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:
There is a high probablity that the patient will respond to tx with appropriate dosage regimen for that antibiotic.
Resistant:
Tx with that antibiotic is 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 adverse reactions o Price o 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
isolates.
in the local 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 o o
Susceptible :
an infection due to a specific isolate can be appropriately treated with the recommended dosage of antibiotic.
Resistant :
isolate will not or is unlikely to respond to achievable concentrations of the antibiotic using normal doses.
Intermediate:
infection caused by the specific isolate can be treated with an antibiotic if treated with high doses or if the
infection is in an anatomic site where the antibiotic is concentrated, for
example, b lactam 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 infecti on
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.
2.
3.
4.
Dilution Testing Epsilometer Disk Diffusion 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