Chapter014


Chapter 14

Mechanisms of Infectious Disease

Chapter 14

Mechanisms of Infectious Disease


Infection or Colonization with MicroorganismsInfection or Colonization with Microorganisms

• “Infection” or “colonization” means that microorganisms are multiplying in or on the host

Discussion:

• Do you have any infections or colonizations at this moment? List as many as you can identify

• Are they normal, or are they making you ill?


Infection or Colonization with Microorganisms (cont.)Infection or Colonization with Microorganisms (cont.)

• Over 300 different species of bacteria live in the large intestine

• Bacteria and fungi live on our skin

• The mouth and pharynx contain many species of bacteria

• The vagina contains acid-producing bacteria


MicrofloraMicroflora

• Are the microorganisms normally living in or on your body

• Some are useful

• Many have no effect

• Pathogens cause disease

• All are capable of causing disease if your health and immunity are weakened

• Opportunistic pathogens


Question Question

Tell whether the following statement is true or false:

All interactions between humans and microorganisms are detrimental.


Answer Answer

False

Some microorganisms perform important functions for their human hosts, like producing vitamins, assisting digestion, or preventing harmful pathogens from entering the host.


Kinds of Infectious AgentsKinds of Infectious Agents• Prions

– Small modified infectious host proteins

– Abnormally shaped versions of your own proteins

– Cause normal proteins to change their shape and become new prions

– Can clump together and damage cells

– Cause degenerative disease in the central nervous system (e.g., mad cow disease)


Kinds of Infectious Agents (cont.)Kinds of Infectious Agents (cont.)

• Viruses

– Protein coat surrounding nucleic acid core

– Have no metabolic enzymes of their own

– Insert their genome into a host cell’s DNA

– Use that cell’s metabolic machinery to make new viruses



• Bacteria

– Cells without membrane-bound organelles (prokaryotes)

– Can live independently

– Use infected organism for food and shelter



• Bacteria

– Can produce toxins

– Exotoxins are proteins released by bacteria

– They damage or kill host cells

– Endotoxins are parts of the bacterial cell wall

– They cause host immune reactions



• Mycoplasmas, rickettsiae, chlamydiae

– Smaller than bacteria

– Mycoplasmas lack cell walls

– Rickettsiae and chlamydiae have to live inside cells to metabolize, like viruses



• Fungi

– Most require a cooler temperature than human core body temperature

– So most infections are on the surface of the body



• Parasites

– Protozoa: malaria, amoebic dysentery, giardiasis

– Helminths: roundworms, tapeworms, flukes

– Arthropods: ticks, mosquitoes, mites, lice, fleas


Question Question

Which pathogen is an intracellular parasite consisting of a protein coat surrounding a nucleic acid?

a. Prion

b. Virus

c. Bacteria

d. Protozoa


Answer Answer

b. Virus

Viruses have no organized cellular structure like bacteria and protozoa. Viruses can only replicate inside another cell; prions cannot reproduce at all.


Discussion:Discussion:• How many ways could you have become infected today? How

could you have experienced:

– Direct contact with a pathogen?

– Ingestion of a pathogen?

– Inhalation of a pathogen?

– Contact with a zoonosis?

– Contact with a nosocomial infection?

– Contact with a fomite?


SymptomatologySymptomatologyInfection

inflammatory and immune

responses attack infective agent

SPECIFIC: signs and symptoms of local damage and

inflammation

NONSPECIFIC: signs and

symptoms of systemic

inflammation


Scenario:Scenario:

A 5-year-old boy has an ear infection…

• He complains of pain in his ear and cannot hear on that side.

• When you look into his ear, you see a red, bulging eardrum with pus behind it.

• He has a fever, sweats, and complains of joint aches.

• Blood tests show an elevated white blood cell count.

Question:

• Use the model of symptomatology to classify these signs and symptoms.


Discussion:Discussion:What Stage of an Infection Are You In?

• How many people in the class are in:

– The incubation stage?

– The prodromal stage?

– The acute stage?

– The convalescent stage?

– The resolution stage?

How can you tell?


Terms for Infection and DamageTerms for Infection and Damage

• -itis means inflammation

– May or may not be due to infection

• -emia means in the blood

• Sepsis or septicemia means bacterial toxins in the blood


Virulence FactorsVirulence Factors

• Make an infection more likely to cause disease

– Toxins: exotoxins and endotoxins

– Adhesion factors help infective organism stick to the body

– Evasive factors help keep immune system from killing infective agent


Question Question

Tell whether the following statement is true or false:

Certain bacterial cells release proteins called endotoxins during growth.


Answer Answer

False

Exotoxins are proteins; endotoxins contain no protein (they are composed of lipids and polysaccharides). Endotoxins are not released during bacterial cell growth.


Scenario:Scenario:

A woman’s stomach contained the bacterium Helicobacter pylori…

• For many years, the woman was healthy

• Then she took on a new stressful job, moved, and began to care for her elderly parents

• A few months later, she began to suffer stomach pains and vomited blood

• She was diagnosed with a bleeding ulcer


Scenario (cont.)Scenario (cont.)

Question:

• How does each of these terms relate to her case?

– Portal of entry

– Site-specific pathogen

– Opportunistic pathogen

– Evasive factors

– Invasive factors


SerologySerology

• After exposure to an infectious agent, the body produces antibodies

• Antibody titer rises

• IgM: rises during the acute phase, then falls

• IgG: remains elevated after the acute phase


Scenario:Scenario:

A month-old baby is ill…

• Serum analysis shows that she has IgG against HIV and IgM against Pneumocystis

Question:

• What inferences can you make?


• We have more drugs to kill bacteria than to kill viruses, and more drugs to kill viruses than to eradicate prions

Question:

• Why has it been easier to develop antibacterial drugs than antiviral drugs?

• Why not use antibacterial or antiviral drugs to destroy prions?

Antibacterial and Antiviral DrugsAntibacterial and Antiviral Drugs


Antibiotics Kill Bacteria by Targeting:Antibiotics Kill Bacteria by Targeting:• Cell wall synthesis

• Protein synthesis

• Nucleic acid synthesis

• Bacterial metabolism

Bacteria Fight Back by:Bacteria Fight Back by:

• Inactivating antibiotics

• Changing antibiotic binding sites

• Using different metabolic pathways

• Changing their walls to keep antibiotics out


Antiviral Agents Kill Viruses by:Antiviral Agents Kill Viruses by:

• Blocking viral RNA or DNA synthesis

• Blocking viral binding to cells

• Blocking production of the protein coats (capsids) of new viruses


Question Question

What type of infections are treated based upon the results of a Gram stain?

a. Fungal

b. Viral

c. Bacterial

d. Parasitic


Answer Answer

c. Bacterial

Bacteria are commonly classified according to Gram stain. Gram-positive and gram-negative organisms are treated with specific antibiotics that target that type of infection. For example, penicillin targets gram-positive organisms.

If the cause of bacterial infection is unknown, broad-spectrum antibiotics may be prescribed, targeting both gram-positive and gram-negative organisms.

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