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  • Explain the differences between Koch’s original postulates and the molecular Koch’s postulates.

Pathogenicity and virulence

The ability of a microbial agent to cause disease is called pathogenicity , and the degree to which an organism is pathogenic is called virulence . Virulence is a continuum. On one end of the spectrum are organisms that are avirulent (not harmful) and on the other are organisms that are highly virulent. Highly virulent pathogens will almost always lead to a disease state when introduced to the body, and some may even cause multi-organ and body system failure in healthy individuals. Less virulent pathogens may cause an initial infection, but may not always cause severe illness. Pathogens with low virulence would more likely result in mild signs and symptoms of disease, such as low-grade fever, headache, or muscle aches. Some individuals might even be asymptomatic.

An example of a highly virulent microorganism is Bacillus anthracis , the pathogen responsible for anthrax . B. anthracis can produce different forms of disease, depending on the route of transmission (e.g., cutaneous injection, inhalation, ingestion). The most serious form of anthrax is inhalation anthrax. After B. anthracis spores are inhaled, they germinate. An active infection develops and the bacteria release potent toxins that cause edema (fluid buildup in tissues), hypoxia (a condition preventing oxygen from reaching tissues), and necrosis (cell death and inflammation). Signs and symptoms of inhalation anthrax include high fever, difficulty breathing, vomiting and coughing up blood, and severe chest pains suggestive of a heart attack. With inhalation anthrax, the toxins and bacteria enter the bloodstream, which can lead to multi-organ failure and death of the patient. If a gene (or genes) involved in pathogenesis is inactivated, the bacteria become less virulent or nonpathogenic.

Virulence of a pathogen can be quantified using controlled experiments with laboratory animals. Two important indicators of virulence are the median infectious dose (ID 50 ) and the median lethal dose (LD 50 ) , both of which are typically determined experimentally using animal models. The ID 50 is the number of pathogen cells or virions required to cause active infection in 50% of inoculated animals. The LD 50 is the number of pathogenic cells, virions, or amount of toxin required to kill 50% of infected animals. To calculate these values, each group of animals is inoculated with one of a range of known numbers of pathogen cells or virions. In graphs like the one shown in [link] , the percentage of animals that have been infected (for ID 50 ) or killed (for LD 50 ) is plotted against the concentration of pathogen inoculated. [link] represents data graphed from a hypothetical experiment measuring the LD 50 of a pathogen. Interpretation of the data from this graph indicates that the LD 50 of the pathogen for the test animals is 10 4 pathogen cells or virions (depending upon the pathogen studied).

A graph with “number of pathogenic agents (cells or virions)” on the X axis and Percent mortality in experimental group on the Y axis. The graph begins at 0,0 and increases until there is nearly 100% death at 10 to the 5. The line then plateaus at 100%.  A 50% death rate occurs at 10 to the 4. This is the LD 50.
A graph like this is used to determine LD 50 by plotting pathogen concentration against the percent of infected test animals that have died. In this example, the LD 50 = 10 4 pathogenic particles.

Questions & Answers

introduction to microbiology
Shweta Reply
Study the life of microorganisms present in the environment their survivals etc..
ayesha
Microscopic organisms, commonly known as microorganisms or microbes, are found all around us and even inside our bodies. The category 'Microbes' includes a massive range of organisms including bacteria, fungi, viruses, algae, archaea and protozoa.
Amre
fermented foods likely benefited ancestors to preserve foods and make other foods like milk, cheese, and bread using microbes.
Zinnia Reply
the evidence that I would support the statement regarding ancient people ideas that disease was transmitted by things they could not see is by the discoveries under a microscope. Many microbiologists have discovered certain diseases caused by microbes.
Zinnia
plz give the information about glyoxylate cycle
kamini Reply
what type of information?
Sapiens
I need medical microbiology mcqs books for Mbbs
Kisota
for pakistani mbbs or other?
Sapiens
what is microbial soup?
Osborn Reply
😃😂funny question isn't it?!!! sorry for inconvenience 😊
ayesha
most bacterial pathogens that cause disease in humans are classified in terms if their preferable ph as
Beesan Reply
what is micro
Kabul Reply
extremely small in size
Swetha
almost invisible to the naked eye
Peter
thanks
Sadiq
uwc
Peter
invisible sized objects
Sivasri
invisible sized object or specimens.which is used to see in only the microscope that specimens or objective called the micro
Sivasri
small organism that can be seen by use of Microbes
Manyang
why do you need to study microorganisms
Sala
what is microbiology?
Green Reply
microbiology is a branch of biology .it deals with the study of microorganisms life cycle , uses, disadvantages,and it impact in any other fields .which is used to view in microscope .
Sivasri
Microbiology is the study of microorganisms, those being unicellular, multicellular, or acellular. Microbiology encompasses numerous sub-disciplines including virology, bacteriology, protistology, mycology, immunology and parasitology. 
Raj
In short , it is the study of micro organisms and types , characteristics.
Kaviya
can you explain the structure of rna in detail
Dhanalakshmi Reply
RNA is of many types....like mRNA,tRNA,rRNA,snRNA,guideRNA..etc .
Swetha
but generally RNA is single stranded
Swetha
It contains adenine guanine cytosine , instead of thymine it contains uracil.
Swetha
ribonucleic acid present in RNA.. where as in DNA it is deoxyribonucleic acid...and ribonucleotide is present in RNA
Swetha
RNA is a blue print of DNA. it has the information from DNA....and we can predict the base panirs in DNA if we have the RNA....copy of that DNA
Swetha
can either have positive or negative polarity
Ernestine
Diatoms need..... With the help of which they can construct their beautiful cell wall
Prathmesh Reply
Microbial growth curve shows a.... Curve.
Shambhuraj Reply
diminishing curve
Oluwapamilerin
It represents the decreasing growth of an organism.
Swetha
what is meant by reverse isolation in tuberculosis treatment?
Swetha Reply
why do we study microbiology
HABIBA Reply
Microbiology is the way of gaining knowledge about those organisms which are surround us but we can't see them with our naked eye,such organisms called microorganisms..We can see them only under microscope.
ayesha
from our birth to death microorganisms are with us in the environment as well as on our body which are called Microflora.
ayesha
Microfloras are non pathogenic
ayesha
what is microbiology
Anji Reply
It is the study of microorganisms and their structures functions and their interactions with environment ...
Swetha
it is the study of microorganisms which are invisible to backed eye but we can see them under microscope then microbiology is the study of microorganisms including their structures , functions,and their interactions with environment
Swetha
tq
Anji
tqq...not backed eye sorry nacked eye
Swetha
it's ok
Anji
where r u from
Anji
thank you
Swetha
from India
Swetha
which state
Anji
Hyderabad .
Swetha
so telugu vosthadhi kadha
Anji
chelusu nakku kochum
Swetha
thank you
Swetha
what is reverse isolation in tuberculosis treatment?
Swetha
Reverse isolation is used to protect you from germs when your immune system is not working properly. Germs can be carried on droplets in the air, medical equipment, or another person's body or clothing.
Kaviya
will you repeat this again?
Swetha
Reverse isolationis used to protect you from germs when your immune system is not working properly. Germs can be carried on droplets in the air, medical equipment, or another person's body or clothing.
Kaviya
kill you?
Swetha
Y
Kaviya
nothing to see the whole answer that is why I sent u like that then only I can see the full answer la
Swetha
What is allosteric site?
Hetshree Reply
I don't know how to explain but it's a deep concave site of an enzyme it's slightly or more slightly similar to the active site on an enzyme
ayesha
a site other than enzyme active site
Muhammad
describe the gram staining procedure as used in identification and classification of bacteria and it's interpretation
tabitha Reply
Gram staining method found by Hans Christian Gram. Classified into Gram Positive and Gram Negative Positive would be purple or blue stained bacteria Negative would be Pink or Red stained bacteria Both are observed under microscope This procedure performed on glass slides by making a smear of sample.
ayesha
Thanks.. nicely u defined
shamim
jazakallah Khair
ayesha
hmmm
shamim
Masha allah
Khamis
jazakallah Khair
ayesha
hi
Nafiu

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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