Lecture 1: Intro and Host-Pathogen Interactions

What are the stages of a bacterial colonization?

1. Entry

2. Adherence

3. Invasion

4. Colonization

5. Growth

Health

The balance between normal microbiota and host

Disease

The imbalance between immune system and normal microbiota overwhelmed by pathogens

What are Koch’s Postulates?

1. The same pathogen must be present in every case of the disease

2. Pathogen must be isolated in pure culture

3. Pathogen isolated from pure culture must cause the same disease in lab animal

4. Pathogen must be re-isolated from inoculated lab animal

Mutualism/Symbiosis

Bacteria and host are benefited (rumen bacteria)

Commensalism

Bacteria are benefited and host is unaffected (normal flora)

Parasitism

Bacteria are benefited and the host is harmed

Bacterial Antagonism

A protective function against pathogens by competing for attachment, nutrients, and producing substances that inhibit pathogens

Probiotics

Involved in competitive exclusion

Pathogenesis

Process/mechanisms of disease development (toxins, attacking immune system, etc.)

Pathogenicity

Capacity/potential of an organism to cause the disease

Virulence

Relative pathogenicity (LD50, ID50)

Invasiveness

Ability to enter, survive, multiply, and spread in the host

LD50

Number of microbes in a dose that will kill 50% of inoculated test animals

ID50

Dose required to produce demonstrable infection in 50% of test animals

Frank (True) Pathogen

Cause disease in normal hosts

Opportunistic Pathogen

Causes disease only when conditions are favorable (stress, vaccination, etc.)

Extracellular Pathogen

Grows and multiples in the space and fluids surrounding cells

Intracellular Pathogen

Grows and multiples inside the cells

Facultative

Grows/multiplies inside and outside cells; can be cultured in bacteriological media

Obligate

Grows/multiplies only inside cells; must be cultured in tissue culture media

Infection

Invasion or colonization by a pathogen; may or may not result in a disease

Disease

Change from a state of health

Primary Infection

Infection in a previously healthy host

Secondary infection

Occurs along with or immediately following another infection

Exogenous Infection

Bacteria originate outside the animal

Endogenous Infection

Bacteria originate from within the animal

Sporadic Disease

Occurs occasionally

Endemic Disease

Consistently present in a population

Epidemic Disease

Consistently present in a population

Pandemic Disease

Occurs worldwide

Acute

Rapid onset, usually severe, and lasts for a short period

Peracute

Higher degree of acute disease

Chronic

Slow onset, less severe, and lasts longer

Subclinical

Mild with no overt signs/symptoms

Localized

Confined to a relatively small area

Systemic

Spreads throughout the body. Bacteria enters lymph/circulation

Focal

Bacteria from a local infection enter lymph or blood to set up local infection in other parts of the body

Bacteremia

Bacteria circulating in the blood

Septicemia

Bacteria multiplying in the blood

Toxemia

Toxin is circulating in the blood

Incubation Period

Interval between the entry and the appearance of first symptoms

Illness

Signs and symptoms are evident

Symptoms

Effects of the disease experience by the patient (pain, nausea)

Signs

Effects of the disease (fever, swelling)

Convalescence

Period of recovery

What are the stages of an infection?

1. Incubation Period

2. Prodromal Stage

3. Period of Invasion

4. Convalescent Period

Incubation Period

From initial contact with the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause clinical signs; can be several hours-years

Prodromal Stage

Vague feelings of discomfort; nonspecific

Period of Invasion

Multiplies at high levels and well-established; more specific signs/symptoms; height of infection

Convalescent Period

As host begins to respond to the infection, symptoms decline