CHAPTER 14 PRINCIPLES OF DISEASE AND EPIDEMIOLOGY I Pathology infection and disease a Pathology scientific study of disease II i Etiology cause of disease ii Pathogenesis manner in which a disease develops iii Structural and functional changes brought about by disease and the final effects on the body b Infection invasion or colonization of the body by pathogenic microorganisms This can be the presence of a type of microorganism in a part of the body where it is not usually found and can lead to disease c Disease occurs when an infection results in any change from a state of health Abnormal state when the body is not properly adjusted or is incapable of performing its normal functions Normal Microflora microorganisms are introduced to the newborn s body from the environment and from their mothers and they remain in the body throughout the infant s life decreasing or increasing in number A typical body harbors an estimated 104 bacterial cells 10 times more bacterial cells than human cells Microorganisms that establish a more or less permanent residence without producing disease under normal conditions are members of the body s normal flora Transient flora may be present for several days and then disappear Factors that determine the distribution and composition of normal flora include nutrients physical chemical factors defenses of host and mechanical factors Nutrients may be derived from secretory and excretory products of cells substances in body fluids dead cells and foods in the GI tract Physical and chemical factors include pH temperature available carbon dioxide and oxygen salinity and sunlight Mechanical factors can play into colonization also as seen with flushing of urine removing unattached microbes Factors that also affect the normal flora are age nutritional status diet health status disability hospitalization emotional state stress climate geography personal hygiene living conditions occupation and lifestyle a Relationships between the normal flora and the host i Microbial antagonism competitive exclusion involves competition among microbes which prevents the overgrowth of harmful microbes They compete for nutrients produce substances that are harmful to invading microbes and affect conditions like pH and available oxygen When this balance is upset disease can result For example if the vaginal bacterial population is eliminated by antibiotics the pH of the vagina reverts to neutral and C albicans yeast can flourish and become the dominant microorganism there causing vaginitis ii Symbiosis a relationship between two organisms in which one organism is dependent on the other unaffected 1 Commensalism one of the organisms benefits and the other is 2 Mutualism benefits both organisms For example E coli in the large intestine synthesized vitamin K and some B vitamins which are then absorbed in the bloodstream and distributed for use by body cells a Probiotics live microbial cultures applied to or ingested that are intended to exert a beneficial effect i LAB lactic acid bacteria ingestion can alleviate diarrhea and prevent colonization by Salmonela enteric during antibiotic therapy III 3 Parasitism one organism benefits by deriving nutrients at the expense of the other Many disease causing bacteria fall under this category b Opportunistic microbes these microbes don t ordinarily cause disease in their normal habitat in a healthy person but may do so in a different environment For example microbes that gain access through broken skin or mucous membrane scan cause opportunistic infections i AIDS patients are often accompanied by a Pneumocytis pneumonia caused by an opportunistic organism that takes advantage of the depleted immune system of these patients ii Many people carry other microbes that are generally regarded as pathogenic but may not cause disease in those people including echoviruses adenoviruses Neiserria meningitis and Streptococcus pneumonia c Cooperation among microbes cooperation can be a factor in causing disease For example pathogens that cause periodontal disease and gingivitis have been found to have receptors for the oral streptococci that colonize the teeth Etiology of Infectious Disease a Koch s postulates Koch was a German physician who played a major role in establishing that microorganisms cause specific diseases In 1877 he published early papers on anthrax and demonstrated that this specie of microorganisms were always present in the blood of animals that had this disease and were not present in healthy animals i Process he took a sample from a sick animal and injected it into a healthy one The second animal developed the same disease and died He also demonstrated that the bacterium would cause anthrax even after many culture transfers ii Postulates 1 The same pathogen must be present in every case of the disease 2 The pathogen must be isolated from the diseased host and grown in pure culture 3 The pathogen from the pure culture must cause the disease when it is inoculated into a healthy susceptible lab animal 4 The pathogen must be isolated from the inoculated animal and must be shown to be the original organism b Exceptions to Koch s postulates some microbes have unique culture requirements and cannot grow on artificial media Alternative methods of culturing and detecting certain microbes must be use i Ex Researchers were unable to isolate the microbe causing Legionellosis directly so they inoculated a victim s lung tissue into guinea pigs The pigs developed the symptoms whereas guinea pigs inoculated with issued from an unafflicted person did not The tissue samples from diseased guinea pigs were then cultured into yolk sacs of chic embryos revealing growth of very small microbes Modern immunological techniques were used to show that the bacteria in the embryos were the same bacteria as those in the guinea pigs and in afflicted humans In a number of situations humans exhibit certain symptoms and signs that are associated with only a certain pathogen and its disease Some aren t as clear cut and provide another exception to Koch s postulates An example is nephritis which can involve several different pathogens all of which have the same signs ii IV and symptoms so it s difficult to know which particular organism is causing a disease iii Another example of an exception results because some pathogens cause several disease conditions Ex Streptococcus pyogenes can cause sore throat scarlet fever skin infections osteomyelitis among other diseases
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