Microbe-Human Interactions
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Transcript of Microbe-Human Interactions
Microbe-Human Interactions
A Continuum of Interactions Exists
• Routine Contact with Microorganisms
• Infection – a condition in which pathogenic microbes penetrate host defenses, enter tissues and multiply
• Disease – Deviation from Normal Health
The Normal State is that Humans are Host to a Variety of
Microorganisms
Humans are Colonized from the Moment of Birth
Babies Readily Acquire Flora During Delivery and from the
External Environment
Regions that Host Flora
• Skin
• Oral cavity
• Lower GI tract
• Upper Respiratory
• Genitourinary Tract
Skin Flora
Staphylococcus, Micrococcus
Propionibacterium
Flora of the Mouth and Pharynx
Streptococcus, Neisseria, Staphylococcus, Lactobacillus,
Bacteroides
Colonized Regions of the Respiratory Tract
Flora of the GI Tract
Bacterioides, Lactobacillus, Enterococcus, Coliforms
Distribution of Flora
Flora of the Genitourinary Tract
Lactobacillus, Streptococcus, E. coli, Staphylococcus
Maintenance of the Normal Resident Flora
• Normal flora is essential to the health of humans
• Flora create an environment that may prevent infections and can enhance host defenses
• Antibiotics, dietary changes, and disease may alter flora
• Probiotics – introducing known microbes back into the body
The Process of Disease
Types of Pathogens
True vs. Opportunistic Pathogens
True Pathogens
• Capable of causing disease in a healthy person
• Generally associated with a recognizable disease
• Can be weak to fatal in their effects
Opportunistic Pathogens
• Cause disease in weakened host
• Sometimes cause disease when they colonize a different region of the body
• See Table 13.4 for pre-disposing conditions
Pathogenicity vs. Virulence
• Pathogenicity = ability to cause disease
• Virulence = relative capacity of a pathogen to invade and harm host cells
Gaining Access – Portal of Entry
The Skin or Membrane Barrier through which Pathogens Gain
Entry to the Body
Pathogens May be Exogenous or Endogenous
Some Agents that Enter the Skin
• Staphylococcus aureus
• Streptococcus pyogenes
• Clostridium sp.
• Insect-borne diseases
• Hypodermic needle contaminants
GI Tract Entrants
• Salmonella sp.
• Vibrio sp.
• E. coli
• Shigella sp.
• Entamoeba sp.
• Giardia sp.
Respiratory Entrants
• This is the most frequent route of entry
• Streptococcus (Strep throat)
• Influenza viruses
• C. diptheriae
• B. pertussis
• Many others
Urogenital Entry Agents
• Syphilis
• Gonorrhea
• Human Papilloma Virus
• HIV
• Chlamydia
• Hepatitis B
Placental & Neonatal Agents
STORCH = Syphilis, toxoplasmosis, others (hepatitis), rubella,
cytomegalovirus, herpes simplex
Infectious Dose
Minimum number of organisms required for an infection to be
successful; lack of ID will not result in infection
To be Successful, a Pathogen Must
• Adhere to the host
• Penetrate host barriers
• Establish a colony in host tissue
Some Methods of Adhesion
• Fimbriae
• Flagella
• Adhesive Capsules
• Binding to Receptors (Viruses)
Virulence factors
Traits used by pathogens to invade and establish themselves in the
host; also determine the degree of tissue damage that occurs
Some Virulence Factors
• Extracellular Enzymes
• Exotoxins
• Endotoxins
• Anti-phagocytic Factors
Exotoxins – Secreted by Living Cells
Examples: Botulin Toxin, Hemolysins (Strep and Staph)
Endotoxins are Released by Dead Cells
Example: Lipopolysaccharides from Gram- cells
Antiphagocytic Factors
Ex.: Leukocidins, Slime Layers
The Process of Infection and Disease
• Distinct stages of clinical infections:– incubation period - time from initial contact with
the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause symptoms; several hours to several years
– prodromal stage – vague feelings of discomfort; nonspecific complaints
– period of invasion – multiplies at high levels, becomes well established; more specific signs and symptoms
– convalescent period – as person begins to respond to the infection, symptoms decline
Patterns of Infection
• Localized - Contained
• Systemic – Agent Circulates Freely
• Focal – Localized infection breaks loose or attacks a distant target with toxin
• Mixed – more than one agent invades
Temporal Patterns of Infection
• Acute = short-lived but severe
• Chronic = long-lived, persistent
Signs of Disease vs Symptoms
• Signs = objective evidence of disease
• Symptoms = subjective evidence sensed by patient (discomfort)
Some Signs of Infection
• Inflammation
• Skin lesions
• Elevated WBC count
• Bacteria or virus in blood
Portals of Exit for Pathogens
• Respiratory/ Salivary
• Skin
• GI tract
• Urogenital
• Blood
Persistence of Infection
• Latent infections & recurrent disease
• Sequelae – long term damage to organs/tissues
Epidemiology
The Study of Disease in Populations
Classifications of Diseases
• Endemic – exhibits a relatively stable frequency in a particular location over time
• Sporadic – occasional cases at irregular intervals
• Epidemic – increase beyond what might be expected in a given population
• Pandemic – spread of epidemics across continents
Patterns of Diseases
Pathogen Reservoirs vs Sources
• Reservoir = habitat where the organism occurs
• Source = Actual contact which provides the infection
Disease Carriers
Shelter and Spread a Pathogen Inconspicuously
Carrier States
• Asymptomatic – No signs of infection• Incubation – Spreads infection while it
is incubating (still no symptoms)• Convalescent – Sheds microbes while
recuperating• Chronic – Latent infections can be
sheltered after apparent recovery• Passive – Mechanically picks up and
transfers microbes
Disease Vectors
Animals that transport an infection from one host to
another
Biological Vectors
• Actively participate in pathogen’s life cycle
• Can inject infected saliva (mosquito)
• May defecate around bite wound (flea)
• Regurgitate blood into a wound (tsetse fly)
Mechanical Vectors
Move the Pathogen from one Host to Another Without being
Infected
Zoonosis
An infection naturally found in animals, but transmissible to
humans (See Table 13.10)
Communicable & Contagious Diseases
• Communicable diseases can be transferred from one host to another and infection established
• Contagious diseases are highly transmissible and move readily from host to host
• Non-communicable diseases are not transmitted from one host to another
Mechanisms of Transmission
• Direct – Portal of Exit from one Individual contacts Portal of Entry of another
• Indirect – Pathogen is carried by intermediary vehicle from one host to another
Fomites
Inanimate objects that harbor and transmit pathogens
Airborne Spread
• Droplet nuclei from sneezes and coughs
• Aerosols
Nosocomial Infections
• Diseases that are acquired during a hospital stay
• Most commonly involve urinary tract, respiratory tract, & surgical incisions
• Most common organisms involved are Gram-negative intestinal flora, E. coli, Pseudomonas, Staphylococcus
Koch’s Postulates
• Find evidence of a particular microbe in all cases of a disease
• Isolate the suspect microbe from an infected individual and culture it in the lab
• Inoculate a healthy individual with the pure lab culture and observe the resulting disease
• Isolate the microbe from the test individual