Chapter 2: Host Parasite Interactions Flashcards
Origins of Microbiota: Fetus
Sterile until birth, exposure to environment leads to colonization
Symbiosis:
Two organisms living together
Commensalism:
One organism benefits, other is unharmed
Mutualism:
Microorganism and host benefit
Parasitism:
One organism benefits while the host is harmed
Indigenous microbiota:
Microbes commonly found on or in healthy persons
Resident microbes:
Colonize area for months or years
Transient microbes:
Temporarily colonizing host
Carrier state:
Pathogenic organism establish themselves in host without symptoms
Factors That Determine the Composition of
the Usual Microbial Biota:
- Specific nutritional factors at that location
- Antibacterial substances that naturally occur (bile, lysozyme, fatty acids)
- Environmental factors (pH, moist or dry, gas atmosphere)
- Changes due to age, nutritional status, disease state, and drug therapy
Normal microbiota:
Significant role in providing host resistance to
infections
- Change in environmental conditions may predispose an individual to infection by the normal microbiota (opportunistic infection)
- A Clinical Microbiologist should be able to
recognize and identify the types of microorganisms found at various body site
Normal Microbiota: Skin
- Generally superficial organisms
– Skin surface and hair follicles
– Scrub 90% away by washing
– Composition depends on activity
of sebaceous or sweat glands
– Skin has shedding mechanism - Apocrine sweat glands
– Secrete substances metabolized
by bacteria (especially armpit, groin, and
perineum) - Release of odorous amines
Normal Microbiota: Mouth
- Streptococcus predominates
- Plaque (Biofilm) - Low oxidation reduction potential so anaerobes grow
- Buccal mucosa and tooth surface
– Production of acids by
microorganisms, leading to tooth decay
Upper respiratory tract:
Mouth, nasopharynx, oropharynx, larynx
- colonized by viridans streptocci
Lower respiratory tract:
Trachea, bronchi, pulmonary parenchyma
-protected by ciliary epithelial cells and mucus, normally considered sterile
GI tract:
Comprises esophagus, stomach, small intestine, and colon
Stomach is low in microbes due to pH=2, what are some exceptions?
– Some Streptococcus, Enterococcus, Prevotella and H. pylori can live here
– Endospores, parasitic cysts, and microbes enmeshed in food - Escape stomach and enter the intestine
Colon has largest number of microbes in GI tract:
– 10^12 bacteria per gram of solid material
– contains over 70% of all microbes in the body.
– Most are obligate anaerobes
– Many use pili to hold
Antibiotics affect:
Can change usual biota. Can cause severe necrotizing enterocolitis, diarrhea, or superinfection.
Sterile sites in GU tract:
Kidneys, bladder, cervix, and fallopian tubes
Nonsterile sites of GU tract:
Distal centimeter of urethra, vagina
Opportunistic infections:
Cause disease when habitat is changes, may occur due to weak immune system.
Immunosuppression caused by:
– Immunosuppressive drugs
– Chemotherapy
– Radiation
– Immune defects
The normal microbiota benefits the host by ___
Priming the immune system, outcompeting potential pathogens for nutrients, and creating a hostile environment for other microbes.
Pathogenicity:
Ability of an organism to produce disease
Opportunistic pathogens:
Usually do not cause infection unless special circumstance such as invasive medical procedures, immunosuppressed person, etc
True pathogens:
Organism that cause disease in healthy immunocompetent hosts, cause disease a high percentage of the time
Iatrogenic infection:
Occurs as a result of medical treatment or procedure
Direct transmission:
Congenital contact, close contact (salivary, skin), droplet infection
Indirect transmission:
Fomites, water, food, airborne, cuts/bites. arthropods
Zoonoses:
Contact with animals or animal products
Incidence:
Refers to number of individuals infected in a population
Prevalence:
Percentage of infected in a given population at a given time
Endemic:
Disease is consistently present in given location
Epidemic:
Larger numbers of cases of disease in given location
Pandemic:
Epidemic around the world
Infection vs Disease:
Infection: invasion and replication of pathogens within the body
Disease: condition that impairs normal functioning and typically has signs, symptoms
Virulence:
Relative ability of microbe to cause disease, measured by number of organism required to cuase infection in host
Virulence factors:
Traits that allow microbe to persist in a host and cause disease
- Resist phagocytosis
- Toxins and extracellular enzymes
- Adhesive fimbriae and pili
Prevention of phagocytosis:
Capsule (masks cell surface structures)
Protein A (interferes with binding of host antibodies, binds Fc portion of IgG)
Panton-Valentine leukocidin:
Killing of phagocytes, causes discharge into cytoplasm and kills cell
Adhesins:
Surface polysaccharides, fimbriae/pili in bacteria
- enable attachment to host surface structure to increase ability to colonize
Break antibody:
IgA protease
Hide from antibody:
Change surface antigen
Invasion:
Penetrate and grow in tissues
Dissemination:
Spread to distant sires in body
Exotoxins:
Both gram negative and positive cells
– Secreted by cell or released with cell lysis
– Have enzyme activity and specificity in activity
– Potent
– Destroyed by heat
- Binding subunit and toxic subunit
Endotoxins:
LPS, cell wall component of gram-negative bacteria
- O-specific polysaccharide-core-lipid A
Toxin activity: Lipid A
Effects: hypotension, fever, initiates coagulation
Host Resistance Factors: physical barriers
Intact skin is effective against most pathogens
Host Resistance Factors: cleansing mechanisms
Desquamation of skin, movement of liquids (tears, urine), cilia
Host Resistance Factors: low pH
stomach, vagina
Host Resistance Factors: antimicrobial substances
- fatty acids on skin
- HCl in stomach
- lysozymes that hydrolyze peptidoglycan
- IgA, antibody found in saliva, secretions
- low molecular weight cationic proteins such as beta-lysins
- interferons that inhibit viral replication
Phagocytic cells:
Engulf and digest lysosomes
*neutrophils, macrophages, monocytes
Steps of phagocytosis:
- chemotaxis
- attachment
- ingestion
- killing
Inflammation:
Nonspecific response to injury or foreign body.
Chemical mediators increases blood flow causing erythema, edema, heat, and pain due to swelling
*increased WBC counts resulting in purulence
Innate immunity:
Natural or nonspecific immunity
- physical or chemical barriers such as skin or mucous membranes
- blood proteins that act as mediators of infection
- little or no specificity
Adaptive/specific immunity:
Passive vs. Active
Humoral vs. Cell-mediated
Highly specialized vs. develop memory
Lymphocytes (adaptive immunity)
B cells: antibodies
T cells : T-helper or cytotoxic T-cells
Mechanisms by Which Microbes May
Overcome the Host Defenses:
- Induce immune tolerance (not recognize pathogen as foreign)
- Immune suppression (actively destroy immune response)
- Antigenic variation
- Intracellular “hiding”
