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Antibiotic Resistance: Understanding and... > Chapter 2. Working with Pathogens - Pg. Vol1-17

Chapter 2 Working with Pathogens Summary: Most pathogens are too small to be seen as individuals with the naked eye; however, on solid surfaces, such as agar, bacteria and yeast grow into visible masses (colonies) containing millions of cells. Many viruses kill the cells they infect, thereby leaving a hole (plaque) in a lawn of host cells growing on a solid surface. Colonies and plaques can be counted to estimate the number of infectious microbes or virus particles present in the cultures. Pathogens can also be detected and identified by nucleic acid hybridization following amplification. Short nucleic acid strands made in the laboratory serve as probes that bind specifically to nucleic acids of particular pathogens. Nucleic acid tests are rapid, specific, and sensitive. Demonstrating that a specific life form is responsible for a particular infection is central to understanding infectious disease. Causality is established by a set of criteria called Koch's postulates. Because new infections continue to emerge, Koch's postulates remain relevant, even though they are more than 120 years old. New disciplines of biology have permitted additional criteria to be considered when investigating causality, which has led to significant modification of the postulates. A key to understanding pathogen biology is the realization that infections often contain huge numbers of pathogen cells; consequently, rare genetic events, such as mutation, occur often enough to be a problem. Differences among the various pathogens require distinct management