Microbial interactions
I.Symbiosis
A.Close physical relationship between organisms of different species
Essential to the survival of one or both
B.Host
Larger organism in the relationship
C.Symbiont
Smaller organism
Symbiont always benefits from its relationship with the host
C.Consequences to host vary
Mutualism
Both host and symbiont benefit from the relationship
E. coli in human intestine makes vitamin K
Commensalism
Symbiont benefitted
Host neither benefitted nor harmed
Many bacteria in human gut obtain nutrients but do not help or injure host
Parasitism
Most common symbiotic relationship
Symbiont (parasite) benefits from the relationship
Host is harmed
D.Parasitism
Parasitic relationship may not seriously injure the host
Good parasite doesn't kill host
Dead host dead parasite
Host may carry small population of parasites without serious consequences
II.General Terms
A.Disease
Occurs when the stress of a parasitic infection becomes obvious in the host
B.Pathogen
Parasite that causes disease
C.Pathogenicity
Ability of a pathogen to cause disease
D.Virulence
Degree or intensity of pathogenicity
Ability of a pathogen to cause disease
Depends on both the pathogen and the host
Depends on pathogen’s toxicity
Ability of the pathogen to poison its host
Also depends pathogen’s Invasiveness
Ability of the pathogen to spread once in the host
Also depends on the host’s resistance
Host’s ability to protect itself from the pathogen
E.Infection
Any situation in which a microorganism is established and growing in a host, whether it damages the host or not
III.Determinants of infectious disease
A.To induce an infectious disease a pathogen must
Be transported to the host
Adhere to, colonize, invade the host
Mechanically
Chemically
Molecularly
B.Transmision of pathogens
Coughing
Sneezing
Body contact
C.Vector:
Cuts
Burns
Abrasions
D.Transmision of pathogens
Vector
Organism that transmits pathogens from one host to another
Fomites
Inanimate objects that harbor and transmit pathogens
E.Attachment and colonization of pathogen
Pathogen must be able to
Adhere to and colonize host cells and tissues
Adhesins
Specialized molecules on pathogens surface
Bind to complementary receptor sites on the host cell surface
F.Entry of pathogens
Respiratory system
Digestive tract
Urogenital tract
Eyes
Ears
G.Invasion - Spread of pathogens
Some pathogens remain localized
Cause disease from a localized area
Many penetrate mucous membranes after attachment to epithelial surfaces
Move deeper into host before causing disease
Active invasion
Secrete invasive enzymes
Hyaluronidase
Dissolves hyaluronic acid
Intercellular cement
Collagenase
Breaks down collagen
Protein component of connective tissue
Produced by some clostridia
Streptokinase
Enzyme produced by Streptococcus
Breaks down fibrin which is produced by host
Fibrin is the material used to form blood clots and scabs.
Fibrin walls off infection.
Dissolving fibrin allows bacteria to spread.
Medical Use of streptokinase
Dissolve blood clots in Coronary arteries
Used immediately after heart attack
Passive invasion
Small breaks in mucous membranes
Wounds
Abrasions
Burns
Insect bites
IV.Growth and Multiplication of the pathogen
A.Pathogens must find suitable environmental conditions in host body
Area of proper pH
Nutrient supply
Trace elements
Ferric iron
Growth factors
Vitamins
Amino acids
B.Types of Pathogenic Growths
Aerobic
Anaerobic
Microaerophilic
C.Areas of Entry
Body surface
Body core
Extremities
Pathogens must avoid host defenses
Capsules allow pathogen to avoid phagocytosis
D.Avoidance of host defenses - Leucocidins
Substances which destroy phagocytes
Some bacteria produces leucocidins when engulfed by a phagocyte
E.Leucocidins
Cause phagocyte to lyse
Bacterium is released unharmed
F.Avoidance of host defenses Intracellular infections
Certain bacteria able to survive and multiply inside phagocyte
Infected white blood cell aids in dispersal of the pathogen
G.Host Damage
Pathogens damage host in two ways
Toxigenicity
Large numbers
H.Toxins
Usually cause the symptoms of disease
Divided into two major categories
Exotoxins
Poisons secreted by actively growing bacteria
Diffuse away from site of infection into Surrounding tissue
Circulatory system of host spreads toxin throughout body of host
Endotoxins
Proteins
Soluble
Heat-labile
Inactivated at 60-80oC
Most lethal substances known µg per kilogram amounts can kill host
Specific exotoxins associated with specific diseases
Highly immunogenic
Stimulate production of antibodies
Inactivated by
Acids
Formaldehyde
Iodine
Alcohols
Heat
Inactivated exotoxins converted to toxoids
Do not cause the disease
Retain ability to induce antibody production
I.Characterization of Exotoxins
Neurotoxins
Cytotoxins
J.Neurotoxins
Inhibit nerve synapsis function
Clostridium produces extremely dangerous neurotoxins
Normally a spore-forming soil anaerobe
Accidental pathogen in man
Clostridium botulinum
Produces most potent toxin known
Seven grams (one teaspoon) enough to kill the entire human population
Toxin produced under anaerobic conditions
Improperly canned non-acid foods contaminated with C. botulinum spores
Clostridium botulinum exotoxin
Affects central nervous system
Prevents muscle contraction
Flaccid paralysis
Fatality rate 100%
Reduced by use of
Antibody against toxin
Respirator to prevent respiratory failure
Death due to
Cardiac arrest
Respiratory failure
Botulism in infants
Babies may get botulism from honey
Honey may contain large numbers endospores
pH in infant's stomach is too high to destroy endospores
Germinate in baby's intestine
Produce neurotoxin
Baby gets botulism
Pacifiers dipped in honey may give botulism in infants
Clostridium tetani
Enters host via deep dirty woundBacteria remain localized at site of woundDo not spread through body
Anaerobic conditions develop in wound
Bacteria produce toxin
Toxin diffuses away from its site of origin
Picked up by circulatory system spread throughout body
Affects central nervous system
Binds to nerve synapsis
Causes tetany
Prolonged contractions of skeletal muscles - Tetanus
Death due to
Cardiac arrest
Respiratory failure
I.Cytotoxins
Inhibit protein synthesis
Damage plasma membrane
Corynebacterium toxin
Produced by Corynebacterium diphtheriae
Causes diphtheria
Hemolysins
Act on the cell membrane
Cause cells to lyse and die
Produced by pathogenic bacteria
Staphylococcus
Streptococcus
Streptococcus durans
On blood agar completely destroy red blood cells
Some destroy intestinal mucosa
Causes inflammatory colitis
Staphylococcus aureus
Clostridium difficile
Affect the intestine
Disruptmembrane transportChange permeability of the membrane
Allow large amounts of water to be secreted into the lumen of the intestine
Results in diarrhea and loss of electrolytes
Staphylococcus aureus
Clostridium perfrigens
Vibrio cholerae
Bacillus cereus
Escherichia coli
Salmonella
J.Endotoxins
Second largest class of toxins
Produced only by Gram negative bacteria
Released when cell dies and disintegrates
Part of the outer membrane of the Gram negative cell wall
Complex molecules
Lipopolysaccharides
Phospholipids
Carbohydrates
Proteins
Not inactivated by substances that denature protein
Heat stable Do not form toxoids
Less toxic than exotoxins
Do not induce formation of antibodies
Endotoxins causes
Fever
Nausea
Diarrhea
Inflammation
Abortion
V.Invasive versus noninvasive pathogens
A.Toxin producing organisms
Do not have to be invasive
Toxins do the damage
Organisms that do not produce toxins must be invasive
Must grow in large numbers to cause damage
Streptococcus pneumoniae grows in large numbers in the lungs
Doesn't produce toxin
Large numbers of bacteria block alveoli (oxygen exchange surfaces)
VI.Koch postulates
A.Developed by Robert Koch in 1880-82
B.First direct demonstration that bacteria cause disease
Experimental procedures provide a frame work for the study of any infectious disease
C.Means of demonstrating that a particular organism causes a particular disease
Bacillus antharacis
Anthrax (1876)
Vibrio cholerae
Cholera (1880) Mycobacterium tuberculosis
Tuberculosis (1884)
D.Koch postulates
1.The organism must always be present in individuals uffering from the disease and in a logical pathological location
2. The suspected organism must be isolated in pure culture and grown away from the host body
3. The same disease must result when the isolated microorganism is inoculated into a healthy host.
4. The organism must be reisolated from such experimental infections and must retain the same characteristics as the original isolations