Transcript Prokaryotes, Viruses, and Protistans

Life At Risk:
Infectious Disease
Chapter 18
Virus
• Noncellular infectious agent
• Consists of protein wrapped around a
nucleic-acid core
• Cannot reproduce itself; can only be
reproduced using a host cell
Viral Body Plans
• Genetic material is
DNA or RNA
• Coat is protein
Helical virus
Polyhedral virus
Complex virus
(bacteriophage)
Enveloped Virus (HIV)
lipid envelope;
proteins span the
envelope, line its
inner surface, spike
out above it
viral coat
(proteins)
Replication
of an
Enveloped
Virus
Transcription
of viral genes
DNA replication
Assembly
Translation
Proteins
Prions
• Small proteins
• Linked to human diseases
– Kuru
– Creutzfeldt-Jakob disease (CJD)
• Animal diseases
– Scrapie in sheep
– Bovine spongiform encephalopathy
(mad cow disease)
Bacteria
• Were first living organisms
• Prokaryotic
Bacterial Characteristics
• No membrane-bound nucleus
• Single chromosome
• Cell wall in most species
• Prokaryotic fission
• Metabolic diversity
Bacterial Shapes
coccus
bacillus
spirillum
Prokaryotic Body Plan
capsule
pilus
bacterial
flagellum
DNA
cell wall
plasma
membrane
ribosomes
in
cytoplasm
Bacterial Genes
• Bacteria have a single chromosome
– Circular molecule of DNA
• Many bacteria also have plasmids
– Self-replicating circle of DNA that has a
few genes
– Can be passed from one cell to
another
Prokaryotic Fission
Antibiotics
• Can destroy or inhibit the growth of
bacteria and some other
microorganisms
• Have no effect on viruses
• Antibiotic-resistance is a growing
problem
Parasitic Protozoa
• Entamoeba histolytica - amoebic dysentery
• Giardia lamblia - giardiasis
Parasitic Worms
• Pinworms
• Tapeworms
• Hookworms
• Ascaris roundworms
Malaria
• Symptoms known for more than 2,000 years
• Most prevalent in tropical and subtropical
parts of Africa
• Kills a million Africans each year
• Caused by four species of Plasmodium
• Transmitted by Anopheles mosquitoes
Plasmodium Life Cycle
sporozoites
sporozoites
Gametes form
in mosquito gut,
combine to form
zygotes
merozoite
Male and female
gametocytes
in blood
Offspring enter
blood, cause
malarial
symptoms
Disease Transmission
• Direct contact with a pathogen
• Indirect contact
• Inhaling pathogens that have been
spewed into the air
• Contact with a vector
Disease Patterns
• Sporadic disease (whooping cough)
• Endemic disease (common cold)
• Epidemic (cholera in Peru)
• Pandemic (AIDS)
Virulence
• Relative ability of a pathogen to cause
serious disease
• Low virulence - common cold
• High virulence - rabies
AIDS
• Combination of disorders that follows
infection with HIV
• Includes
– Yeast (Candida) infections
– Pneumocystis pneumonia
– Karposi’s sarcoma
HIV Replication (1)
• RNA retrovirus
• A protein (gp120) at virus surface binds to
host cells with CD4 and chemokine receptors
• These receptors occur on helper T cells
• Once bound, RNA and viral enzymes enter
the host cell
HIV Replication (2)
• Viral RNA is reverse-transcribed to DNA
• HIV DNA is called provirus; it inserts
into host DNA
• The host cell makes copies of viral DNA
and viral proteins that assemble to form
new virus particles
T-Cell Decline
• Release of new viral particles kills the
host T cell
• The body is constantly making new T
cells, but cannot outpace the rate of
destruction
• As infection proceeds, T-cell numbers
inevitably decline
Effect of T-Cell Decline
• CD4 helper T cells play a vital role in
immune function
• They are required for both cell-mediated
and antibody-mediated immunity
• Infected individual becomes vulnerable
to other infections, which eventually
result in death
Transmission of HIV
• HIV does not live long outside human
body
• Most often spread by exchange of
bodily fluids with an infected person
• Sexual intercourse and needle sharing
are main modes of transmission
Human Papillomaviruses
• HPV can cause bumplike warts on the
genitals and anus
• One strain, 16 HPV, does not cause
symptoms
• It can lead to cancers of cervix, vagina, vulva,
penis, and anus
• There is no cure