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ÿþConcepts of Disease 1. Variety of Human-Microbe Interactions - Differences in susceptibility from person to person -Variations among different strains of the same bacterial species Lost of virulence Differences between isolates The outcome of microbe-human encounter depends on the infected person s defenses against disease and on the traits of the infecting strain 2. Views of the Microbe-Human Interaction 1) Disease-causing bacteria evolved specifically to cause human disease. 2) Disease-causing bacteria are actually trying to achieve an equilibrium with humans that does not result in disease, and that disease symptoms result when this equilibrium is not achieved. 3) Humans are more often than not accidental hosts of bacteria that may be able to cause human disease but have actually evolved to occupy some other niche. In this view, bacteria entering the humen body react by activating stress responses, producing disease symptoms in the process Ad.1. A bacterium that causes disease only in human and has no external reservoir Ad.2. A bacterium that causes an asymptomatic carrier state in most of humans it infects Ad.3. A bacterium that spends most of its time outside human body and only occasionally causes human disease 3. Terminology 1) The term  host-parasite is widely used to describe the human- microbe interaction 2) Colonization of the body by bacteria capable of causing disease is called infection 3) An infection producing symptoms is called disease 4) The word colonization means that a bacterium occupies and multiplies in a particular area of the human body. Colonization is not synonymous with disease, nor it is necessarily synonymous with infection. 5) The terms colonization, infection, and disease may vary in their applicability depending on the status of the person colonized. 6) People, who are infected but do not have detectable symptoms are called asymptomatic carriers (example of Typhoid Mary) 3. Terminology, cont. 7) Symptoms of bacterial infection are defined as effects of bacterial infection that are apparent to infected person (Example: Chlamydia trachomatis / cervix infection / no pain and vaginal discharge / inflammation / fallopian tubes shut / infertility) 8) Virulence (or pathogenicity) is defined as the ability of a bacterium to cause infection 9) Virulence factor (or mechanism of pathogenesis or virulence mechanism) denotes a bacterial product or strategy that contributes to virulence or pathogenicity An experimental definition of virulence factor: A loss of the factor by the bacterium results in a decrease in its ability to cause disease Problems:  Two or more factors  A virulence factor in one type of bacteria is not a virulence factor in another type  Is a housekeeping protein a virulence factor?  The loss of trait makes a bacterium more virulant 4. Opportunists Bacteria that normally do not cause disease in healthy people but can cause disease in people whose defenses have been impaired Example: Pseudomonas aeruginosa and burns or cystic fibrosis Opportunists vs. primary pathogens, definition problems Example: Streptococcus pneumoniae

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