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MEDICAL MICROBIOLOGY
Medical microbiology is basically branch of medicine and microbiology. There in medical
microbiology we discus about pathogens, parasites, disease causer or infection causer and
their prevention diagnoses too by using microorganisms. There are four types of
microorganisms those cause infection are bacteria, fungi(yeast), Viruses, parasites.
Medical microbiologist also doing work in all teaching levels and research.it also providing
vaccines against the infections. Deadly and debilitating diseases such as small pox, polio, and
tuberculosis have been either eradicated or become more treatable. There are claims that
consuming probiotics (bacteria potentially beneficial to the digestive system) and/or
prebiotics (substances consumed to promote the growth of probiotic microorganisms)
contributes to human health.
Microorganisms could be useful in the treatment of cancer. Strains of non-pathogenic
clostridia can infiltrate and replicate within solid tumours, deliver therapeutic proteins.
HISTORY OF MEDICAL MICROBIOLOGY
In 1546, Girolamo Fracastoro proposed that epidemic diseases were caused by transferable
seed like entities through direct, indirect contact and contact over long distances with an
infection.
Louis Pasteur and Robert Koch are the founders of medical microbiology. Louis Pasteur is
famous for his experiments when he disproved the theory of spontaneous generation. He
offered method for food preservation (pasteurization) and vaccines against anthrax, fowl
cholera and rabies.
Robert Koch contributed to the germ theory of disease, provided that specific diseases were
caused by specific microbes. He developed criteria known as the Koch's postulates and was
among first to isolate bacteria in pure culture resulting in his description of several bacteria
including Mycobacterium tuberculosis, the causative agent of tuberculosis
In 1676, Anton van Leeuwenhoek observed bacteria and other microorganisms, using a
single-lens microscope of his own design.
In 1857 Louis Pasteur also designed vaccines against several diseases such as anthrax, fowl
cholera and rabies as well as pasteurization for food preservation.
1867 Joseph Lister is considered to be the father of antiseptic surgery. By sterilizing the
instruments with diluted carbolic acid and using it to clean wounds, post-operative infections
were reduced making surgery safer for patients.
Robert Koch contributed to the germ theory of disease, provided that specific diseases were
caused by specific microbes. He developed criteria known as the Koch's postulates and was
among first to isolate bacteria in pure culture resulting in his description of several bacteria
including Mycobacterium tuberculosis, the causative agent of tuberculosis.
Now a day’s microbiologist also doing work on more microorganisms and producing
morevaccines.in medical microbiology there more than 4 fields doing work as well
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Fields in medical microbiology
Microbial physiology is the study of microbial growth, microbial metabolism and microbial
cell structure.
Microbial genetics is the study of how genes are organized and regulated in microbes in
relation to their cellular functions.
Parasitology investigates parasites. The specimen here is feces, blood, urine, sputum, and
other samples.
Virologyidentifies viruses in specimens of blood, urine, and cerebrospinal fluid.
Immunology/Serology uses antigen-antibody interaction as a diagnostic tool, determines
compatibility of transplanted organs.
A medical microbiologist is a specialist in medical (clinical) microbiology.
Fungal infectious diseases
The diseases that are caused by the pathogenic fungi species such as candida,histoplasmia etc
Transmission of infectious diseases:-
There are various ways in which disease can be transmitted between individuals. These
include:
• Direct contact - Touching an infected host.
• Indirect contact - Touching a contaminated surface
• Droplet contact - Coughing or sneezing
• Fecal–oral route - Ingesting contaminated food or water sources
• Airborne transmission - Pathogen carrying spores
• Vector transmission - An organism that does not cause disease itself but transmits
infection by conveying pathogens from one host to another
• Fomite transmission - An inanimate object or substance capable of carrying infectious
germs or parasites
• Environmental - Nosocomial infections
Fungal infectious disease:-
1-Histoplasmosis:-
Alternative Names
Ohio River Valley fever; Fibrosing mediastinitis
Causes
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Histoplasmosis is an infection caused by a fungus called Histoplasma. The fungus lives in
the environment, particularly in soil that contains large amounts of bird or bat droppings. In
the United States, Histoplasma mainly lives in the central and eastern states, especially areas
around the Ohio and Mississippi River valleys. The fungus also lives in parts of Central and
South America, Africa, Asia, and Australia
Histoplasmosis is also caused by the reproductive cells (spores) of the fungus Histoplasma
capsulatum. The spores are extremely light and float into the air when dirt or other
contaminated material is disturbed. Even if you've had histoplasmosis in the past, you can
still get the infection again. However, if you contract histoplasmosis again, the illness will
likely be milder than the initial infection.
. People can get histoplasmosis after breathing in the microscopic fungal spores from the
air,or often after participating in activities that disturb the soil.. Although most people who
breathe in the spores don’t get sick, those who do may have a fever, cough, and fatigue.
Many people who get histoplasmosis will get better on their own without medication, but in
some people, such as those who have weakened immune systems, the infection can become
severe.
Symptoms of histoplasmosis:-
Most people who are exposed to the fungus Histoplasma never have symptoms. Other people
may have flu-like symptoms that usually go away on their own.
Symptoms of histoplasmosis include:
Fever
cough
Fatigue (extreme tiredness)
Chills
Headache
Chest pain
Body aches
In some people, histoplasmosis can also produce joint pain and a rash. People who have an
underlying lung disease, such as emphysema, may develop a chronic form of histoplasmosis
that can additionally feature weight loss and a cough that brings up blood. Symptoms of
chronic histoplasmosis sometimes can mimic those of tuberculosis. The infection may be
active for a short period of time, and then the symptoms go away. Sometimes, the lung
infection may become long-term (chronic). Symptoms include:
Chest pain and shortness of breath
Cough, possibly coughing up blood
Fever and sweating
In a small number of patients, histoplasmosis may spread throughout the body, causing
irritation and swelling (inflammation) in response to the infection. Symptoms may include:
Chest pain from swelling in the lining around the heart (pericarditis)
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Headache and neck stiffness from swelling in the covering of the brain and spinal
cord
High fever
Life cycle of Histoplasma:-
Histoplasma spores circulate in the air after contaminated soil is disturbed. The spores are too
small to see without a microscope. When people breathe in the spores, they are at risk for
developing histoplasmosis. After the spores enter the lungs, the person’s body temperature
allows the spores to transform into yeast. The yeast can then travel to lymph nodes and can
spread to other parts of the body through the bloodstream.
Complications
Histoplasmosis can cause a number of serious complications, even in otherwise healthy
people. For infants, older adults and people with compromised immune systems, the potential
problems are often life-threatening. Complications can include:
Acute respiratory distress syndrome (ARDS).Histoplasmosis can damage lungs to the point
that the air sacs begin filling with fluid. This prevents efficient air exchange and can deplete
oxygen levels in the blood.
Heart problems. Inflammation of the pericardium, the sac that surrounds your heart, is called
pericarditis. When the fluid in this sac increases, it can interfere with the heart's ability to
pump blood efficiently.
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Adrenal insufficiency. Histoplasmosis can harm your adrenal glands, which produce
hormones that give instructions to virtually every organ and tissue in your body.
Meningitis. In some cases, histoplasmosis can cause meningitis, an infection and
inflammation of the membranes surrounding your brain and spinal cord.
Exams and Tests
Histoplasmosis is diagnosed by:
Biopsy of the lung, skin, liver, or bone marrow
Blood or other tests to detect histoplasmosis proteins or antibodies
Cultures of the blood, and other things (this test provides the clearest diagnosis of
histoplasmosis, but results can take 6 weeks)
To help diagnose this condition, your doctor may perform:
Bronchoscopy
Chest CT scan
Chest x-ray
Spinal tap to look for signs of infection in cerbrospinal fluid (CSF)
Treatment:-
Most of the time, this infection goes away without treatment.
If you are sick for more than 1 month or are having trouble breathing, your doctor
May prescribe medication. The main treatment for histoplasmosis is antifungal drugs.
Amphotericin B, itraconazole, and ketoconazole are the usual treatments.
Antifungals may need to be given through a vein, depending on the form or stage of disease.
Some of these medicines can have side effects.Sometimes, long-term treatment with
antifungal drugs may be needed. You may need to take these medications for up to 1 to 2
years.
Prevention:-
Histoplasmosis may be prevented by reducing exposure to dust in chicken coops, bat caves,
and other high-risk locations. Wear masks and other protective equipment if you work in
these environments.
PARASITIC DISEASE
Definition
A parasitic disease is an infectious disease caused or transmitted by a parasite.
Many parasites do not cause diseases
Parasitic diseases can affect practically all living organisms, including plants and
mammals.
The study of parasitic diseases is called parasitology.
Symtoms:
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Abdominal pain
Weight loss
Increased appetite
Sleeping problems
Vomiting
Fatigue
dehydration
Cause:
Mammals can get parasites from contaminated food or water andbug bites.Other risks are
walking barefeet, inadequate disposal of faeces and eating undercooked or exotic
food.Parasites can also be transferred to their host by the bite of an insect vector, i.e.
mosquitos.
ENTAMOEBA HISTOLYTICA-AMOEBIASIS:
Entamoeba histolytica is a protozoan parasite responsible for a disease called amoebiasis. It
occurs usually in the large intestine and causes internal inflammation as its name suggests
(histo = tissue, lytic = destroying)E histolytic is transmitted via ingestion of the cystic form
(infective stage) of the protozoa
Inside humans Ent amoeba histolytica lives and multiplies as a trophozoite. Trophozoites are
oblong and about 15–20 µm in length. In order to infect other humans they encyst and exit
the body.
The life cycle of Entamoeba histolytica does not require any intermediate host.
Mature cysts (spherical, 12–15 µm in diameter) are passed in the feces of an infected
human. Another human can get infected by ingesting them in fecally contaminated
water, food or hands.
If the cysts survive the acidic stomach, they transform back into trophozoites in the
small intestine. Trophozoites migrate to the large intestine where they live and
multiply by binary fission.
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Infection leads to amoebiasis
DIAGNOSIS:
Amoebiasis can be diagnosed under a microscope by finding cysts and (rarely trophozoites)
from a stool sample.
TREATMENT;
Treatment can occur by using two antibiotics;
Metronidazole
Iodoquinol
Virus infectious diseases:
A viral infection occurs when an organism's body is invaded by pathogenic viruses.
Structural characteristics, such as genome type, virion shape and replication site, generally
have the same features among virus species within the same family.
There are currently 21 families of viruses known to cause disease in humans.
Study of viral infection is also known as virology.
Diagnosis & treatment:
Viral disease can be detected by severe muscle and joint pains before fever and also by skin
rash and lymph gland swelling.
Laboratory investigations is not necessary to detect viral infections, because no increase in
the white blood cells. But it is used for bacterial infection, if it is sucepted.
Viruses commonly have self-limited life, so treatment is usually reduce the symptoms only.
Antipyretic (greek word anti ,against and pyreticus, pertaining to fever.those substances that
reduce fever, e.g. ibuprofen) and Analgesic(without pain.e.g. paracetamol) drugs are
commonly be used.
1. Measles:
Measles is an infectious disease caused by a measle virus.
It is a single-stranded, negative-sense, enveloped RNA virus of the genus
Morbillivirus within the family Paramyxoviridae.
Virus lives in the mucous of nose and throat of infected person.
It spreads easily from person to person by coughing and sneezing into the air.
Infected droplets of mucous can remain active and contagious for around two hours.
Means, virus can live outside the body .e.g on surfaces and door handles.
Symptoms:
Conjunctivitis (pink eye), runny nose, fever, cough, ichy rash skin and meningitis
(Inflammation of the brain and spinal cord that can result in permanent brain damage and
death).
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Structure:
The measles virus has two envelope glycoproteins on the viral surface - hem agglutinin (H)
and membrane fusion protein (F). These proteins are responsible for host cell binding and
invasion. Three receptors for the H protein have been identified that are: complement
regulatory molecule CD46,signaling lymphocyte molecule and the cell adhesion molecule
Nectin-4
Life cycle:
Measle virus is transmitted by aerosol inhalation or contact with respiratory secretions. Main
target cells are immune cells like T and B cells,macrophages and dendritic cells that express
CD150 which serves as an entry receptor. Then infects epithelial cells using nectin-4.
Endothelial cells and neurons are also infectable, but entry receptors are unknown.it is
consider that first cells infected are alveolar macrophages and dendritic cells that transfer
virus to regional lymph nodes where T and B cells become infected. Then virus transfer to
spleen, lymphatic tissues, liver, thymus, skin, and lungs. Skin rash occur due to infection of
endothelial cells and immune complex formation. Infected epithelial cells permits
transmission to other hosts.
Treatment:
Measles can be diagnosed by symptoms like,rash and small spots on body. But,a
simple blood or saliva test can also be diagnosed.
No specific medicine that kills measles virus.
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Treatment with vitamin A offered to infected people because measle occurs due to deficiency
of vitamin A.
2. Diarrhea
It is caused by Rotavirus among infants and young children.
Name comes from its characteristic wheel-like appearance of virus when viewed by
electron microscopy.
It is a genus of non-enveloped, double-stranded RNA virus in the family Reoviridae.
Virion shape is icosahedral and replication site is cytoplasm.
It infects & damages the cells of small intestine causes gastroenteritis.
Symptoms:
Vomiting, Low fever, dizziness while standing, decrease in urination & dry mouth and throat.
Transmission:
Rotavirus infection is highly contagious.
The primary mode of transmission of rotavirus is the passage of the virus in stool to
the mouth of another child. This is known as a fecal-oral route of transmission.
Virus is stable (remains infective) in the environment, transmission can occur through
ingestion of contaminated water or food and contact with contaminated surfaces.
Rotavirus can survive for days on hard and dry surfaces, and it can live for hours on
human hands
Diagnosis & treatment:
Infection can be diagnosed by gastroenteristics as the cause of severe diarrhoea.
Also by finding in the child’s stool by enzyme immunoassay.
Treatment is non-specific .
involves management of symptoms and, most importantly, maintenance of hydration.
If untreated, children can die from the resulting severe dehydration.
ORS (oral rehydration solution) also given to child in which small amounts of salt
and sugar containing extra water, for treatment depending on severity of diarrhoea.
In some cases, fluids are given by intravenous (within vein) drip.
Tuberculosis
Tuberculosis, commonly known as TB, is a bacterial infection that can spread through the
lymph nodes and bloodstream to any organ in your body. It is most often found in the lungs.
Most people who are exposed to TB never develop symptoms because the bacteria can live in
an inactive form in the body. But if the immune system weakens, such as in people with HIV
or elderly adults, TB bacteria can become active. In their active state, TB bacteria cause death
of tissue in the organs they infect. Active TB disease can be fatal if left untreated.
Tuberculosis (TB) is a potentially serious infectious disease that mainly affects your lungs.the
bacteria that cause tuberculosis are spread from one person to another through tiny droplets
released into the air via coughs and sneezes.
Bacterium
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Mycobacterium tuberculosis bacteria, the cause of TB. Tuberculosis is caused by the
infectious agent known as Mycobacterium tuberculosis (Mtb). This rod-shaped bacterium,
also called Koch's bacillus, was discovered by Dr. Robert Koch in 1882.
Characteristics
Mycobacteria are aerobic meaning it needs oxygen to survive. For this reason, during
active TB disease, Mtb complexes are always found in the upper air sacs of the lungs.
Mycobacteria is a small, slow-growing bacterium that can live only in people.
Mycobacteria have thicker cell wall is than in many other bacteria.
Mycobacteria are found in habitats such as water or soil.
Types
Although body may harbor the bacteria that cause tuberculosis, your immune system usually
can prevent you from becoming sick. For this reason, doctors make a distinction between:
Latent TB. In this condition, you have a TB infection, but the bacteria remain in your body in
an inactive state and cause no symptoms. Latent TB, also called inactive TB or TB infection,
isn't contagious. It can turn into active TB, so treatment is important for the person with latent
TB and to help control the spread of TB in general. An estimated 2 billion people have latent
TB.
Active TB. This condition makes you sick and can spread to others. It can occur in the first
few weeks after infection with the TB bacteria, or it might occur years later.
Symptoms
Coughing that lasts three or more weeks
Coughing up blood
Chest pain, or pain with breathing or coughing
Unintentional weight loss
Fatigue
Fever
Night sweats
Chills
Loss of appetite
Tuberculosis can also affect other parts of your body, including your kidneys, spine or brain.
When TB occurs outside your lungs, signs and symptoms vary according to the organs
involved. For example, tuberculosis of the spine may give you back pain, and tuberculosis in
your kidneys might cause blood in urine.
Causes
Tuberculosis is caused by bacteria that spread from person to person through microscopic
droplets released into the air. This can happen when someone with the untreated, active form
of tuberculosis coughs, speaks, sneezes, spits, laughs or sings.
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Although tuberculosis is contagious, it's not easy to catch. You're much more likely to get
tuberculosis from someone you live with or work with than from a stranger. Most people with
active TB who've had appropriate drug treatment for at least two weeks are no longer
contagious.
Treatment
Medications are the cornerstone of tuberculosis treatment. But treating TB takes much longer
than treating other types of bacterial infections.
With tuberculosis, you must take antibiotics for at least six to nine months. The exact drugs
and length of treatment depend on your age, overall health, possible drug resistance, the form
of TB (latent or active) and the infection's location in the body.
Recent research suggests that a shorter term of treatment — four months instead of nine —
with combined medication may be effective in keeping latent TB from becoming active TB.
With the shorter course of treatment, people are more likely to take all their medication and
the risk of side effects is lessened.
Prevention
If you have active TB, keep your germs to yourself. It generally takes a few weeks of
treatment with TB medications before you're not contagious anymore. Follow these tips to
help keep your friends and family from getting sick:
Stay home. Don't go to work or school or sleep in a room with other people during the first
few weeks of treatment for active tuberculosis.
Ventilate the room. Tuberculosis germs spread more easily in small closed spaces where air
doesn't move. If it's not too cold outdoors, open the windows and use a fan to blow indoor air
outside.
Cover your mouth. Use a tissue to cover your mouth anytime you laugh, sneeze or cough. Put
the dirty tissue in a bag, seal it and throw it away.
Wear a mask. Wearing a surgical mask when you're around other people during the first three
weeks of treatment may help lessen the risk of transmission
Diagnosis in medical microbiology
Identification of an infectious agent for a minor illness can be as simple as clinical
presentation; such as gastrointestinal disease and skin infections. In order to make an
educated estimate as to which microbe could be causing the disease, epidemiological factors
need to be considered; such as the patient's likelihood of exposure to the suspected organism
and the presence and prevalence of a microbial strain in a community
There are four diagnosis test in medical microbiology
1. Microbial culture
2. Microscopy
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3. Biochemical test
4. Serological test
1. Microbial culture
Microbiological culture is the primary method used for isolating infectious disease for study
in the laboratory. Tissue or fluid samples are tested for the presence of a specific pathogen,
which is determined by growth in a selective or differential medium.
The 3 main types of media used for testing are:
Solid culture: A solid surface is created using a mixture of nutrients, salts and agar. A single
microbe on an agar plate can then grow into colonies (clones where cells are identical to each
other) containing thousands of cells. These are primarily used to culture bacteria and fungi.
Liquid culture: Cells are grown inside a liquid media. Microbial growth is determined by the
time taken for the liquid to form a colloidal suspension. This technique is used for diagnosing
parasites and detecting mycobacteria.[26]
Cell Culture: Human or animal cell cultures are infected with the microbe of interest. These
cultures are then observed to determine the effect this new microbe has on the cell. This
technique is used for identifying viruses.
Despite the need for a fast alternative method clinical laboratories still have little choice but
to wait days for positive identification of viable pathogenic bacteria via conventional Petri
methods. To give just a few examples: waiting period is 18-48 hours for Methicillin-resistant
Staphylococcus aureus (MRSA) and Staph, and Petri cultures take 72 hours to return results
for Group B Streptococcus.
2- Microscopy
Microscopy can be done quickly, but accuracy depends on the experience of the microscopist
and quality of equipment. Regulations often limit physicians' use of microscopy for
diagnostic purposes outside a certified laboratory. Microscopic examination of tissue may be
required to distinguish invasive disease from surface colonization—a distinction not easily
achieved by culture methods.
Most specimens are treated with stains that color pathogens, causing them to stand out from
the background, although wet mounts of unstained samples can be used to detect fungi,
parasites (including helminth eggs and larvae), vaginal clue cells, motile organisms (eg,
Trichomonas), and syphilis (via darkfield microscopy). Visibility of fungi can be increased
by applying 10% potassium hydroxide (KOH) to dissolve surrounding tissues and nonfungal
organisms.
Gram stain: The Gram stain classifies bacteria according to whether they retain crystal
violet stain (gram-positive—blue) or not (gram-negative—red) and highlights cell
morphology (eg, bacilli, cocci) and cell arrangement (eg, clumps, chains, diploids). Such
characteristics can direct antibiotic therapy pending definitive identification. Finding a
mixture of microorganisms with multiple morphologies and staining characteristics on Gram
stain suggests a polymicrobial bacterial infection. To do a Gram stain, technicians heat-fix
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specimen material to a slide and stain it by sequential exposure to Gram crystal violet, iodine,
decolorizer, and counterstain (typically safranin).
Acid fast strainThese stains are used to identify acid-fast organisms (Mycobacterium sp) and
moderately acid-fast organisms (primarily Nocardia sp). These stains are also useful for
staining Rhodococcus and related genera, as well as oocysts of some parasites (eg,
Cryptosporidium).
Flourescent strainThese stains allow detection at lower concentrations (< 1 × 104 cells/mL).
Examples are acridine orange (bacteria and fungi), auramine-rhodamine and auramine O
(mycobacteria), and calcofluor white (fungi, especially dermatophytes).
Trichome strainThese stains are used to detect intestinal protozoa.
Warthin-Starry stain and Dieterle stain: These silver stains are used to visualize bacteria
such as spirochetes, Helicobacter pylori, microsporidia, and Bartonella henselae (the cause of
cat-scratch disease).
Diagnosis test
Enzymatic activities are widely used to differentiate bacteria.Even closely related bacteria
can usually be separated into distinct species by subjecting them to biochemical tests, such as
one to determine their ability to fe rment an assortment of selected carbohydrates. For one
example of the use of biochemical tests to identify bacteria (in this instance, in marine
mammals) the. Moreover, biochemical tests can provide insight into a species' niche in the
ecosystem. For example, a bacterium that can fix nitrogen gas or oxidize elemental sulfur will
provide important nutrients for plants and animals. This will be Enteric, gram-negative
bacteria are a large heterogeneous group of microbes whose natural habitat is the in testinal
tract of humans and other animals. This family contai ns several pathogens that cause
diarrheal illness. A number of tests have been developed so that technicians can quickly
identify the pathogens, a clinician can then provide appropriate t reatment,and
epidemiologists can locate the source of an illness. All members of the fam ily
Enterobacteriaceae are oxidase-negative. Among the enteric bacteria are members of the
genera Escherichia, Ellterobacter, Shigclla, Citrobacter, andSa/monel/a . Escherichia,
Enterobacter, and Citrobacter,which ferment lactose to produce acid and gas, can be
distinguished from Sa/monella and Shigclla, which do not. Further biochemical testing, as
represented in Figure 10.8, can differentiate among the genera. The time needed to identify
bacteria cabe reduced considerably by the use of selective and differ rential media or by rapid
identification methods
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Rapid identification methods are manufactured for Groups of medically important bacteria,
such as the enterics. Such tools are designed to perform several biochemical tests
simultaneously and can identify bacteria within 4 to 24 hours. This is sometimes called
numeral identification because the results of each test are assigned a number. In the simplest
form, a positive test would be assigned a value of 1, and a negative is assigned a value of O.
In most commercial testing testresults are assigned numbers ranging from I to 4 that are
basedon the relative reliability and importance of each test, and theresulting total is compared
to a database of known organisms. An unknown enteric bacterium is inoculated into a tube
designed to perform 15 biochemical tests. After incubation, results in each compartment are
recorded. Notice that each test is assigned a value; the number Figure A slide agglutination
test. (a) In a positive test. The grainy appearance is due to the clumping (agglutination) of the
bacteria_
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(b) In a negative test, the bacteria are still evenly distributed in the saline and antiserum_
Derived from scoring all the tests is called the ID value. Fermentation of glucose is
important, and a positive reaction is valued at 2, compared with the production of acetoin (V-
P test, or the Voges-Proskauer test), which has no value. A computerized interpretation of the
simultaneous test results is essential and is provided by the manufacturer. A limitation of
biochemical testing is that mutations and plasmid acquisition can result in strains with
different characteristics. Unless a large number of tests is used, an organ ism could be
incorrectly identified.
Agglutination test
Slide agglutination test, samples of an unknown bacterium are placed in a drop of saline on
each of several slide Different known antiserum is added to each sample.
The bacteria agglutinate (clump) when mixed with antibodies that were produced in response
to that species or strain of bacterium; a positive test is indicated by the presence of
agglutination.
Serological test
that are evident in serum (see Chapter 18). Microorganisms are antigenic; that is,
microorganisms that enter an animal's body stimulate it to form antibodies. Antibodies are
proteins that circulate in the blood and combine in a highly specific way with the bacteria that
caused their production. For example, the immune system of a rabbit injected with killed
typhoid bacteria (ant igens) responds by producing antibodies against typhoid bacteria.
Solutions of such antibodies used in the identification of many medically important
microorganisms are commercially available; such a solution is called an antiserum (plural:
antisera). If an unknown bacterium is isolated from a patient, it can be tested against known
antisera and often identified quickly.