that holds two or more identified, easily differentiated species of microorganisms o A contaminated culture was once pure or mixed (and thus a known entity) but has since had contaminants o Media can be classified according to three properties: o 1. physical state o 2. chemical composition o 3. functional type (purpose) Defined(synthetic)media may contain nothing more than a few essential compounds such as salts and amino acids dissolved in water or may be composed of a variety of defined organic and inorganic chemicals Complex media contain extracts of animals, plants, or yeasts, including such materials as ground-up cells, tissues, and secretions. General-purpose media are those that will allow the growth of as broad a spectrum of microbes as possible. As a rule, they are of the complex variety. Examples include nutrient agar and broth, brain-heart infusion, and trypticase soy agar (TSA)                        enriched medium contains complex organic substances such as blood, serum, hemoglobin, or special growth factors Bacteria that require growth factors and complex nutrients are termed fastidious selective medium contains one or more agents that inhibit the growth of certain types of microbe or microbes (call them A, B, and C) Differential media do not inhibit the growth of any particular microorganisms but are designed to display visible differences in how they grow. Differentiation shows up as variations in colony size or color a single medium can be both selective and differential Media that are both selective and differential allow for microbial isolation and identification to occur at the same time, which can be very useful in the screening of patient specimens as well as food and water samples. MacConkey agar contains neutral red, a dye that is yellow when neutral and pink or red when acidic A common intestinal bacterium such as Escherichia coli that gives off acid when it metabolizes the lactose in the medium develops red to pink colonies reducing medium contains a substance (sodium thioglycollate or cystine) that absorbs oxygen or slows the penetration of oxygen in a medium, thus reducing its availability Carbohydrate fermentation media contain sugars that can be fermented Assay media are used by technologists to test the effectiveness of antimicrobial drugs If it was formed from a single cell, a colony consists of just that one species and no other if an individual bacterial cell is separated from other cells and provided adequate space on a solid nutrient surface, it will grow into a discrete mound of cells called a colony 2.2 The very smallest of the microbes are the viruses The objective forms the initial image of the specimen, called the real image When this image is projected up through the microscope body to the plane of the eyepiece, the ocular lens forms a second image, the virtual image. The virtual image is the one that will be received by the eye and converted to a retinal and visual image That property is resolution, or resolving power. Resolution is the capacity of an optical system to distinguish two adjacent objects or points from one another The oil immersion lens (100× magnification) uses oil to capture some of the light that would otherwise be lost to scatter In general, organisms that are 0.5 μm or more in diameter are easily seen. This includes fungi and protozoa, some of their internal structures, and most bacteria. However, a few bacteria and most viruses are far too small to be resolved by the optical microscope and require electron microscopy The contrast is measured by a quality called the refractive index. Refractive index refers to the degree of bending that light undergoes as it passes from one medium, such as water or glass, to another medium, such as bacterial cells. The higher the difference in refractive indexes (the more bending of light), the sharper the contrast that is registered by the microscope and the eye.          bright-field microscope forms its image when light is transmitted through the specimen Transmission electron microscopes are the method of choice for viewing the detailed structure of cells and viruses. The scanning electron microscope provides some of the most dramatic and realistic images in existence. This instrument is designed to create an extremely detailed three-dimensional view of all kinds of objects—from plaque on teeth to tapeworm head A more permanent slide for long-term study can be obtained by preparing fixed, stained specimens. “Fixed” in this context means that the organisms are dried and attached to the glass slide no matter how great the magnification or how fine the resolving power of the microscope. To solve this problem, stains can be added to the fixed smear, either coloring the objects Dyes can be classified as basic (cationic) dyes, which have a positive charge, or acidic (anionic) dyes, which have a negative charge. Bacteria that stain purple are called gram-positive, and those that stain red are called gramnegative. The different results in the Gram stain are due to differences in the structure of the cell envelope and how it reacts to the series of reagents applied to the cell This century-old staining method remains a universal basis for bacterial classification and identification. The Gram stain can also be a practical aid in diagnosing infection and in guiding drug treatment.