Chapter 14 Pathology. Definitions! Pathology – study of disease Etiology – cause of disease Pathogenicity – how a pathogen overcomes host defenses to.

Chapter 14

Pathology

Definitions!• Pathology – study of disease• Etiology – cause of disease• Pathogenicity – how a pathogen

overcomes host defenses to produce disease

• Pathogenesis – development and progression of disease

• Epidemiology – occurrence and spread of disease

Symbiosis

• A relationship between two organisms

• Mutualism – both benefit

• Commensalism – one benefits, other is unaffected

• Parasitism – one benefits, other is harmed

Normal Flora in Humans

• Resident flora – permanent microbes in the body– Most are

commensals or mutualistic

• Transient flora – temporary microbes in the body

Symbiosis – Special Cases

• Microbial antagonism – competition between microbes. Normal flora outcompete pathogens in our bodies. This is a form of mutualism between human and normal flora.

• Opportunism – disease caused when organism is in a new environment, the host is immunocompromised, or normal flora disturbed– E.g. E. coli in UTI, Pneumocystis in immunodeficient

patients, overuse of antibiotics

Koch’s Postulates – Proves Etiology

Summary of Koch’s Postulates

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

2. The pathogen must be able to be isolated and cultured in pure media.

3. The cultured pathogen from step 2 must be able to cause disease again.

4. The same pathogen must be able to be isolated from the organism given disease.

Exceptions to Koch’s Postulates

• The pathogen may not be able to be cultured in pure media (e.g. viruses, Rickettsia).

• Diseases may have multiple causes (e.g. nephritis, UTI)

• One pathogen causes multiple diseases/symptoms (e.g. Mycobacterium tuberculosis affects lung, brain, kidney, skin and bones)

Pathogenicity in Bacteria

• Adherence – binding to host cells

• Penetration – entry into host cells

• Toxins – compounds that harm the host

Adherence

• Bacterial structures (pili, fimbriae, capsules, slime layers) allow attachment to host

• Specific proteins such as adhesins may be used for specific attachment to receptors on the host

Biofilms

• Mass of pathogens in cooperative adherence. • First organisms that attach secrete materials that

assist others to attach and colonize• E.g. dental plaques on teeth.

Penetration

• Endocytosis – adherence can trigger endocytosis in host cell. Invasins may be involved that help rearrange cytoskeleton to facilitate entry and intracellular movements.

Penetration

• Tissue degradation – enzymes secreted that dissolve barriers. E.g. hyaluronidase breaks down hyaluronic acid, a sugar that holds cells together. This is the cause of gangrene.

Penetration• Clotting

– Coagulase produced by staph allows clots to form, protecting the colony.

– Streptokinase can dissolve clots to free pathogen so they can spread.

Toxins

• Endotoxins– Produced by Gram- – Part of the LPS– Nonspecific– Low toxicity (fever,

aches)– Released when

bacteria die or divide

• Exotoxins– Produced mostly by

Gram+– Secreted– Specific targets– High toxicity

Exotoxins Types

• Hemolysins – destroy red blood cells.

• Neurotoxins – attacks nervous system.

• Superantigens – provoke intense immune response.

Pathogenesis

• Types of Infections– Duration and severity– Placement of infection– Sequence

• Disease Progression– Stages of infection

Severity and Duration of Disease

• Acute – quick (weeks) but severe– E.g. flu, ebola

• Chronic – long lasting (months-years) and continuous– E.g. tuberculosis

• Latent – has an inactive phase– E.g. HIV

Placement

• Local – confined to one area

• Focal – localized to one area but toxins/pathogens can affect other areas

• Systemic – affects entire body

Sequence of Infections

• Primary infection – the first infection of a healthy person

• Secondary infection – the second pathogen. Usually opportunistic– Superinfection – a type of opportunistic

infection when the normal flora is destroyed.

Disease Progression

1. Incubation – infection, no symptoms

2. Prodromal – early/mild symptoms

3. Invasive – most acute and dangerous. Acme is the peak

4. Decline – begin recovery, symptoms subside

5. Convalescence – fully recovered, body regains strength

Disease Progression