General characteristics of Viruses
A. Unique
characteristics
1. Filterable
– very small (go through filter)
2. Obligate
intracellular parasites – require host cells in order to multiply
3. Contain
single type of nucleic acid, DNA or RNA
4. Have
protein coat surrounding nucleic acid
5. Uses
cell’s machinery to multiply
B. Host Range
Viruses infect all organisms, but usually specific
Viruses that infect bacteria à bacteriophages
C. Viral size
(Fig. 13.1)
20 – 1000 nm in length
Viral Structure
DRAW
A. Components
of viruses
1. Nucleic
Acid – consists of one of the following
ds-DNA
ss-DNA
ds-RNA
ss-RNA
2. Capsid
protein coat
made of capsomeres (protein subunits)
3. Envelope
Usually lipid bilayer obtained from host plasma
membrane
Contains both host and viral proteins
B. Two types
of viruses
1.
Nonenveloped (naked) viruses (Fig. 13.2)
2. Enveloped
viruses (Fig. 13.3)
General Morphology of Viruses
A. Helical
Viruses
Long rods
Ex: Ebola
virus (Fig. 13.4)
B. Polyhedral
viruses
Many sided
Most common à icosahedral (20-sided)
Ex:
Adenovirus (Fig. 13.2b)
C. Complex
viruses
Complicated structures
Ex: T-even
bacteriophage (Fig. 13.5a)
The isolation cultivation, and identification of
viruses
A. Growing
bacteriophages in the lab
1. Infect
bacterial culture with bacteriophage
2. Put on
agar plate
3. Plaques
form (areas of destroyed bacteria) (Fig. 13.6)
B. Growing
Animal viruses in the lab
1. In Living
animals
mice, rabbits, guinea pigs, etc.
2. In
embryonated eggs
Inject into fertile chicken egg
3. In cell
cultures
Put virus on monolayer of cells à plaques form
Multiplication of bacteriophages
A. Lytic
cycle (by lytic phages, ex: T-even bacteriophages) (Fig 13.10)
1. Attachment
Phage attaches to host cell
2.
Penetration
Phage penetrates host cell and injects its DNA
3.
Biosynthesis
Phage DNA directs synthesis of viral components by
host cell (viral enzymes, nucleic acids, Capsid proteins)
4. Maturation
Viral DNA and capsids are assembled into complete
virions (infectious particles)
5. Release
Viral enzymes (lysozymes) cause host cell lysis
New virions are released
B. Lysogenic
Cycle
Performed by temperate phages
Ex: Bacteriophage Lambda (Fig 13.12)
1. Lysogenic
Cycle
(* indicates a step different from the bacteriophage
lytic cycle)
a. Attachment
Phage attaches to host cell
b.
Penetration
Phage penetrates host cell and injects its DNA
c. Phage DNA
integration*
Phage DNA is spliced into bacterial chromosome à prophage
Most viral genes repressed à no viral activity
d. Host cell
replication*
Each new cell carries prophage
e. Induction*
(onset of productive lytic cycle)
Occurs spontaneously or as a result of injury (UV,
chemicals)
Phage DNA excised
Lytic cycle proceeds (Biosynthesis à Maturation à release)
2. Three
results of lysogeny
a. Immunity
Lysogenic cells immune to reinfection by the same
phage
b. Phage
conversion
Host cell may exhibit new properties
Ex: Corynebacterium
diphtheriae
C. diphtheriae à non pathogenic
C. diphtheriae + lysogenic phage à
diphtheria
(phage carries gene for a toxin, bacteria expresses
gene)
c.
Specialized transduction (Fig. 13.13)
Certain
bacterial genes are transferred from one bacterium to another via a phage.
During induction, some of the adjacent bacterial
genes can be excised from host chromosome and packaged in capsid.
When phage infects new cell, bacterial genes are also
transferred
Multiplication of Animal Viruses
(* indicates a step different from the bacteriophage
lytic cycle)
1. Attachment
Virus attaches to host cell
2.
Penetration*
Capsid enters by
a.
Endocytosis (Fig. 13.14)
Occurs with naked and enveloped viruses
b. Fusion
Occurs with some enveloped viruses
Viral envelope fuses with plasma membrane
DRAW
3. Uncoating*
Capsid removed by enzymes
4.
Biosynthesis
Viral nucleic acid directs synthesis of viral
components by host cell (viral enzymes, nucleic acids, Capsid proteins)
5. Maturation
Viral DNA and capsids are assembled into complete
virions
6. Release*
a.
Nonenveloped viruses à rupture of plasma membrane
b. Enveloped
viruses à budding (Fig. 13.20)
HIV (Retroviridae) multiplication (Fig. 13.19)
1. Attachment
2.
Penetration
3. Uncoating
4.
Biosynthesis
a. Reverse transcription
Viral ss-RNA
viral reverse transcriptaseà Viral
ds-DNA
b.
Integration
Viral ds-DNA integrates into host cell DNA (provirus)
c. Provirus
latent
No viral activation
Provirus replicates with cell
d. Provirus
expression
Viral components synthesized
5. Maturation
6. Release
(by budding)
Viruses and Cancer
Oncogenic viruses
Viral nucleic acid integrates into host DNA à mutation à cancer
Capable of inducing tumors
10 % of cancers are induced by viruses
Ex:
Epstein-Barr virus à
Burkitt’s lymphoma, Nasopharyngeal carcinoma
Hepatitis B virus à liver cancer
Human Papilloma virus (HPV) à cervical cancer
Latent Viral infections
Viral infection but no disease for long period, then
disease appears suddenly
Ex: Herpes
simplex virus
Inhabits host’s nerve cells, no symptoms
Stimulus activates virus à cold sore
Persistent Viral infections
Viral infection that causes viral load and disease
that develops gradually over long period
Ex: Subacute
sclerosing panencephalitis (SSPE)
Caused by measles virus
Results in mental deterioration
Prions (proteinaceous infectious particle)
Causes spongiform encephalopathy (“spongy brain”)
(Fig. 22.17)
Loss of motor control à mental deterioration à coma à death
Ex: in
animals à Scrapie, “mad cow
disease”
Ex: in humans
à Kuru,
Creutzfeldt-Jacob disease (Fig. 22. 18)
Can have very long incubation times (> 20 years)
Disease mechanism:
(Fig. 13.21)
PrPC is a normal protein in brain
PrPSc is an abnormal protein, same primary
structure but folded differently
PrPSc contacts PrPC à causes PrPC to refold into PrPSc
PrPSc taken in by endocytosis and
accumulates in lysosomes
Actual cause of cell damage unknown
Viroids
Cause plant disease (including important crops)
Short pieces of naked RNA that do not code for
protein
Host cell makes many copies of RNA à plant growth stunted
Chapter
14 Principles of Disease and
Epidemiology
Pathology
Scientific study of disease
Includes
Etiology – the cause of disease
Pathogenesis – the manner in which disease develops
Final effects on body
Infection – invasion or colonization of the body by
pathogens
Disease – Change in health, may be due to infection
Normal Flora (Fig. 14.1, 14.2)
Harmless microbes in body
Benefit host by preventing overgrowth of harmful
microorganisms
We carry 10 times the bacterial cells as we have
normal cells
Symbiosis – relationship between Microbiota and host
A.
Commensalism
One organism benefits, the other unaffected
Ex: Corynebacteria
inhabit surface of eye
B. Mutualism
Both organisms benefit
Ex: E.
coli inhabiting human intestine
E. coli
gets nutrients
Human gets Vitamin K, some Vitamin B, and proteins
that inhibit growth of pathogens
C. Parasitism
One organism benefits at the expense of another
Ex:
disease-causing bacteria
Opportunistic Microorganisms
Cause no disease in normal person but might in
different environment
Ex: Gain
access through broken skin à cause disease
Host is immune suppressed à cause disease
The Etiology of Infectious Diseases
A. Koch’s
postulates (Fig. 14.3)
Used to determine the etiology of an infectious
disease
1. The same
pathogen must be present in every case of the disease.
2. The
pathogen must be isolated from the diseased host and grown in pure culture.
3. The
pathogen from the pure culture must cause the diseases when it its inoculated
into a healthy, susceptible lab animal.
4. The
pathogen must be isolated from the inoculated animal and must be shown to be
the original organism.
B. Exceptions
to Koch’s postulates
1. Some
microorganisms will not grow on artificial media
Ex: Treponema
pallidum cannot be grown on artificial media
2. The same
disease may be caused by several different pathogens
Ex: pneumonia
à caused by many
pathogens
3. The same
pathogen may cause several different diseases
Ex: Steptococcus
pyogenes causes sore throat, scarlet fever, skin infections, ….
4. May not be
ethical to test
Ex: HIV only
grows in humans, not ethical to test HIV on humans
Classifying Infectious Diseases
A.
Definitions
Symptoms
Subjective changes in body function
Not apparent to observer
Ex: pain,
malaise
Signs
Objective changes in body function
Can observe and measure
Ex: lesions,
swelling, fever, paralysis
Syndrome
Group of symptoms or signs that accompany a
particular disease
Ex: Acquired
Immunodeficiency Syndrome (AIDS)
B.
Classification based on how disease is transmitted
1.
Communicable disease
Spreads from one host to another
Ex:
chickenpox, measles, tuberculosis, typhoid
Contagious diseases
Easily
spreads from one host to another
Ex:
chickenpox, measles
2.
Noncommunicable disease
Not spread from one host to another
Caused by
a. Normal
Flora
b. Microbes
that normally are outside body but are introduced into the body
Ex: Clostridium
tetani in soil
àIntroduced through wound or abrasion
à Tetanus
C.
Classification based on the frequency of occurrence of disease
1. Sporadic
disease
Occurs only occasionally
Ex: typhoid
fever
2. Endemic
Disease
Constantly present in the population
Ex: common
cold
3. Epidemic
disease
Many people acquire in a short amount of time
Ex: gonorrhea
4. Pandemic
disease
Epidemic disease that occurs worldwide
Ex:
influenza, AIDS
D.
Classification based on severity or duration of a disease
1. Acute
disease
Develops rapidly, last a short time
Ex: influenza
2. Chronic
Disease
Develops more slowly
Likely to be continual or recurrent for long periods
Ex:
infectious mononucleosis, TB
3. Latent
disease (sequelae)
After initial infection, causative agent remains
inactive, then becomes an active disease, producing symptoms
Ex: Shingles
E.
Classifications based on the extent of host involvement
1. Local
infection
Microorganisms limited to small area of the body
Ex: boil or
abscesses
2. Systemic
infection
Microorganisms or products spread throughout body by
blood or lymph
Ex: measles
3. Blood
infections
a. Bacteremia
– bacteria in blood
b. Septicemia
– bacteria multiplying in blood
c. Toxemia –
toxins in blood (Ex: tetanus)
d. Viremia –
viruses in blood
4. Primary
infection
Acute infection that causes initial illness
5. Secondary
infection
Caused by opportunistic pathogen after primary
infection weakened body’s defenses
Ex:
streptococcal pneumonia (secondary)
following influenza (primary)
6.
Subclinical infection
No apparent disease
Ex:
poliovirus, hepatitis A carriers
The stages of a disease (Fig. 14.5)
A. Incubation
period
Time between initial infection and first appearance
of signs or symptoms
B. Prodromal
Period
Early mild symptoms, such as general aches and
malaise
C. Period of
Illness
Overt signs and symptoms of disease
Immune response overcomes pathogens or death occurs
D. Period of
Decline
Signs and symptoms subside
Patient vulnerable to secondary infections
E. Period of
Convalescence
Person regains strength
Body returns to prediseased state
The Spread of Infections
A. Reservoirs
of Infection – continual source of disease organisms
1. Human
Reservoirs
People that harbor pathogens, often asymptomatic
carriers
Is principle living reservoir of human disease
Ex: people
infected with measles
2. Animal
Reservoirs
Wild or domestic animals that harbor pathogens
Zoonoses – diseases that can be transmitted from
animals to humans
Ex: rabies,
Lyme disease
3. Nonliving
Reservoirs
a. Soil
Ex: Soil that
contains fungi, C. botulinum, C. tetani
b. Water
Ex: fecally
contaminated water
Vibrio cholerae à cholera
c. Food
Ex: Food that
contains Salmonella typhimurium
B. The
Transmission of Disease (Fig. 14.6)
1. Contact
Transmission (Fig. 14.7)
a. Direct
Contact Transmission
Person to person transmission
Ex: touching,
kissing, sexual intercourse
Ex of diseases transmitted: common cold, influenza, STD’s
b. Indirect
contact transmission
Transmitted from reservoir to host by means of a nonliving
object (Fomite)
Ex. Of Fomite:
tissues, towels, toys, syringes
Ex. of diseases transmitted: Hepatitis B
c. Droplet
Transmission (Fig. 14.8)
Spread in mucus droplets that travel short distances
Ex: Produced
in sneezing, coughing
Ex. of diseases transmitted: influenza, pneumonia
2. Vehicle
Transmission
Transmission of disease agent by medium
Ex: by water,
food, air, blood, body fluids
Types of vehicles
a. Waterborne
transmission
Contaminated water
Ex. of diseases transmitted: Cholera
b. Foodborne
transmission
Unsanitary or incompletely cooked food
Diseases trans:
Food poisoning, tapeworm infestation
c. Airborne
transmission
Droplet nuclei in dust that travel more than 1 m
Ex. of disease trans.: measles, TB
Also fungal spores
Ex. of diseases trans.: histoplasmosis
3. Vector
Transmission
Transmission via animals, esp. arthropods
Types of vector transmission
a. Mechanical
Transmission
Passive transport of pathogen on feet or body parts
Ex:
houseflies transfer infected feces to food
Diseases trans:
typhoid fever, shigellosis
b. Biological
Transmission
Active process
Arthropod bites infected host à arthropod infected
Infected arthropod bites uninfected host à host infected
Ex: Anopheles
mosquito is vector for malaria
Nosocomial Infections
Infections acquired as a result of a hospital stay
5-15% of hospital patients acquire
A. Examples
of nosocomial infections
Staphylococcus aureus and E. coli à surgical would infections, pneumonia
Clostridium difficile à diarrhea
B. Causes of
Nosocomial infections (Fig. 14.9)
1.
Microorganisms in the hospital
Hospitals are a major reservoir for pathogens
2. The
compromised host
Resistance to infection impaired by …
Broken skin
Suppressed immune system
3. The chain
of transmission
Patient to patient
Staff to patient
C. The
control of nosocomial Infections
Reduce number of pathogens
à aseptic techniques, hand washing, etc,
àkeep areas clean and disinfected
Emerging infectious diseases
Diseases that are new, or changing, or showing an
increase in incidence
(Table 14.6)
Epidemiology
Study of when and where diseases occur and how
transmitted
Central source of epidemiological info. in U.S.
à The Centers for Disease Control and Prevention (CDC)
Chapter
15 Microbial Mechanisms of
Pathogenicity
How Microorganisms enter a host à portals of entry
A. Portals of
entry
1. Mucous
Membranes
a.
Respiratory tract
Diseases:
cold, pneumonia
b.
Gastrointestinal tract
Diseases:
poliomyelitis, Hepatitis A
c. Urogenital
tract
Diseases:
AIDS, syphilis
d.
Conjunctiva
Diseases:
conjunctivitis
2. Skin
Impenetrable by most microorganisms
Diseases:
Hookworms, fungi
3. The
Parenteral Route
Deposit microorganisms directly into tissues
Punctures, injections, bites, wounds
B. Factors
that determine if infection causes disease
(microorganisms in body à may or may not cause disease)
1. The
preferred portal of entry
Many pathogens only cause disease through one portal
of entry
Ex: Streptococci, inhaled à pneumonia
swallowed à no disease
2. Numbers of
invading microbes
As number of microorganisms increase, probability of
disease increases
3. Adherence
For most pathogens…. adhere to host cells à disease
no adherence à no disease
Adhesins on microorganism bind to receptors on host
cells (Fig. 15.1)
How Bacterial Pathogens penetrate host defenses
A. Capsules
Outer gelatinous cover on some bacteria
Can resist host defenses …
à Phagocytic immune cells can’t adhere
à can’t phagocytize
Ex: Steptococcus
pneumoniae
with capsule à Pneumococcal pneumonia
w/o capsule à harmless
B. Components
of the cell wall
May contain chemicals that enhance virulence
Ex: S.
pyogenes produces M-protein
à helps attachment to host cells
à resists phagocytosis by white blood cells
Ex: Mycobacterium
tuberculosis has waxes (Mycolic acid) in the cell wall
à resists digestion by phagocytes
C. Enzymes
Some bacteria produce harmful extracellular enzymes
Ex: Clostridium
perfringens
à produces collagenase