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