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Antibiotic-susceptibility-testing



Why Get Tested

To determine the likelihood that a particular antibiotic or antifungal drug will be effective in stopping the growth of the bacteria or fungi causing your infection

When To Get Tested

As follow up to a positive bacterial or fungal culture when you have an infection and one or more types of bacteria or fungi have been grown and isolated in a culture from a sample obtained from the site of suspected infection when your infection is not responding to treatment

Sample Required

A sample of a pure culture of bacteria or fungi grown and isolated from an infected body site

Test Preparation Needed

None




Common Questions


Susceptibility testing is used to determine which antimicrobials will inhibit the growth of the bacteria or fungi causing a specific infection The results from this test will help a health practitioner determine which drugs are likely to be most effective in treating a person s infection Like bacteria viruses can develop resistance to the drugs used to treat them but that type of testing is performed differently so this article is limited to the discussion of bacterial and fungal susceptibility testing Some types of bacteria and fungi are known to be susceptible to certain antimicrobials so routine testing may not always be necessary For example it is well known that most streptococci including the type that causes strep throat can be treated with penicillin so these types of infections may be treated without susceptibility testing However if the usual drug of choice fails to treat the infection then susceptibility testing may be necessary to determine a more effective drug Other types of infections may require testing because the bacteria or fungi isolated from an infection site are known to have unpredictable susceptibility to the drugs usually used to treat them Some examples include staphylococci staph and Pseudomonas aeruginosa Sometimes there may be more than one type of pathogen isolated from an infected site Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating all of the different types of bacteria causing the infection This may be true for example with wound infections

Susceptibility testing is often ordered at the same time as a culture of a potentially infected site such as a wound urine or blood culture However the test will usually only be performed when the culture is positive for one or more pathogens The test may also be ordered when an infection does not respond to treatment to see if the pathogen has developed resistance and to determine which antimicrobial agent would be more effective in treating the infection

Results of the testing are usually reported as Susceptible likely but not guaranteed to inhibit the pathogenic microorganism may be an appropriate choice for treatment Intermediate may be effective at a higher dosage or more frequent dosage or effective only in specific body sites where the antibiotic penetrates to provide adequate concentrations Resistant not effective at inhibiting the growth of the organism may not be an appropriate choice for treatment Sometimes results are reported as minimum inhibitory concentration MIC in units such as milligrams per microliter mg mcL This is the highest dilution lowest concentration of an antibiotic that will still be effective in inhibiting growth of the bacteria Though results may be expressed this way the laboratory will often include in the report an interpretation of what the results mean e g susceptible intermediate or resistant If there is more than one pathogen identified in a culture the laboratory will report results for each one A health practitioner will choose an appropriate drug from those on the report that were categorized as Susceptible If there are no Susceptible choices then the practitioner may select one categorized as Intermediate This may require a higher dosage and may involve a longer duration of therapy as well as a higher risk for medication side effects A pathogen may be Resistant to all of the drugs that are usually used to treat that type of infection If this is the case then the practitioner may prescribe a combination of antibiotics that work together to inhibit the bacteria when neither one alone will be effective These drug therapies may be more expensive and have to be given intravenously sometimes for extended periods of time Some infections caused by resistant bacteria have proven very difficult to treat

A sample for culture and susceptibility testing should be collected before the start of any treatment with an antimicrobial unless the test is used to monitor the effectiveness of treatment

Cultures usually require 24-48 hours before results are available Once the culture is complete and a pure sample of the microorganism is obtained the susceptibility testing may take about another 24-48 hours depending on the method used There are commercial tests available that offer rapid susceptibility testing and that may produce results in less than 24 hours Cultures for fungus and tuberculosis may take much longer up to 6 to 8 weeks

Yes In certain situations a health practitioner may choose a therapy while a culture is incubating and in others may prescribe therapy without ever ordering a culture based on knowledge and experience While it is impossible to predict which microorganism is causing an infection unless a culture is performed some organisms are seen more frequently than others For instance most urinary tract infections UTIs are caused by the bacterium Escherichia coli Knowing this a practitioner may rely on current susceptibility patterns for this bacterium to choose an antibiotic that is effective in most cases In addition there are certain life-threatening infections that must be treated immediately with no time to wait for the results of a culture In other instances a culture would not be attempted because a specimen may not be obtainable such as with otitis media ear infections or the pathogen may not be easily isolated from other flora in the specimen such as with community-acquired pneumonia In these cases the practitioner chooses therapy to cover the most common pathogens that cause these infections

Resistance may be innate natural or acquired Natural resistance is part of the microorganism s normal physical characteristics Since microorganisms multiply very rapidly they go through many generations in a short period of time There is always the potential for antimicrobial resistance to arise through a genetic change mutation If this change gives the microorganism a survival advantage it may be passed on to subsequent generations An acquired resistance may develop through a selection process When someone is treated with an antimicrobial agent the most susceptible microorganisms are the ones that are killed first If treatment is stopped before all of the pathogens are killed the survivors may develop a resistance to that particular antimicrobial agent The next time they are exposed to the same drug it may be ineffective as the bacteria and their progeny are likely to retain resistance to that antimicrobial agent Resistance can also develop when microorganisms that are resistant share their genetic material with susceptible ones This may occur more frequently in a health care setting where many patients are treated with antimicrobial agents For instance resistant strains of bacteria such as MRSA methicillin resistant Staphylococcus aureus have been a problem in hospitals for decades and are increasingly common in the community When a resistance trait arises in bacteria for whatever reason the resistant organism may spread to other people throughout a community Once a strain of bacteria has become resistant to one or more antimicrobial agent the only recourse is to try to inhibit its spread and to try to find another one that will kill it The second or third choice antimicrobial agents that are available are often more expensive and associated with more side effects This presents a challenge that is compounded by the fact that microorganisms are becoming resistant faster than new antimicrobial agents are being developed

Another way to test for resistance is by using molecular methods to test for changes mutations in a microorganism s genetic material that enables it to grow in the presence of certain antimicrobial agents Methicillin-resistant Staphylcoccus aureus MRSA contain the mecA gene that confers resistance to the antibiotics methicillin oxacillin nafcillin and dicloxacillin Detection of the mecA gene using a molecular based test allows the rapid detection of MRSA prior to culturing the bacteria Someone carrying this organism in their nasal passages can be isolated from other patients in the hospital so that the resistant staph are not transmitted to others Another example of testing for resistance is a test for beta-lactamase an enzyme produced by some bacteria that makes penicillin ineffective This test can be used to determine whether the bacteria produce this enzyme and are therefore resistant to penicillin and other similar drugs However this test is rarely performed and only on certain groups of bacteria for example Haemophilus influenzae and anaerobic bacteria