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)
Viral Multiplication - Basic Steps
• Virus attaches to host cell
• Whole virus or genetic material enters host
• Viral DNA or RNA directs host to make viral
genetic material and protein
• Viral nucleic acids and proteins are
assembled
• New viral particles are released from cell
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
• Trypanosoma brucei - African trypanosomiasis
• Cryptosporidium parvum - cryptosporidosis
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
• In the U.S., anal intercourse and needle
sharing are main modes of transmission
Transmission of HIV
• Less commonly transmitted by vaginal
intercourse and oral sex
• Can travel from mothers to offspring
during pregnancy, birth, or breastfeeding
• Not known to be transmitted by food,
air, water, casual contact, or insect bites
AIDS Test
• Should know HIV status of potential
partner
• A person can test negative and still
have and transmit the virus
• Test detects antibodies that appear
weeks to months after infection
Treatment
• No cure
• Once HIV genes are incorporated, no
way to get them out
• AZT and other drugs slow the course of
the disease and increase life span
• Researchers continue to develop drugs
and to work toward an AIDS vaccine
Safer Sex
• Use a latex condom
• Limit partners
• Get to know a prospective partner
before sex
• Be alert to signs of ailments of the
genitals
• Avoid abuse of alcohol and drugs
Chlamydial Infections
• Most common reported STD in U.S.
• A variety of diseases caused by
bacterium
• Leads to inflammation of cervix in
female, burning urination in both sexes
• In females, can spread to uterus and
oviducts to cause PID
Pelvic Inflammatory
Disease (PID)
• Complication of many bacterial STDs
• Bacteria infect uterus, oviducts, ovaries
• Symptoms include bleeding, vaginal
discharge, pain in lower abdomen
• Increases likelihood of ectopic
pregnancy
• Can cause sterility
Gonorrhea
• Caused by the bacterium Neisseria
gonorrhoeae
• Females often symptom-free in early
stages, males discharge pus
• Can cause sterility if untreated
• Can be cured with antibiotics
Syphilis
• Caused by the spirochete Treponema
pallidum (a kind of bacterium)
• Early symptoms are painless chancres;
later an extensive rash
• In some, immune response to infection
causes damage to brain and spinal cord
• Passage from mother to infant can
cause stillbirth, infection of newborn
Genital Herpes
• Caused by Herpes simplex Type II
• Periodic eruption of small, painful
blisters on genitals
• Infection requires contact with fluid from
these sores
• Antiviral drugs can reduce pain but
there is no cure
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
Type B Hepatitis (HBV)
• DNA virus attacks liver cells
• Transmitted in blood or body fluids such
as saliva, vaginal secretions, and semen
• Blood test can reveal HBV or antibodies
• HBV vaccination for people at risk
Vaginitis
• Vaginal yeast infection (candidiasis)
– Treated with antifungal medication
– Both partners may need to be treated to
prevent reinfection
• Trichomoniasis
– Both partners are treated with an antibiotic