Background to ID Flashcards
1. Define the following terms: host, pathogen, pathogenicity, virulence, colonization, infection, symbiosis and opportunistic pathogen. 2. Define and contrast the various types of symbiosis (mutualism, commensalism, parasitism). 3. Understand the concept of normal flora and provide the names of the common normal flora found in specific regions of the human body. 4. Understand the various modes of entry, exit and transmission of human pathogens. 5. Contrast intracellular and extracellular pat
Define host.
An animal capable of supporting growth of a microbe.
Define pathogen.
Any virus, bacterium, or other agent that causes disease.
Define pathogenicity.
Condition or quality of being pathogenic, or the ability to cause disease.
Define virulence.
The degree or intensity of pathogenicity of an organism as indicated by case fatality rates and/or ability to invade host tissues and cause disease.
Define colonization.
Establishment of a site of microbial reproduction on an “external” host surface (transiently or permanently) without observable clinical symptoms or immune reaction.
Define infection.
Presence and replication of a microbe in or on a host.
Define infectious disease.
Where the interaction between microbe and host leads to pathologic process characterized by damage to the host.
- may result from microbial factors (proliferation, production of cytotoxic enzymes or toxins)
- may result from host’s immune response
Define symbiosis.
All associations in which one species lives in or on the body of another.
Define strict pathogens.
Pathogens always associated with human disease.
Examples: Mycobacterium tuberculosis (TB), Neisseria gonorrhoeae (gonorrhea)
Define opportunistic pathogen.
A microbe which is not pathogenic under normal conditions, but may cause infection and disease if introduced into a normally sterile body site (or into a host with a compromised immune system(
- may be members of patient’s normal microbial flora
Ex. Staphylococcus aureus, Escherichia coli, Candida albicans
Define commensalism.
Symbiotic relationship in which one benefits and the other neither benefits nor is harmed.
Define mutualism.
Symbiotic relationship in which both members benefit from their interaction.
Define parasitism.
Symbiotic relationship in which one organism derives benefit while harming its host.
Contrast the various types of symbiosis (mutualism, commensalism, parasitism).
Commensalism, mutualism and parasitism form a continuum as associations can merge and change given the circumstances of the relationship.
What is normal flora?
- = total microbial population routinely found associated with a healthy human
- found in parts of body that are exposed to or communicate with the external environment (skin, nose and mouth, intestinal and genitourinary tracts)
- primarily bacteria (although minor numbers of viruses, fungi and protozoans may be present)
- ~10^14 bacteria cells (~1.2 kg) associated with the ~10^13 cells that make up the human body
Describe the development of the normal flora.
- human fetus lives in protected, sterile environment
- during and after birth, colonization of skin, oropharynx, gastrointestinal tract and other mucosal surfaces occurs (from mother’s genital tract flora, flora of medical personnel, environmental flora)
- diet influences normal flora of GI tract:
Ex. breastmilk is high in lactose –> GI flora of breastfed infants contains lactic acid streptococci and lactobacilli; bottle-fed infants show greater variety of organisms - normal flora may be: resident (months to life) or transit (hours to days)
What determines the composition of normal flora?
Composition of normal flora is in a continual state of flux determined by:
- local physiological conditions:
- types and amounts of nutrients, pH, oxygen concentration, etc.
- presence of local antibacterial substances such as lysozyme in tears
- types of other flora present (competition for nutrients, inhibition by metabolites etc.) - systemic host factors (age, diet, hormonal state, health, personal hygiene)
What are the beneficial effects of normal flora?
- participate in metabolism of food products –> degradation of dietary oxalates (helps prevent formation of calcium oxalate kidney stones)
- gut bacteria provide essential growth factors (vitamin K and B vitamins)
- stimulates normal development of immune system –> germ-free animals have poorly developed immune systems –> more susceptible to normally non-pathogenic organisms
- protects against infection with virulent microbes
Describe the mechanisms of how normal flora protect against virulent microbes.
- exclusion of potential pathogens –> competition for attachment sites, nutrients, etc.
- production of toxic metabolites
- Bifidobacterium in gut of breast fed infants –> lowers pH due to acetic acid production –> protects against GI pathogens (can’t grow at low pH)
- skin bacteria produce fatty acids and vaginal lactobacilli maintain acid environment –> discourage other species from invading
- production of antibacterial factors
- ex. Bacteriocins
What are the potential negative effects of normal flora?
- source of pathogens: normal flora often has the potential to cause disease if presented with the right opportunity
- skin flora enters blood/deeper tissues after cuts, bites
- peritonitis caused by intestinal flora after ruptured colon
- oral Streptococcus enter bloodstream (extraction of teeth) –> infect heart valves
- Pseudomonas infections in burn victims
- Perianal organisms ascend urethra –> urinary tract infection
- overgrowth of certain organisms when normal flora changes (ex. after antibiotics) or when immune system becomes compromised
- metabolic activity of intestinal flora may be detrimental
- biotransformation of natural compounds into carcinogens
- release of toxic waste products from bacterial metabolism
- relationship of normal flora to obesity and type II diabetes
How can normal flora be unintentionally altered?
- longterm antibiotic use –> indigenous flora in GI tract is eliminated –> Clostridium difficile proliferates –> diarrhea, colitis
- hospitalization –> replacement of normally avirulent microbes in the oropharynx with gram-negative rods (ex. Klebsiella, Pseudomonas) that can invade lungs –> pneumonia
How can normal flora be intentionally altered?
- bone marrow transplant patients (immunocompromised): give antibiotics to remove normal flora and minimize risk of opportunistic infections
- “probiotics”: consumption of live bacteria so as to alter the balance of intestinal flora and improve health
- Lactobacillus and Bifidobacterium are commonly used (gram positive, lactic acid producing species)
- considered “safe” (already a part of normal GI flora)
- already routinely consumed with many different food products (sauerkraut, yogurt, etc.)
- Enteric coating may ensure better delivery to colon
What are “prebiotics”?
Non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, thus improving host health
- selectively fermented by desired microbes
- “soluble fiber”
- “dietary fiber”
- Ex. Inulin (a plant polysaccharide), raw oats, unrefined wheat
What are the benefits of probiotics?
May help reduce:
- bloating and diarrhea from lactose intolerance
- diarrhea that is a side effect of antibiotics
- diarrhea in infants with rotavirus enteritis
- infections in the digestive tract
