Download ENVR 112 Microbial Agents of Infectious Diseases

Microbial Agents of
Infectious Diseases
ENVR 412 – Ecological Microbiology
04/09/09
Human-Microbial
Interactions

Microbes colonize and inhabit humans and
other living things.

So-called “Normal Flora” colonize the skin,
the oral cavity, the gastrointestinal tract,
the upper respiratory tract (e.g.
nasopharynx), and parts of genitourinary
tract (urethra and vagina).

Colonization by normal flora is either
neutral or beneficial.

“Pathogens” are microorganisms that
colonize, invade and damage the body.
Pathogens cause disease.
Leading causes of death in the US:
1900 and 2008
Deaths due to infectious disease
in the United States
(1918 flu pandemic)
Africa
• 10.7 million deaths
• 6.7 million (63%) due
to infectious disease
Americas
• 6.0 million deaths
• 623,000 (10%) due to
infectious disease
Sanitation
Global Water Supply and Sanitation
Assessment (2000)








2.4 billion people have inadequate sanitation
1.1 billion people have inadequate or unsafe water
4 billion cases of diarrhea every year
2.2 million deaths from diarrheal disease every year
Most illness and death in children <5 years old
Less services in rural than in urban areas
Urban settlement/slums remain a problem
In the developing world wastewater treatment is rare
Infectious disease risks from water, poor
sanitation and hygiene, food and air are still
with us the developed and developing world
Outline
 Short
history of public health microbiology
 Types
of pathogens
 Infection
dynamics
 Host-pathogen
 Routes
interactions
of transmission
 Indicators
of water quality
Early History of Public Health

The first recording of laws concerning public health
appears in the Book of Leviticus in The Bible,

the Hebrew people were told to practice personal
hygiene by washing and keeping clean.

they were instructed to bury their waste materials
away from their camp sites, to isolate those who were
sick and to burn soiled dressings.

they were prohibited from eating animals that had
died of natural causes.

the procedure for killing animals was described and
the edible parts were designated.
Early History of Public Health

During Middle Ages, most of the knowledge about
sanitation and public health was lost and there was a
general stagnation of culture for almost 1,000 years.

However, during Renaissance, wide spread epidemics
of small pox, syphilis, rabies and other diseases
prompted people to search for explanations as to how
diseases were contracted and transmitted.

Most of the people believed that diseases were caused
by the curse of Gods and as a result many bizarre
treatments were used to drive away the devils or evil
spirits and relieve symptoms.
Detail from Peter S. Bregel
painting shows rich man standing
near an outhouse which empties
directly into the river.
In Middle Ages, people used to
throw excreta from their houses
on the roads below.
Early History of Public Health

Outbreaks commonly decimated the
populations of cities and countrysides

Plague

Syphilis

Cholera

Smallpox

By the middle of the 1800s, the new industrial cities were
not only jam-packed, they were filthy, and full of garbage,
dead animals, and sick people
John Snow (1854)

Proved cholera is
waterborne

Fathered modern
epidemiology

Provided basis of
public sanitation
movement
1840s
 Public
health began to slowly be
redefined from a moral problem to an
engineering and social reform problem


Chadwick Report (1842) – “The Sanitary
Conditions of the Labouring Population”
John H. Griscom (1845) – “The Sanitary
conditions of the Laboring Population of
New York”
Sanitation
Movement
(1850s – 1900)
“Our safety depends
upon official vigilance”
The Angel of
Cleanliness bars the
gate to cholera, yellow
fever and smallpox.
Harpers Weekly 1885, National Library
of Medicine
Institutionalization of solid waste
management in the U.S.
late 1700s
Urban populations
explode due to
immigration and
industrialization
early 1800s
Water piped in (fire
protection) then
contaminated water
sent to streets, rivers
1890s
1920s
treatment of public
(drinking) water
supplies
cities take responsibility
for waste management,
water supplies, etc.
1980s
Federal support
switches from
subsidies to loans
Poor sanitation leads to
disease outbreaks, high
rates of mortality and
morbidity
1950s
1980s
Federal government
begins requiring water
treatment
Federal subsidies for
wastewater treatment
Antony Van Leeuwenhoek (1685)
 First
to observe live microorganisms,
using a simple microscope
Louis Pasteur
Father of Microbiology

Disproved the theory of spontaneous generation (1861).
Louis Pasteur
The Germ Theory of
Disease

Contributed to the understanding of fermentation
(1858).

Developed techniques for selective destruction of
microorganisms (pasteurization) (1866).

Attenuated vaccines for chicken cholera (1881).

Immunization against rabies (1885).
Ignaz Swmmelweis (1850)

Advocated hand
washing to stop the
spread of “child bed
fever”

Fired twice and generally
ridiculed for his ideas,
despite drop in mortality
rates
Robert Koch
The Germ Theory
of Disease

discovered anthrax bacilli (1876).

developed solid culture media (1882).

discovered organisms causing tuberculosis
(1882).
Koch’s Postulates (1884)
The Germ Theory of Disease
1.
Isolate the suspected agent from a disease
victim.
2.
Grow the agent in pure culture.
3.
Infect a healthy host and show that the
organism produces the classical clinical
disease.
4.
Isolate the "same" organism from the new
victim.
Golden Age of Microbiology
1850-1920

Advances led to the development of new
branches of microbiology such as immunology
and virology.

1915 M. H. McCrady establishes a quantitative
approach for analyzing water samples using the
most probable number, multiple-tube
fermentation test.
What causes infectious diseases?
Pathogens: disease causing organisms

Human pathogens include:
1.
2.
3.
4.

bacteria
viruses
fungi
protozoa
E. coli
Hepatitis A Virus
Disease requires:
1.
2.
3.
an infectious pathogen
a susceptible host
a suitable environment
Giardia lamblia
Classes or Categories of
Pathogenic Microorganisms
Viruses: smallest (0.02-0.3 µm diameter); simplest:
nucleic acid + protein coat (+ lipoprotein envelope)
Bacteria: 0.5-2.0 µm diameter; prokaryotes; cellular; simple internal
organization; binary fission
Protozoa: most >2 µm- 2 mm; eukaryotic; uni-cellular; nonphotosynthetic; flexible cell membrane; no cell wall; wide range of
sizes and shapes; hardy cysts
Groups: flagellates, amoebae, ciliates, sporozoans (complex life
cycle) and microsporidia.
Helminths (Worms): multicellular animals; some are parasites; eggs
are small enough (25-150 µm) to pose health risks from human
and animal wastes in water.
The Microbial World:
Sizes of Microbes
Types of Pathogens:
Pathogen: a microorganism capable of
causing disease.
True pathogen (frank pathogen):
Presence is always considered
abnormal (e.g. Mycobacterium
tuberculosis, Yersinia pestis). Causes
disease in hosts with normal immune
defenses
Opportunistic pathogen: causes
infection/disease in immunocompromised hosts. May be
member of host’s normal flora.
Yersinia pestis
Pseudomonas aeruginosa
Seven Capabilities of a Pathogen:
1. Maintain a reservoir
2. Be transmitted to and enter a host
3. Adhere to a body surface
4. Invade the body
5. Evade the Host’s Defenses
6. Multiply in the Host
7. Leave the Body
The Process of Infection

The presence of microbes in normally sterile
sites is indicative of an infection




May involve normal flora or pathogens
Generally involves growth and multiplication of
the microbe in a host
Infection does not always result in injury of the
host (disease)
Two main classes of infection:
1.
localized
2.
generalized (systemic)
Localized Infections
1.
Organism enters the body and reaches target site of infection
2.
Organism adheres to or enters host cells and multiplies at site of
infection
3.
Infection spreads within the site (e.g., respiratory tract; intestines)
4.
Symptoms of illness appear
5.
Organism does not spread through the lymphatic system or reach
the bloodstream
6.
Infection subsides due to host defenses (e.g., immunity)
7.
Agent eliminated from the body; infected cells replaced; "cure"
Generalized Infections
Organism enters the body and reaches target site of initial infection
2. Organism adheres to or enters host cells and multiplies at initial site
of infection
3. Infection spreads within site and to other sites via tissues, lymphatic
system, bloodstream (bacteremia, viremia, etc.) and possibly other
routes
4. Symptoms of illness may appear
5. Organisms infect other organs, tissues and cells; more spread via
bloodstream
6. Symptoms of illness become severe
7. Host defenses eliminate organisms leading to cure or disease
continues, possibly leading to irreversible damage or death
1.
Microbial Multiplication and Clinical
Manifestation of Disease
Infection: Host-Pathogen Interaction
Variables of the Host
 Age
 General health
 Pregnancy
 Medications--OTC or
prescription
 Metabolic disorders
 Alcoholism, cirrhosis,
hemochromatosis
 Malignancy
 Amount of food consumed
 Gastric acidity variation:
antacids, natural variation,
achlorhydria
 Genetic disturbances
 Nutritional status
 Immune competence
 Surgical history
 Occupation
Variables of the Pathogen
 Variability of gene expression
of multiple pathogenic
mechanism(s)

Potential for damage or stress
of the microorganisms

Interaction of organism with
food menstruum and
environment

pH susceptibility of organism

Immunologic "uniqueness" of
the organism

Interactions with other
organisms
Factors Influencing Exposure and
Infection: Agent Factors

Sources, Reservoirs, Transport and Persistence
(in the Environment)

Ability to Enter a Portal in the Human or Other
Host

Ability to Reach and Proliferate at Site(s) of
Infection in the Host

Excretion of the Agent from the Host

Quantity and "Quality" (including virulence) of the
Infectious particles
Individuals at Increased Risk
For Infections And Illness
• Young
-immune systems not
fully developed
• Old
-immunosenescence
(aging immunity)
• Pregnant
• Immunocompromised
-chemotherapy
-existing infection or
chronic medical
disorder (e.g., liver
disease)
• Proper diet especially
important for these
individuals
• Some of these individuals
represent rapidly growing
segments of the
population
Virulence
Virulence: A quantitative measure of the
ability of a microorganism to cause
disease (pathogenicity).
LD50 (lethal dose 50%): The number of
organisms needed to cause death in
50% of the infected hosts.
ID50 (infectious dose 50): The number of
organisms needed to cause an infection
in 50% of the hosts.
Infectious Dose
Microbes differ in their infectious dose:

Enteric and respiratory viruses are infectious at very
low doses




as little as one cell culture infectious dose has a high
probability of infecting an exposed human.
But, this may still require exposure to many virus particles.
Most enteric bacteria are infectious at moderate (10s100s of cells) to high (1,000 cells) doses.
Protozoan cysts may be infectious at low doses

A few as 1-10 cysts of Giardia lamblia) or oocysts of
Cryptosporidium parvum
Factors Influencing Exposure and
Infection: Environmental Factors

Reservoirs: where organisms can live,
accumulate or persist outside of the host
of interest; could be another organism or
the inanimate environment.

Vehicles: inanimate objects/materials by
which organisms get from one host to
another; includes water, food, objects
(called fomites) and biological products
(e.g., blood).
Factors Influencing Exposure and
Infection: Environmental Factors

Amplifiers: Types of reservoirs
where organisms proliferate; A
host in which infectious agents
multiply rapidly

Vectors: Living organisms bringing
infectious organisms to a host.

Mechanical vectors: Microbes do
not multiply in the vector
• ex: biting insects infected with the
infectious organism

Biological vectors: Microbes must
propagate in the vector before
they can be transmitted to a host.
Environmental Factors Influencing Survival
or Proliferation of Infectious Agents

Physical: temperature, relative humidity,
sunlight, moisture content or water activity,
climate and weather, etc.

Chemical and Nutritional: Antimicrobial
chemicals, nutrients for microbial
proliferation.

Biological: Antagonistic activity by other
organisms: antimicrobial agents, parasitism,
etc.; presence and state of a vector
Penicillin
Disease Frequency
1.
Sporadic

occurs occasionally in a population (tetanus)
2.
Endemic

always present in a population at about the same
level (gonorrhea)
3.
Epidemic

outbreak that affects many members of a
population in a short time (cholera, influenza)
4.
Pandemic

An epidemic affecting a large geographical area;
often on a global scale (HIV, influenza)
Transmission

Person-to-person microbial diseases

Airborne, direct contact & sexually
transmitted diseases

Animal-transmitted diseases

Arthropod-transmitted diseases

Soilborne diseases

Waterborne diseases
Airborne Transmission
•
Bacterial and viral respiratory
diseases are transmitted in air
•
Most respiratory pathogens are
transferred from person to person
via respiratory aerosols
generated by coughing, sneezing,
talking or breathing
•
A few respiratory pathogens can
be transmitted by water or soil
and do not require person-toperson propagation (e.g.
Legionella pneumophila)
Airborne Transmission
The upper and lower respiratory tracts offer different environments,
favoring different microorganisms
Direct Contact Transmission
•
Diseases spread by direct contact with an
infected person or by contact with blood or
excreta from an infected person
•
Staphylococcal infections – acne, boils, pimples,
impetigo, pneumonia, osteomyelitis, carditis,
meningitis, and arthritis
•
Helicobacter pylori – gastric ulcers
•
Hepatitis viruses – hepatitis, cirrhosis
Sexually Transmitted Diseases

Caused by a wide variety of bacteria, viruses,
protists and even fungi

WHO estimates 1 million new cases daily

About 60% of these infections occur in young
people <25 years of age

Chlamydia is the most common bacterial STI in
humans

HIV infection in humans is now pandemic, and
has killed more than 25 million people since it
was first recognized in 1981
Animal-Transmitted Pathogens
• Zoonosis – animal disease transmissible to
humans
• Rabies – reservoirs in wild animals, primarily
raccoons, skunks, coyotes, foxes and bats; a
viral neuroinvasive disease that causes acute
encephalitis (inflammation of the brain)
• Hantavirus – reservoirs in rodents, primarily
mice and rats; can cause several severe
diseases including viral hemorrhagic fever
Vectorborne Pathogens
• Pathogens spread from
the bite of a pathogeninfected arthropod
vector, such as ticks,
mosquitoes and fleas
• Humans can either be accidental hosts (e.g.
rickettsias) or they can be required hosts in
the life cycle of the pathogen (e.g. malaria)
Soilborne Pathogens
• Accidential agents of infection, with no life cycle
dependency on the accidental host
• Soil is an unlimited reservoir of these pathogens,
and so they cannot be eliminated
• Includes fungi (e.g. ringworm) and bacteria
(tetnus)
• Overall incidence of serious infections from the
soil is low, although certain superficial fungal
infections are common
Waterborne Pathogens
Bacteria
1Escherichia
coli
Protozoa
1Shigella
rod ~ 1.5 - 2 mm
dysenteriae
Giardia lamblia
rod ~ 1.5 - 2 mm
Viruses
Caliciviruses
virion ~ 35 nm
1Copyright
Hepatitis A
virion ~ 27 nm
trophozoite ~ 15 mm
Dennis Kunkel Microscopy, Inc.; http://www.denniskunkel.com
Virulence properties of
Enteropathogenic E. coli
• Enteroaggregative strains (EAEC) – adhere to enterocytes,
causing cell death
• Enterotoxigenic strains (ETEC) – enterotoxin secretions in small
intestine, causing fluid loss and diarrhea; heat stable (ST) and
heat labile (LT) toxins; stimulate guanylate or adenylate cyclase
activity with fluid and electrolyte loss
• Enterohemorrhagic strains (EHEC) – produce shigella-like toxins
(large intestine), causing effacement of microvilli and cell death;
watery and later bloody stools
• Invasive strains (EIEC) – invasion and ability to grow in intestinal
epithelium (large intestine), killing the cells
Microbial Indicators of
Fecal Contamination
Traditional approach to protect/assess the "sanitary" quality
of water (or food) with respect to fecal contamination.
Quantify bacteria commonly present in
warm blooded animals:
1. high numbers
2. easy to measure
3. surrogates for pathogens,
bacterial pathogens.
intestines of
especially
Criteria for an Ideal Indicator of Fecal
Contamination
1. Applicable to all types of water (and other relevant samples).
2. Present in feces, sewage and fecally contaminated samples when
pathogens are present; numbers correlate with amount of fecal
contamination; outnumber pathogens.
3. No "aftergrowth" or "regrowth" in the environment.
4. Survive/persist greater than or equivalent to pathogens.
5. Easily detected and quantified by simple lab tests in a short time.
6. Constant characteristics.
7. Harmless to humans and other animals.
8. Numbers in water (food, etc..) are associated with risks of enteric illness in
consumers (dose-response relationship).
Current and Candid Bacterial Indicators of
Fecal Contamination

Total coliforms: standards for drinking, bathing and shellfish
harvesting waters; not feces-specific (environmental sources).

Fecal ("thermotolerant") coliforms: ditto for total coliforms.

E. coli: the "fecal" coliform; but may occur naturally in tropics.

Fecal streptococci: another group of enteric, fecally excreted
bacteria; not feces-specific (environmental sources).

Enterococci: Streptococcus faecalis and S. faecium; a sub-set of the
fecal streptococci considered more feces-specific; EPA guide-line
for bathing water quality used as standards in some states.

Clostridium perfringens: anaerobe; feces-specific?; very (too?)
resistant spores; candidate indicator for protozoan cysts.
Relationships among Total and
Fecal Coliforms and E. coli
Total Coliforms
Fecal Coliforms
Escherichia coli
• All total and fecal coliforms and E.
coli possess -galactosidase; they
can hydrolyze and ferment lactose
• E. coli also possesses glucuronidase and hydrolyzes
glucuronide substrates
Standard Methods for
Fecal Indicator Bacteria
1.
Multiple Fermentation Tube
2.
Membrane Filtration
3.
Biochemical Assay
Escherichia coli
rod ~ 1.5 - 2 mm
Multiple Fermentation
Tube Method
Calculates the Most Probable
Number (MPN) of bacteria
MPN Index using 5 tubes per dilution (10mL, 1.0mL, 0.1mL)
95% Confidence Limits
Combination
of Positives
MPN Index/
100mL
Lower
Upper
0-0-0
<2
-
-
2-0-0
4
1.0
17
3-2-0
14
6.0
35
4-2-0
22
9.0
56
56
Membrane Filter Method
Membrane filtration apparatus
Fecal coliform colonies detected
by membrane filtration
Colilert™ Method
Colilert™ Method
Total Coliforms Present
E. coli Present
Detection of Microbes from Water/Shellfish
• Traditional methods for detecting
microbes of public health concern are
inadequate
– Assays are slow
– Bacterial indicators do not reliably
model all microbial threats
– Assays do not provide information
about sources of contamination
Fecal coliform
colonies detected
by membrane
filtration
Alternative Approaches
•
Bacteriophages – e.g. coliphages - indicators of
enteric viruses
•
Source-specific markers – “microbial source
tracking”
•
Direct pathogen detection – enteric viruses,
virulence genes of bacteria
•
Rapid Methods – polymerase chain reaction
(PCR)
Study Points:
•
Types of pathogens
•
Routes of transmission
•
Characteristics of and relationships
between fecal indicator bacteria
•
Problems and alternatives associated with
traditional fecal indicator bacteria