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Microbiology
Chapter 18
Copyright © 2005 by Elsevier Inc. All rights reserved.
Introduction
Microbiology is the study of microorganisms.
Micro means microscopically small, and
bio means living organisms.
The dental assistant needs a foundation
in microbiology in order to understand the
nature of pathogens (disease-producing
microorganisms) and how to prevent the
transmission of disease in the dental office.
The two major oral diseases, dental caries (decay)
and periodontitis are bacterial infections.
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Pioneers in Microbiology
• Aristotle (384–322BC) believed that life arose from muck,
decaying food, warm rain, or even dirty shirts.
• Antony van Leeuwenhoek (1632–1723) used a primitive
microscope to observe stagnant water,
hay infusions, and scrapings from the teeth.
• John Tyndall (1820–1893) discovered that some bacteria
existed in two forms: a heat-stable form
and a heat-sensitive form.
• Joseph Lister (1827–1912) was the first to recognize the
role of airborne microorganisms in postsurgical
infections.
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Pioneers in Microbiology-cont’d
• Robert Koch (1843–1910) developed a two-part dish for
growing bacteria and a technique for isolating pure
colonies of bacteria.
• Julius Petri (1852–1921): Petri plates, dishes used to
isolate bacterial colonies, were named after him.
• Louis Pasteur (1822–1895), the Father of Microbiology,
developed the process of pasteurization and discovered
the first vaccine for rabies.
• The Pasteur Institute was built in France by the French
government in 1888 to honor Louis Pasteur.
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Fig. 18-1 Colonies of bacteria are growing in the culture medium in this Petri dish
(From Samaranayake LP: Essential microbiology in dentistry, ed 2, New York, 2002, Churchill Livingstone.)
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Fig. 18-2 Discovery of rabies vaccine by Louis Pasteur, 1885
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Fig. 18-3 Louis Pasteur being honored at the Sorbonne
(Courtesy National Library of Medicine.)
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Major Groups of Microorganisms
• Bacteria
• Algae
• Protozoa
• Fungi
• Viruses
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Bacteria
• A large group of one-celled microorganisms that vary
in shape, size, and arrangement.
• Usually, pathogenic bacteria grow best at 98.6˚F
(37˚C) in a moist, dark environment.
• The skin, respiratory tract, and gastrointestinal tract
are inhabited by a great variety of harmless bacteria
called normal flora.
• An infection occurs when bacteria occurring
naturally in one part of the body invade another part
of the body and become harmful.
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Shapes of Bacteria
• Spherical (cocci)
– The cocci reproduce by dividing into two.
– The cocci that form chains as they divide
are called streptococci.
– The cocci that form irregular groups or
clusters are called staphylococci.
• Rod-shaped (bacilli)
• Spiral (spirilla)
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Fig. 18-4 The three basic shapes of bacteria
(From Stepp CA, Woods M: Laboratory procedures for medical office personnel, Philadelphia, 1998, Saunders.)
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Fig. 18-5 Colonies of streptococci growing on the
agar medium are diagnostic for strep throat
(From Stepp CA, Woods M: Laboratory procedures for medical office personnel, Philadelphia, 1998, Saunders.)
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Fig. 18-6 Golden-yellow colonies of Staphylococcus bacteria
(From Samaranayake LP: Essential microbiology in dentistry, ed 2, New York, 2002, Churchill Livingstone.)
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Gram-Positive and Gram-Negative Bacteria
• Hans Christian Gram (1853-1938) developed a four-step
staining process for separating bacteria into
two groups.
• Gram staining requires the sequential use of a crystal violet
dye, iodine solution, alcohol solution, and a safranin dye.
• The bacteria that are stained by the dye are classified as
gram-positive. (They appear dark purple under the
microscope.)
• The bacteria that are not consistently stained are classified
as gram-variable (Mycobacterium tuberculosis).
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Fig. 18-7 Gram-positive stain
(From De la Maza LM, Pezzlo MT, Baron EJ: Color atlas of microbiology, St. Louis, 1997, Mosby.)
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Fig. 18-8 Gram-negative stain
(From De la Maza LM, Pezzlo MT, Baron EJ: Color atlas of microbiology, St. Louis, 1997, Mosby.)
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Bacteria’s Need for Oxygen
• Aerobes are a variety of bacteria that require
oxygen to grow.
• Anaerobes are bacteria that grow in the
absence of oxygen and are destroyed by
oxygen.
• Facultative anaerobes are organisms that can
grow in either the presence or the absence of
oxygen.
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Capsules
• Some types of bacteria form a capsule that acts as
a protective layer covering the cell wall
(Streptococcus mutans).
• This type of bacteria are generally virulent
(capable of causing serious disease).
• The capsule increases their ability to resist the
defense mechanisms of the body.
• The capsule may also prevent antibiotic agents
from having an effect on the bacteria.
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Spores
• Some bacteria change into a highly resistant form
called spores. The disease tetanus is caused by a
spore-forming bacillus.
• Bacteria remain alive in the spore form but
are inactive.
• Spores represent the most resistant form of
life known.
• They can survive extremes of heat and dryness
and even the presence of disinfectants and
radiation.
• Harmless spores are used to test the effectiveness
of the techniques for sterilizing dental instruments.
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Rickettsias
• The Rickettsias are short, nonmovable rods that
normally live in the intestinal tract of insects such
as lice, fleas, ticks, and mosquitoes.
• They are very small and require host cells in order
to reproduce.
• The diseases caused by rickettsia are typhus and
Rocky Mountain spotted fever.
• These diseases are transmitted to humans via the
bite of an infected insect.
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Chlamydias
• The Chlamydias are the smallest of all bacteria.
• They are important because of their role in sexually
transmitted diseases.
• Chlamydias are also responsible for the human
disease known as trachoma, a leading cause of
blindness in the world.
• They also cause disease in some birds and
mammals.
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Algae
• Algae range from the microscopic single cell
organism to the larger multiple cell organisms
such as seaweed and kelp.
• All algae contain chlorophyll, as well as pigments
that cause them to appear yellow-green, brown,
or red.
• Algae are found in abundance in both freshwater
and marine habitats.
• Most algae do not produce human disease.
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Protozoa
• Protozoa consist of a large group of single cell
organisms.
• Some protozoa can remain viable as cysts for long
periods of time outside their hosts.
• The majority of protozoa do not cause disease, but
some live in hosts and do cause damage.
• A small number of protozoa are responsible for
intestinal infections of humans; others invade the
blood, lungs, liver, or brain.
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Fungi
• Fungi are plants, such as mushrooms, yeasts, and
molds, that lack chlorophyll.
• Candida is a common yeast found in the oral cavity
of about half of the population. It also is found in the
gastrointestinal tract, female genital tract, and
sometimes the skin.
• Cross infection may occur from mother to baby,
and among infant siblings.
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Oral Candidiasis
• Caused by the yeast Candida albicans.
• All forms of candidiasis are considered to be
opportunistic infections, and are found in patients
that are very young, very old, and very ill.
• Oral candidiasis is characterized by white
membranes on the surface of the oral mucosa,
tongue, and elsewhere in the oral cavity.
• The lesions can look like thin cottage cheese and
can be wiped off to reveal a raw, red, and sometimes
bleeding base.
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Fig. 18-9 A, Multiple white plaques of pseudomembranous
candidiasis (thrush) in an HIV-infected individual
(From Regezi JA, Sciubba JJ, Pogrel MA: Atlas of oral and maxillofacial pathology, St. Louis, 2000, Saunders.)
A
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Fig. 18-9 B, Candidiasis-associated denture stomatitis
showing the edentulous maxillary arch.
(From Regezi JA, Sciubba JJ, Pogrel MA: Atlas of oral and maxillofacial pathology, St. Louis, 2000, Saunders.)
B
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Prions
• Small proteinaceous infectious particles.
• Composed entirely of proteins that lack nucleic acids
(DNA or RNA).
• Until the discovery of prions, it was believed that any
agent capable of transmitting disease had to be made
up of genetic material composed of nucleic acids.
• Prions convert normal protein molecules into
dangerous ones simply by causing the normal ones
to change their shape.
• Prions are a new and separate class, unlike bacteria,
fungi, viruses, and all other known pathogens.
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Prion Diseases
• Prions are responsible for “mad cow” disease.
• Prions are linked to human diseases such as CreutzfeldtJacob disease (rare form of dementia) and possibly
Alzheimer’s disease.
• Prion-caused diseases have been found in humans
and animals.
• Prions are highly resistant to heat, chemical agents,
and irradiation.
• There is no treatment or vaccine against prion diseases,
and the only preventive measure is not eating suspect
food.
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Viruses
• Viruses are much smaller than bacteria. Despite their
tiny size, many viruses cause fatal diseases.
• New and increasingly destructive viruses are being
discovered and have caused the creation of a special area
within microbiology called virology.
• Viruses can live and multiply only inside an appropriate
host cell. The host cells may be human, animal, plant, or
bacteria.
• A virus invades a host cell, replicates (produces copies of
itself), and then destroys the host cell so the viruses are
released into the body.
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Virus Specificity
• Viruses can have specificity (preference) for
particular cell types in which to replicate.
• For example, HIV infects cells known as CD4+ cells,
whereas the hepatitis virus infects only liver cells.
• Some other viruses are able to cause disease in
more than one organ.
• Unfortunately, some viruses can cross the placenta
and infect the fetus.
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Virus Latency
• Some viruses establish a latent (dormant) state in
host cells.
• A latent virus can be reactivated in the future and
produce more infective viral particles followed by
signs and symptoms of the disease.
• Stress, another viral infection, and exposure to
ultraviolet light can reactivate the virus.
• HIV has a latency of months to years.
• Hepatitis C is known to have a latency period of
15 to 25 years.
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Treating Viral Diseases
• Viruses cause many clinically significant diseases
in humans.
• General antibiotics are ineffective in preventing or
curtailing viral infections, and even the few drugs
that are effective against some specific viruses
have limitations.
• Viruses are also capable of mutation.
• It is very difficult to develop vaccines against
viruses because of the ability of viruses to change
their genetic code.
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Transmission of Viral Diseases
• Viral diseases are transmitted by direct contact,
insects, blood transfusions, contamination of food
or water, and inhalation of droplets expelled by
coughing or sneezing.
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Viruses in the Environment
• Viruses can be destroyed easily in the external
environment.
• Chemicals such as chlorine (bleach), iodine, phenol,
and formaldehyde easily and effectively destroy
viruses on surfaces and objects.
• These agents, however, are too toxic to be used
internally.
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Viral Diseases
• There are at least five types of viral hepatitis,
each of which is caused by a different virus:
– Hepatitis A virus (HAV)
– Hepatitis B virus (HBV)
– Hepatitis C virus (HCV)
– Hepatitis D virus (HDV)
– Hepatitis E virus (HEV)
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Hepatitis A-(HAV)
• Hepatitis A (HAV) can affect anyone.
• It is spread from person to person by putting something in the
mouth that has been contaminated with the stool of a person
with hepatitis A. This type of transmission is called “fecal-oral.”
• Good personal hygiene and proper sanitation can also help
prevent hepatitis A.
• Always wash your hands after changing a diaper or using the
bathroom.
• Hepatitis A is the least serious from of viral hepatitis.
• There is a vaccine available that provides long-term prevention
in persons over 2 years of age.
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Hepatitis B-(HBV)
• Hepatitis B (HBV) is a very serious disease that may result in
prolonged illness, liver cancer, cirrhosis of the liver, liver
failure, and even death.
• It is a bloodborne disease that may also be transmitted by other
body fluids, including saliva.
• Anyone who has ever had the disease, and some persons who
have been exposed but have not been actually ill, may be
carriers of HBV and may actually be spreading the infection
to others.
• This presents a high risk for dental personnel because dental
treatment beings them into contact with saliva and blood.
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Hepatitis B Vaccination
• There is a highly effective vaccine available to prevent
hepatitis B.
• All dental personnel with a chance of occupational exposure
should be vaccinated against hepatitis B.
• The OSHA Bloodborne Pathogen Standard requires that the
employer offer the hepatitis B vaccination, at no cost to the
employee, within 10 days of initial assignment to a position in
which there is chance of occupational exposure to blood and/or
other body fluids.
• The employee has the right to refuse the offer of vaccination;
however, that employee must sign a release form indicating
that the employer did offer the vaccine, and that the employee
understands the potential risks of contracting hepatitis B.
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Hepatitis C-(HCV)
• Hepatitis C (HCV) is most efficiently transmitted through blood
transfusion or percutaneous exposure to blood.
• This could occur from an accidental needle stick to an
employee in a dental office, or by the sharing of contaminated
needles among injection drug users, or from contaminated
tattoo needles.
• The carrier rate associated with HCV is higher than that
associated with HBV.
• Unfortunately there is no vaccine against hepatitis C at this
time, nor is there a cure for the disease.
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Hepatitis D-(HDV)
• Hepatitis D virus is a defective virus that cannot replicate
itself without the presence of HBV.
• Therefore, infection with HDV may occur simultaneously
as a co-infection with HBV or may occur in an HBV
carrier.
• Persons with a co-infection of HBV and HDV often have
more severe acute disease and a higher risk of death
compared with those infected with HBV alone.
• Vaccination against HBV will also prevent infection
with HDV.
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Hepatitis E-(HEV)
• Hepatitis E is not transmitted through
bloodborne contact.
• It is most frequently transmitted via the fecal-oral
routes through contaminated food or water.
• The disease is most frequently seen in the form
of an epidemic in developing countries.
• Transmission is not a major concern in a standard
dental setting.
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Human Immunodeficiency Virus (HIV)
• HIV is a bloodborne viral disease.
• It is an infection in which the body’s immune system
breaks down. AIDS is caused by HIV.
• When HIV enters the body, it infects special T cells and
slowly kills them. As more and more of the T cells die,
the body’s ability to fight the infection weakens.
• A person with HIV infection may remain healthy
for many years.
• HIV-positive people develop AIDS when they become
sick with serious illnesses and infections that can
occur with HIV.
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HIV-cont’d.
• HIV is spread by sexual contact with an infected person,
by needle sharing among drug users.
• Now that blood is screened for HIV antibodies, the blood
supply is safe in this country.
• Babies born to HIV, infected mothers may become infected
before or during birth, or through breast feeding after birth.
• In (non-dental) healthcare settings, workers have been
infected with HIV after being stuck with needles containing
HIV-infected blood, or less frequently, after infected blood
gets into the worker’s bloodstream through an open cut or
splashes into a mucous membrane (e.g., eyes, or inside of
the nose).
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Types of Human Herpesvirus
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Herpes Simplex Virus-Type 1
• Herpes simplex virus type 1 is a viral
infection that causes recurrent sores on lips.
• Because these sores frequently develop
when the patient has a cold or fever of other
origin, the disease has become commonly
known as fever blisters or cold sores.
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Primary Herpes
• This disease, which is highly contagious, makes its first
appearance in very young children (1 to 3 years of age)
and is known as primary herpes.
• The child may have a slight fever, pain in the mouth,
increased salivation, bad breath, and a general feeling of
illness. The inside of the mouth becomes swollen, and the
gingivae are inflamed.
• Healing begins naturally within 3 days, and the illness is
usually over in 7 to 14 days. During this time, supportive
measures can be taken to make the child more
comfortable, relieve the pain, and prevent secondary
infection.
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Recurrent Herpes Labialis
• After the initial childhood infection, the virus of herpes
simplex lies dormant and reappears later in life as the
familiar recurring fever blister or cold sore.
• Recurrences tend to take place when the patient's general
resistance is lowered as a result of stress, fever, illness,
injury, and exposure to the sun. The use of sunscreen
with a sun protective factor of 15 helps to prevent
sun-induced recurrences of herpes.
• Attacks may recur as infrequently as once a year or as
often as weekly or even daily. As in the case of primary
herpes, recurrent herpes labialis sores heal by
themselves in 7 to 10 days, leaving no scar.
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Fig. 18-10 Herpes labialis. A, 12 hours after onset. B, 48 hours after onset.
(From Ibsen OC, Phelan JA: Oral pathology for the dental hygienist, ed 4, Philadelphia, 2004, Saunders.)
A
B
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Herpes Simplex Virus-Type 2
• Herpes simplex virus type 2, also known as genital herpes, is
one of the most common STDs in the United States.
– Initial symptoms, which generally appear 2 to 10 days after
infection, include tingling, itching, and a burning sensation
during urination.
• Once a person is infected with the virus, outbreaks will recur.
The disease can be transmitted only during these recurrences.
• A mother with active vaginal or cervical herpetic lesions at the
time of delivery can pass the virus to her newborn. About 50%
of such newborns will be infected as they pass through the
birth canal. Of the infants infected, at least 85% will be severely
damaged or killed by the virus.
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Herpes Transmission
• The major transmission route for the herpesvirus is through
direct contact with lesions.
• Even when there are no active lesions, there is still the
possibility of transmission of the virus through saliva or the
aerosol spray from the dental handpiece.
• Because there is no preventive vaccine to protect against
herpes, it is essential that precautions be taken to prevent
exposure.
• Protective eyewear is particularly important because a herpes
infection in the eye may cause blindness. Gloves protect
against infection through lesions or abrasions on the hands.
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West Nile Virus
• The West Nile virus is commonly found in Africa, West Asia,
and in the Middle East.
• It is believed to have been in the U.S. since the early summer
of 1999. The virus is carried by mosquitoes and can infect
humans, birds, horses and some other mammals.
•
It affects a person’s nervous system, causing inflammation
of the brain and spinal cord.
• Symptoms include fever, headache, tiredness, aches, and
sometimes rash. Cases occur primarily in the late summer or
early fall. In the southern climates where temperatures are
milder, West Nile virus can be transmitted year round.
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Severe Acute Respiratory Syndrome (SARS)
• Severe acute respiratory syndrome (SARS) is a viral respiratory
illness caused by a coronavirus. The same type of virus is
responsible for colds.
• SARS was first reported in Asia in 2003 where 774 people died from
the disease. In the U.S., only eight people had laboratory evidence
of SARS infection, and these people had traveled to other parts of
the world.
• SARS is spread by close person-to-person contact. The virus is
thought to be transmitted by respiratory droplets spread when a
person coughs or sneezes
• The Centers for Disease Control and Prevention (CDC) is working
with state and local health departments, and healthcare
organizations to be prepared to respond quickly when, or if, SARS
appears in the U.S.
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Bacterial Diseases
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Tuberculosis
• Tuberculosis, which is caused by the bacterium M. tuberculosis,
is the leading cause of death worldwide from infectious
diseases.
• Because HIV-infected patients have a weakened immune
system, they are highly susceptible to tuberculosis; therefore,
HIV and tuberculosis are often present together.
• Of the two, tuberculosis is a greater health risk for healthcare
workers.
• One reason for this is that the rod-shaped tubercle bacillus is
able to withstand disinfectants that kill many other bacteria.
• Tuberculosis kill time is the benchmark for the effectiveness
of a surface disinfectant.
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Legionnaire’s Disease
• Legionella pneumophila bacteria causes two acute bacterial
diseases: Pontiac fever and Legionnaires' disease.
• The bacteria are transmitted through aerosolization and
aspiration of contaminated water.
• There is no person-to-person transmission.
• Dental personnel have higher antibodies against L. pneumophila
than the general public, indicating occupational exposure and
resistance to this organism.
• The less serious form of infection is called Pontiac fever and the
more serious form of infection is called Legionnaires' disease
and causes a very severe pneumonia. In individuals who are
already immunocompromised or elderly, the disease can be
fatal.
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Fig. 18-11 Bacteria in biofilm taken from dental unit waterlines
(Courtesy Dr. Shannon Mills, USAF.)
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Tetanus
• Tetanus, which is also known as lockjaw, is an extremely
dangerous and often fatal disease that is caused by a sporeforming bacillus found in soil, dust, or animal or human feces.
• This microbe is usually introduced into the body through a
wound or break in the skin (as in a puncture wound from a
soiled instrument).
• The organism causing tetanus produces the severe muscle
spasms and rigidity that give the disease its popular name of
lockjaw. The disease can be prevented by the administration
of a vaccine; however, immunity must be kept current through
booster doses.
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Syphilis
• Syphilis, a sexually transmitted disease (STD), is caused by
Treponema pallidum spirochetes. Although these bacteria are
quite fragile outside of the body, there is danger of direct crossinfection in the dental operatory through contact with oral
lesions.
• The first stage of syphilis is the presence of a painless
ulcerating sore, known as a chancre, which is infectious on
contact. When it occurs on the lip, it may resemble herpes, but
the crusting is darker.
• The second stage is also infectious, and immediate infection
may occur through contact with an open sore.
• The third stage, known as latent syphilis, is usually fatal, and it
may occur after the disease has been dormant for 20 years.
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Fig. 18-12 A, Chancre on tongue seen in primary syphilis
(Courtesy Dr. Norman Trieger; From Ibsen OC, Phelan JA: Oral pathology for the dental hygienist, ed 4, Philadelphia, 2004, Saunders.)
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Fig. 18-12 B, Chancre on lip
(Courtesy Dr. Edward V. Zegarelli; From Ibsen OC, Phelan JA: Oral pathology for the dental hygienist,
ed 4, Philadelphia, 2004, Saunders.)
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