are sterile and do not have a normal flora.Nosocomial InfectionHospital-acquired infectionReservoir of InfectionA continual source of infection. A reservoir of infection can be living organisms such as humans and otheranimals; nonliving objects are substances that are contaminated with the pathogen. A contaminated nonliving object is called a fomite, such as a dirty glass and used needles. Contaminated soil and wateralso serve as inanimate reservoirs of infectionResistanceThe ability to ward off diseases. A
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lack of resistance is called susceptibility.SterilizationA process that destroys all living organismsVectorA carrier of pathogens from host to host. The mosquito is the animal vector carrying the plasmodium (malaria) to humans. A contaminatedsyringe is a nonliving vector (fomite).Types of PathogensThe groups of microorganisms (some of which are pathogens) are bacteria, viruses, fungi, and protozoa. Other larger, disease-causing organisms includeworms and arthropods. Microorganisms (Microbes)Bacteria are single-celled organisms found everywhere. They were firstobserved under the microscope by van Leeuwenhoek, who called them“little beasties.” Most bacteria consider living conditions within the human body to be ideal, so they move right in. Microbiotaor normal flora(microorganisms that normally and harmoniously live in or on the human body without causing disease) prevents the overgrowth of other organisms, keeping them under control. Some bacteria synthesize needed substances such as vitamin K.When bacteria successfully invade the human body, they cause damage in two ways: (1) by entering and growing in the human cell and (2) by secreting toxins that damage the cells.Bacteria are classified into three groups based on shape: (1)coccus (round), (2) bacillus (rod-shaped), and (3) curved rod. Rickettsiae and chlamydiae are also classified as bacteria, although they differ in several important ways from cocci, bacilli, and curved rods.The cocci are round cells and are arranged in patterns. Cocci that are arranged in pairs are called diplococci. Streptococci are arranged in chains, like a chain of beads. Staphylococci look like bunches of grapesand are arranged in clusters. The cocci cause many diseases, includinggonorrhea, meningitis, and pneumonia. The bacilliare long and slender and are shaped like a cigar. Diseases caused by bacilli include tetanus, diphtheria, and tuberculosis. The curved rods include the vibrio, the spirillum, and the spirochete The vibrios have a slight curve and resemble a comma. Cholera is caused by a vibrio (Vibrio cholerae). The spirillum is a long cell that coils like a corkscrew.
Tightly coiled spirilla that are capable of waving and twisting motions are called spirochetes. The most famous spirochete, Treponema pallidum, causes syphilis.There are two clinically important characteristics of bacteria: (1) the presence of a cell wall and (2) the ability to form spores. Although thehuman cell is surrounded by the cell membrane, the bacterial cell is surrounded by two structures: a cell membrane and an outer cell wall.
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