Agents of Disease
Bacteria and Viruses and Parasites, Oh My!
1. What is an infectious agent?
A pathogen that can cause a disease and is communicable
Disease:
any deviation from or interruption of the normal structure or function of any body
part, organ, or system
Communicable:
capable of being spread from organism to organism
Koch's Postulates
1) The microorganism must be present
when the disease is present but absent
in healthy organisms
2) It must be possible to isolate the
microorganism
3) The isolated microorganism must
cause the disease when placed into a
healthy organism
4) It must be possible to re-isolate the
microorganism from the second
diseased host
Purpose of the Host
3. What is the pathogen using the host for?
Food
Shelter
Place to grow/replicate
Place to eliminate its wastes
Three Classes of Disease Agents
6. Order the agents of disease according to size (smallest to largest)
Viruses, Bacteria, Parasites
Smallest
Largest
Bacteria
Biological characteristics of infectious agent
- Single celled, prokaryotic (lack nucleus and organelles)
- Have a polysaccharide cell wall outside of cell membrane
- 3 shapes
Spherical (cocci)
Rod (bacilli)
Spiral (spirochete)
- Can be seen with a light microscope
Peptidoglycan Cell Wall
Bacterial Shapes
Rod-shaped
Bacilli
Spherical
Cocci
Spiral-shaped
Spirochete
Bacteria
What the agent does to cause symptoms
- Secrete toxic substances that damage tissues
- Feed on nutrients intended for host
- Form colonies that disrupt normal functions in the host body
Bacteria
How the infectious agent is transmitted to the host
- Limited mobility
- Rely on carriers such as animals (including insects), water & food
Bacteria
Method(s) of treatment
- Antibiotics
Strep Throat
- Group A streptococcal infection
- Hemolysin - causes breaks down of red blood cells
Cholera
- Vibrio cholerae
- Infects the small intestine
- Causes intense diarrhea and vomiting
- Worldwide, it affects 3–5 million people
and causes 100,000–130,000 deaths a
year as of 2010
Syphilis
- Sexually transmitted infection
- Spirochete bacterium
Treponema pallidum
- Syphilis is believed to have infected 12
million people worldwide in 1999, with
greater than 90% of cases in the
developing world
Typhoid Fever
- Salmonella enterica
- Headache, cough, bloody nose,
abdominal pain, leukopenia, a
decrease in the number of
circulating white blood cells
- Intestinal hemorrhage or
perforation, encephalitis,
neuropsychiatric symptoms
(delirium and muttering)
Mary Mallon ("Typhoid Mary") in a hospital bed
(foreground). She was forcibly quarantined as a carrier
of typhoid fever in 1907 for three years and then again
from 1915 until her death in 1938.
Tetanus
- Tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped,
obligate anaerobic bacterium Clostridium tetani.
Tetanus often begins with mild spasms in the jaw muscles (lockjaw). The spasms
can also affect the chest, neck, back, and abdominal muscles. Back muscle
spasms often cause arching, called opisthotonos. These episodes can cause
fractures and muscle tears.
Parasites
Biological characteristics of infectious agent
- Eukaryotic pathogens
- Live in symbiosis with host
- Types
Protozoa (protists)
Worms
Arthropods (insects)
Fungi
- Can be seen with a light microscope or the naked eye
Parasites
What the agent does to cause symptoms
- Mechanical injury (ex. boring holes through tissue)
- Eating or digesting tissue of host
- Toxin release poisons host
- Robbing nutrients from host
Parasites
How the infectious agent is transmitted to the host
- Contaminated food or water
- Direct contact
- Feces or saliva of insects or other animals
Parasites
Method(s) of treatment
- Depends on parasite
- Medication
- Prevention by good hygiene, water treatment, sanitary conditions
- Insect control
Malaria
- Protist (Phylum Apicomplexa), Genus Plasmodium
- Fever, shivering, joint pain,
vomiting, hemolytic anemia,
jaundice, retinal damage,
convulsions
- Enlarged spleen or liver,
severe headache,
hemoglobinuria with renal failure
may occur(hemoglobin from
lysed red blood cells leaks into
the urine)
- Coma and death
Tapeworm
- Phylum Platyhelminthes, Class Cestoda
- Scolex hooks into wall of
digestive tract
- Can have no symptoms
- Abdominal discomfort, diarrhea,
loss of appetite
- Blockage of intestine
- Rarely, T. solium larvae can
migrate to the brain causing
severe headaches, seizures and
other neurological problems
Schistosomiasis
- Phylum Platyhelminthes, Genus Schistosoma
- Fluke with intermediate snail
host
- Transmitted by playing/drinking
infected water
- Damage to internal organs,
delay in growth/cognitive
development
- Risk of bladder cancer
Viruses
Biological characteristics of infectious agent
- Not a cell, not alive
- Nucleic acid in a protein coat (capsid)
- Genome can be DNA or RNA (retrovirus)
- Cannot replicate without a host cell
- Cellular hijackers
- Too small to been seen in a light microscope
Viral Shapes
Viruses
What the agent does to cause symptoms
- Tend to infect specific cells
- Virus kills host cell during its replication (lytic cycle)
- Virus can insert sections of its genome into the host cell genome and lay
dormant for an extended period of time (lysogenic cycle)
Viral Lytic Cycle
Lysogenic Cycle
Lysogenic to Lytic
Viruses
How the infectious agent is transmitted to the host
- Direct contact
- Exchange of bodily fluids
- Airborne
- Insect and animal vectors
- Contaminated food or water
Viruses
Method(s) of treatment
- Some antiviral drugs can slow action of virus (viricidals)
Ex. Valtrex (valacyclovir) for cold sores (herpes)
- Prevention by vaccines
Vaccine - contains "killed/inactivated" form of microbe or a component
of its antigens or toxins to illicit an immune response
Creates memory cells to provide a faster, secondary immune response
if/when the person encounters the true virus
Viral Diseases
Poliomyelitis
Cold sores (herpes)
Ebola
Small pox
Influenza
Rabies
TMV
a non-living particle composed of a nucleic acid
(DNA or RNA) and a protein coat (capsid)
protein coat that surrounds the viral nucleic acid
virus that contains RNA and the enzyme reverse
transcriptase
virus invades a host cell, produces new viruses, and
kills the host cell upon lysis
viral genome remains within a host cell for an
extended period of time; may revert to lytic cycle
D
C
D
A
C
Stanely was
able to crystallize the tobacco mosaic virus, which suggested
that viruses were chemicals rather than tiny cells
Radiation, sunlight, certain chemicals, stress
RNA is injected into host cell, RNA is transcribed to DNA by reverse
transcriptase, viral DNA is then incorporated into the host cell's genome
Flu viruses mutate quickly, altering the antigens on the viral surface. Flu vaccine
targets a different strain each year.
Bacteriophages are very effective at injecting foreign DNA into
bacteria. Genetic engineers can use bacteriophages to introduce
DNA of interest to bacteria.
2
penetration
5
release
(lysis)
1
attachment
4
assembly
3
synthesis
Prions
- Infectious proteins
- Abnormal prion proteins
accumulate inside brain cells,
causing the cells to die
- The brain becomes riddled
with holes (spongiform
encephalopathy)
- Prions resistant to heat,
radiation, and chemical
treatment