Microbe-Human Interactions. A Continuum of Interactions Exists Routine Contact with Microorganisms...
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Transcript of Microbe-Human Interactions. A Continuum of Interactions Exists Routine Contact with Microorganisms...
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
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
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
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
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
Virulence factors
Traits used by pathogens to invade and establish themselves in the
host; also determine the degree of tissue damage that occurs
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
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
Persistence of Infection
• Latent infections & recurrent disease
• Sequelae – long term damage to organs/tissues
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
Pathogen Reservoirs vs Sources
• Reservoir = habitat where the organism occurs
• Source = Actual contact which provides the infection
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
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)
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
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