Antibiotic sensitivity test
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Transcript Antibiotic sensitivity test
In Vitro Testing of
Antimicrobial Agents
Maryam Monajemzadeh , Pathologist
Children Medical Center Hospital
Tehran University Of Medical Sciences
History
Paul Ehrlich
1910
Upon making the observation that some dyes stained
bacterial cells, but not animal cells, Ehrlich
determined that there was a fundamental difference
between the 2 types of cells. He began the search for
the “magic bullet”- a drug that would kill a
microbial pathogen without harming the human
host.
History
After 605 tests to find a cure for syphilis,
Ehrlich was successful in 1910. He proved
that arsphenamine, a compound of arsenic,
was effective in treating lab animals. The
ability of the new drug, named Salvarson, to
cure syphilis, proved that chemicals could be
used to selectively kill microorganisms
without harming the human host
permanently.
Paul Ehrlich
History
Gerhard Domagk 1932
accidentally discovered the first sulfa drug,
sulfanilamide, while testing a red dye called
Prontosil on streptococci
Gerhard
Domagk
History
Fleming 1928
noticed that colonies of Staphylococcus were
inhibited by mold
Fleming identified the mold as a species of
Penicillium
with further testing, it was shown that Penicillium was
a bacteria-killing substance
Alexander
Fleming
History
Ernst Chain & Howard Florey
successfully purified penicillin
1941 - 1st test on an ill human
the patient improved dramatically,
but died when the penicillin ran out
mass development of penicillin
was spurred on by WW2
first antibiotic developed for the
general public
Sir Howard Florey
Dr. Ernst Chain
Importance:
More important than pathogen
identification
Uses of Antibiotic Sensitivity
Testing
Antibiotic sensitivity test: A laboratory test
which determines how effective antibiotic
therapy is against a bacterial infections.
Antibiotic sensitivity testing will control the
use of Antibiotics in clinical practice
Testing will assist the clinicians in the choice
of drugs for the treatment of infections.
Components of Antibiotic
Sensitivity Testing
1.The identification of relevant pathogens in
exudates and body fluids collected from
patients
2. Sensitivity tests done to determine the
degree of sensitivity or resistance of
pathogens isolated from patient to an
appropriate range of antimicrobial drugs
3. Assay of the concentration of an
administered drug in the blood or body fluid of
patient required to control the schedule of
dosage.
Antibiotic Sensitivity Testing Is
Essential of selection of
Antibiotics
The main objective:
To predict the outcome of treatmentwith the
antimicrobial agents
Susceptible:
Resistant:
There is a high probablity
Tx with that antibiotic is
that the patient will
respond to tx with
appropriate dosage
regimen for that antibiotic.
likely to fail
90-60 rule
For many infections we can expect tx success
about 90% of the time when the organism
tests as susceptible to that tx
Success will occur in around 60% of cases
when the organism tests as resistant to that tx
60% response= the natural response to many
bacterial infections in the immunogically nl
host
Intermediate:
Higher doses
Normally concentrated
effective
Intermediate:
Higher doses is dangerous
Limited penetration
ineffective
Buffer zone:
Prevents strains with borderline
susceptibilities from incorrectly categorized
as resistant
Patient has been given the
empirical therapy before report of
lab
Confirm the susceptibility:
•
Alternatives
o
o
o
adverse reactions
Price
Narrow spectrum
Indicate resistant
Susceptibility tests:
Only with pathogens:
for
which standard methods are
available
Resist or suscep is a major concern, not
normal flora or contaminations
Uses of Antibiotic Sensitivity
Testing
Helps to guide the Physician in choosing
Antibiotics
The accumulated results on different
pathogens their sensitivity will guide the
physician in choosing empirical treatment in
serious patients before the individual’s
laboratory results are analyzed in the
Microbiology laboratory.
Reveals the changing trends in the local
isolates.
Helps the local pattern of antibiotic
prescribing.
Why Need continues for testing for
Antibiotic Sensitivity
Bacteria have the ability
to develop resistance
following repeated or
subclinical (insufficient)
doses
so more advanced
antibiotics and synthetic
antimicrobials are
continually required to
overcome them.
Testing Antibiotic
Susceptibility
Antibiotic sensitivity test: A laboratory test
which determines how effective antibiotic
therapy is against a bacterial infections.
Antibiotic sensitivity test: the in vitro
testing of bacterial cultures with antibiotics to
determine susceptibility of bacteria to
antibiotic therapy.
Definitions:
o Susceptible : an infection due to a specific
isolate can be appropriately treated with the
recommended dosage of antibiotic.
o Resistant : isolate will not or is unlikely to
respond to achievable concentrations of the
antibiotic using normal doses.
o Intermediate: infection caused by the specific
isolate can be treated with an antibiotic if treated
with high doses
or if theinfection is in an anatomic site where
the antibiotic is concentrated, for example, blactam antibiotics in the urine.
Host factors affecting treatment
Dosage
route of administration
immune status
distribution space of the antibiotic
pharmokinetic characteristics of the antibiotic
and the hepatic and/or renal functional status of the
patient
Serum protein binding
Drug interactions
Multiple simultaneous infections
Virulence of organism
Site and severity of infection
The pharmacological concept for
breakpoints
The concentration range tested for a drug and the
interpretative criteria for various categories are based
on extensive studies that correlate with:
Serum achievable levels for each antimicrobial agent
Particular resistance mechanisms
Successful therapeutic outcome
From breakpoints to interpretation
Sensitive strain
Intermediate strain
Resistant strain
Measuring antimicrobial sensitivity of a strain
isolated from a patient, to determine its status
as S, I or R is an individual problem
Defining the status of a bacterial species or genus
is an epidemiological problem distributed
across time and space that requires monitoring
In order to have intra- and interlaboratory
reproducibility, and have different
laboratories obtain the same results, these
variables must be standardized.
Inhibitory Methods for Susceptibility
Testing
1. Dilution Testing
2. Epsilometer
3. Disk Diffusion
4. Direct Tests for β-lactamase
Recent years:
Trend to commercial broth microdilution
Automated instrument methods
Disk diffusion:
Flexibility in drug selection
Respond quickly to changes in breakpoints
Low cost
Standard
Readily understood by clinicians
Microdilution and agar gradient
Quantitative results
Fastidious or anarobic
Automated
There is no clear evident that MICs are more
relevant than susceptiblity category results