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