Antibiotic Susceptibility Testing
- Also Known As:
- Sensitivity Testing
- Drug Resistance Testing
- Culture and Sensitivity
- C & S
- Antimicrobial Susceptibility
- Formal Name:
- Bacterial and Fungal Susceptibility Testing
At a Glance
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 selecting the best treatment for a bacterial or fungal infection requires laboratory testing; when your infection is not responding to treatment
Test Preparation Needed?
What is being tested?
Susceptibility is a term used when microbe such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed on bacteria or fungi causing an individual’s infection after they have been recovered in a culture of the specimen. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.
Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective.
Although viruses are microbes, testing for their resistance to antiviral drugs is performed less frequently and by different test methods. This article is limited to the discussion of bacterial and fungal susceptibility testing.
During the culture process, pathogens are isolated (separated out from any other microbes present). Each pathogen, if present, is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, it is possible to determine whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. For example, strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus), can be treated with ampicillin and does not require a test to predict susceptibility to this class of antibiotics.
Susceptibility testing is performed on each type of bacteria or fungi that may be relevant to the individual’s treatment and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria. Resistance can also be determined by detection of a gene that is known to cause resistance to specific antibiotics.
How is it used?
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 care practitioner determine which drugs are likely to be most effective in treating a person’s infection.
Some 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, such as a wound infection. Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating all the different types of bacteria causing the infection.
When is it ordered?
Susceptibility testing is usually 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 drug would be more effective in treating the infection.
What does the test result mean?
Results of the testing are usually reported as:
- Susceptible — likely, but not guaranteed to inhibit the pathogenic microbe; 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 in a laboratory test; may not be an appropriate choice for treatment
These categories are based on the minimum inhibitory concentration (MIC). This is the lowest concentration of an antibiotic that will be effective in inhibiting growth of the bacteria. Though results may be expressed as the MIC, in units such as micrograms/milliliter, 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 contain the results for each one.
A healthcare practitioner will choose an appropriate drug from those on the report that were categorized as “Susceptible.” If there are no “Susceptible” choices, or the patient is allergic to drugs in the susceptible category, 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 may have to be given intravenously, sometimes for extended periods of time. Some infections caused by resistant bacteria have proven very difficult to treat.
Is there anything else I should know?
A sample for culture and susceptibility testing should be collected before the start of any treatment with an antimicrobial drug, unless the test is used to monitor the effectiveness of treatment.
How long will it take to get my results?
Bacterial cultures usually require 24-48 hours to grow the pathogen and obtain a pure culture for further testing. Cultures for fungus and tuberculosis may take much longer — up to 6 to 8 weeks since these microbes grow more slowly. Traditional susceptibility testing assays require 18-24 hours of incubation; more rapid assays are becoming available that may provide results in less than 24 hours. Molecular tests to detect resistance genes vary from same day results to several days.
Since the wait time for results is a limitation of susceptibility testing, developing new, faster, susceptibility testing techniques is an active area of research. As the FDA approves more of these rapid tests, they are being added to the testing menu in clinical microbiology laboratories.
Would a doctor ever prescribe an antibiotic without or before performing a culture?
Yes. In certain situations, a health care practitioner may choose a therapy while a culture is incubating; and in other cases, the practitioner may prescribe therapy without ever ordering a culture based on knowledge and experience. While it is impossible to predict which microbe 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 E. coli to choose an antibiotic that is likely to be effective in most cases.
In addition, there are certain life-threatening infections, such as meningitis, 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 – inner ear infections. In these cases, the practitioner chooses therapy to cover the most common pathogens that cause these infections.
How do microbes become resistant to antimicrobial drugs?
Resistance may be innate (natural) or acquired. Natural resistance is part of the microbe’s normal physical and genetic characteristics. Since microbes 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 microbe 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 drug, the most susceptible microbes 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 drug. 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 drug.
Resistance can spread when resistant microbes share their genetic material with susceptible ones. This may occur more frequently in a healthcare setting, where many patients are treated with antimicrobial drugs. For instance, resistant strains of bacteria, such as methicillin resistant Staphylococcus aureus (MRSA), have been a problem in hospitals for decades and are increasingly common in the community.
A resistant microbe may spread to other people throughout a community. Once a strain of bacteria has become resistant to one or more antimicrobial drug, 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 drugs that are available are often more expensive and associated with more side effects. This presents a challenge that is compounded by the fact that microbes are becoming resistant faster than new antimicrobial drugs are being developed.
Are there other ways of testing for resistance?
Sometimes molecular methods are used to look for changes (mutations) in a microbe’s genetic material that enables it to grow in the presence of certain antimicrobial drugs. For instance, 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 microbe in their nasal passages can be isolated from other patients in the hospital so that the resistant staph are not transmitted to others.
Sources Used in Current Review
Street, T. (Updated 2014 March 13). Antimicrobial Susceptibility. Medscape. Available online at https://emedicine.medscape.com/article/2103786-overview?pa=rOBA1lN0uZhjg0bJO6bD2AgpF2o0ma5fK6AeuJMPuepOQ4%2B%2BoTWGQm%2Be5C5zB%2BCc43mU9jD%2B1DtnxY47OmyybA%3D%3D. Accessed April 2018.
Arena, F. et al. (2015). Antibiotic Susceptibility Testing: Present and Future. Medscape. Available online at https://www.medscape.com/viewarticle/851528_1. Accessed April 2018.
Grigorenko, E. and Stalons, D. R. (2016 October 5). Addressing Antibiotic Resistance with Molecular Diagnostics. Clinical Lab Products. Available online at http://www.clpmag.com/2016/10/addressing-antibiotic-resistance-molecular-diagnostics/. Accessed April 2018.
Syal, K. et al. (2017). Current and Emerging Techniques for Antibiotic Susceptibility Tests. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479269/. Accessed April 2018.
Vorvick, L. J. (Reviewed 2017 January 26). Sensitivity Analysis. MedlinePlus. Available online at https://medlineplus.gov/ency/article/003741.htm. Accessed April 2018.
Maurer, F. P. et al. (2017 March 30). Advances in Rapid Identification and Susceptibility Testing of Bacteria in the Clincal Microbiology Laboratory: Implications for Patient Care and Antimicrobial Stewardship Program. Infectious Disease Reports. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5391540/. Accessed April 2018.
(Updated 2017 December 7). Antibiotic Resistance Questions and Answers. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/antibiotic-use/community/about/antibiotic-resistance-faqs.html. Accessed April 2018.
(© 1995-2018). Antimicrobial Susceptibility, Anaerobic Bacteria, MIC. Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/56031. Accessed April 2018.
(© 2018). Anitmicrobial Susceptibility – mecA/mecC Genes by PCR. Arup Laboratories. Available online at http://ltd.aruplab.com/Tests/Pub/0060211. Accessed April 2018.
Rifai, N. et al. (© 2018). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, Sixth Edition. Antimicrobial Resistance and Testing Considerations for Key Bacterial Pathogens, pp . 173700014 – 173700024. Available online at https://expertconsult.inkling.com/read/rifai-tietz-textbook-clinical-chemistry-molecular-diagnost-6e/chapter-75/antimicrobial-resistance-and. Accessed April 2018.
(Updated 2018 April 2). About AR Laboratory Testing and Resources. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/drugresistance/laboratories.html. Accessed April 2018.
(2018 April 3). Containing Unusual Resistance. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/vitalsigns/containing-unusual-resistance/index.html. Accessed April 2018.
Antimicrobial Resistance. Association of Public Health Laboratories. Available online at https://www.aphl.org/programs/infectious_disease/Pages/Antimicrobial-Resistance.aspx. Accessed April 2018.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
(2004 April). The Problem of Antibiotic Resistance. National Institute of Allergy and Infectious Diseases [On-line information]. Available online at http://www.niaid.nih.gov/factsheets/antimicro.htm.
(2003 April 4). Antibiotic Resistance, A Growing Threat. US Food and Drug Administration [On-line information]. Available online at http://www.fda.gov/oc/opacom/hottopics/anti_resist.html.
(2004 February 19). Using Antibiotics Sensibly. Mayoclinic.com, Infectious Disease Center [On-line information]. Available online at http://www.mayoclinic.com/invoke.cfm?id=FL00075.
(2003 November). Multidrug-Resistant Tuberculosis Fact Sheet. American Lung Association [On-line information]. Available online at http://www.lungusa.org/site/pp.asp?c=dvLUK9O0E&b=35815.
Bren, L. (2003 September, Revised). The Battle of the Bugs: Fighting Antibiotic Resistance. U.S. Food and Drug Administration, FDA Consumer magazine [On-line information]. Available online at http://www.fda.gov/fdac/features/2002/402_bugs.html.
Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson RA and Pincus MR, eds. Philadelphia: 2007, Pp 1048-1057.
(March 14, 2009) MedlinePlus Medical Encyclopedia. Sensitivity Analysis. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003741.htm. Accessed May 2009.
Forbes, B. et. al. (© 2007). Bailey & Scott’s Diagnostic Microbiology, Twelfth Edition: Mosby Elsevier Press, St. Louis, Missouri. Pp 187-214.
(2007 August 6). Get Smart: Know When Antibiotics Work, Frequently Asked Questions. CDC [On-line information]. Available online at http://www.cdc.gov/getsmart/index.html. Accessed June 2009.
Sutphen, S (2007 August 30). Antibiotic Resistance in the Emergency Room: The First Line of Defense. Medscape CME [On-line information]. Available online at http://www.medscape.com/viewarticle/562056. Accessed 8-24-08.
Barclay, L. (2008 July 3). Medscape Medical Household Antibacterial Product Use May Promote Microbial Resistance. Medscape Medical News [On-line information]. Available online at http://www.medscape.com/viewarticle/577055. Accessed June 2009.
Nicasio, A. et. al. (2008 May 13). The Current State of Multidrug-Resistant Gram-Negative Bacilli in North America. Medscape from Pharmacotherapy [On-line information]. Available online at http://www.medscape.com/viewarticle/572674. Accessed June 2009.
(2009 August 27). Treatment Multi-drug Resistant and Extensively Drug-Resistant Tuberculosis: Current Status and Future Prospects. Medscape Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/706826. Accessed June 2009.
(© 1995-2013). Antimicrobial Susceptibility, Aerobic Bacteria, MIC. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/8073. Accessed August 2013.
Vorvick, L. (Updated 2013 January 22). Sensitivity analysis. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003741.htm. Accessed August 2013.
Hazen, K. (Revised 2013 February). Susceptibility Testing. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed August 2013.
Street, T. and Schmidt, S. (Updated 2012 October 18) Antimicrobial Susceptibility. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2103786-overview. Accessed August 2013.
McPherson, R. and Pincus, M. (© 2011). Henry’s Clinical Diagnosis and Management by Laboratory Methods 22nd Edition: Elsevier Saunders, Philadelphia, PA. Pp 11117-1128.