Microbial Interactions with Humans
Pathogenicity
Pathogens: Microorganisms that cause harm.
Pathogenicity: the ability of a pathogen to cause disease.
Opportunistic : causes disease only in the absence of normal host resistance.
Host-parasite interactions: The ability of a microorganism to cause or prevent disease is influenced by complex
Wound
Normal Flora
Moist areas, especially around sweat glands, are colonized by gram-positive Bacteria and other members of the skin normal flora.
Environmental and host factors influence the quantity and quality of the normal skin microflora.
Normal Flora
Influences on composition:
Weather
Age
Personal Hygiene
Antibiotic Therapy
Transient
Colonizers/Residents
Bacteremia
Normal Flora
Chemoorganotrophs
Fermenters/Non-fermenters
Methanogenic
Pathogenesis
Exposure
Adherence
Invasion
Colonization/growth
Toxicity or Invasion
Damage and Disease
Dental Caries
Plaque microorganisms produce adherent substances.
Acid produced by microorganisms in plaque damages tooth surfaces, and dental caries result.
A variety of microorganisms contribute to caries and periodontal disease.
Intestinal Tract
The stomach is very acidic and is a barrier to most microbial growth.
The intestinal tract is slightly acidic to neutral and supports a diverse population of microorganisms in a variety of nutritional and environmental conditions.
Upper Respiratory Tract
In the upper respiratory tract (nasopharynx, oral cavity, and throat), microorganisms live in areas bathed with the secretions of the mucous membranes.
The normal lower respiratory tract (trachea, bronchi, and lungs) has no resident microflora, despite the large numbers of organisms potentially able to reach this region during breathing.
Adherence
Pathogen invasion starts at the site of adherence and may spread throughout the host via the circulatory systems.
Capsule: A polymer coat consisting of a dense, well-defined layer surrounding the cell.
Slime Layer: A loose network of polymer fibers extending outward from a cell
Fimbriae and pili
Colonization factors
Colonization and Growth
A pathogen must gain access to nutrients and appropriate growth conditions before colonization in host tissue can occur.
Organisms may grow locally at the site of invasion or may spread through the body.
If extensive bacterial growth in tissues occurs, some of the organisms are usually shed into the bloodstream in large numbers, a condition called bacteremia
Virulence
Virulence is determined by invasiveness, toxicity, and other factors produced by a pathogen.
Various pathogens produce proteins that damage the host cytoplasmic membrane, causing cell lysis and death.
Because the activity of these toxins is most easily detected with red blood cells (erythrocytes), they are called hemolysins.
In most pathogens, a number of factors contribute to virulence.
Attenuation is loss of virulence
Virulence
Pathogens produce a variety of enzymes that enhance virulence by breaking down or altering host tissue to provide access and nutrients.
Still other pathogen-produced virulence factors provide protection to the pathogen by interfering with normal host defense mechanisms. These factors enhance colonization and growth of the pathogen
Clots, enzymes, Toxins
Exotoxins
Toxicity: ability to casue disease by preformed toxin that inhibits or kills cells.
Invasiveness: ability of an organism to grow in host tissue
The most potent biological toxins are the exotoxins produced by microorganisms.
Each exotoxin affects specific host cells, causing specific impairment of a major host cell function.
Enterotoxins
Enterotoxins: toxins that specifically affect the small intestine, causing changes in intestinal permeability that lead to diarrhea.
Many enteric pathogens colonize the small intestine and produce A-B enterotoxins.
Food-poisoning bacteria often produce cytotoxins or superantigens.
Endotoxins
Endotoxins are lipopolysaccharides derived from the outer membrane of gram-negative Bacteria.
Released upon lysis of the Bacteria, endotoxins cause fever and other systemic toxic effects in the host.
Endotoxins are generally less toxic than exotoxins
Limulus amebocyte lysate assay
The presence of endotoxin detected by the Limulus amebocyte lysate assay indicates contamination of a substance by gram-negative Bacteria.
White blood cells of Horseshoe Crab
Debilitation
Conditions of age, stress, diet, general health, lifestyle, prior or concurrent disease, and genetic makeup may compromise the host's ability to resist infection.
Many hospital patients with noninfectious diseases (for example, cancer and heart disease) acquire microbial infections because they are compromised hosts.
Nosocomial Infections: hospital-acquired infections are called
Innate Resistance to Infection
Nonspecific physical, anatomical, and chemical barriers prevent colonization of the host by most pathogens.
Lack of these defenses results in susceptibility to infection and colonization by a pathogen.
Definitions
Parasites: organisms on or in a host that cause damage
Opportunistic pathogen: causes disease only in absence of normal resistance
Infection: when a microbe lives and grows in a host.
Disease: damage to the host that impairs body function.
Definitions
Mucous membranes: epithelial cells that line the lumen(opening) of various tissues
Mucus: glycoproteins
Digestive tract: stomach, small intestine (duodenum, ileum and jejunum)
Methane producers: H2 + CO2=CH4
Upper Respiratory tract: nasopharynx, oral cavity and throat
Lower Respiratory tract: trachea, bonchi and lungs
Ciliated epithelial cells: have cilia on surface/line upper respiratory tract
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