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Resistance Testing: Methods & Clinical Relevance

Resistance testing is a laboratory procedure that determines whether pathogens are insensitive to specific medications. It is essential for targeted and effective treatment of infections.

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Resistance testing is a laboratory procedure that determines whether pathogens are insensitive to specific medications. It is essential for targeted and effective treatment of infections.

What is resistance testing?

Resistance testing is a microbiological laboratory procedure used to determine whether a pathogen – most commonly a bacterium, and less frequently a fungus or virus – is resistant (insensitive) to specific medications, especially antibiotics. If a pathogen is resistant, the medication is either ineffective or only marginally effective. The results of resistance testing form the basis for targeted, effective therapy.

Why is resistance testing important?

The overuse and inappropriate use of antibiotics has led many bacteria to develop resistance. Without targeted resistance testing, there is a significant risk of prescribing an ineffective medication, which prolongs illness and further promotes the development of resistance. Resistant pathogens represent a growing global health concern, particularly in hospital settings.

How is resistance testing performed?

Sample collection

First, a sample is collected from the affected area of the body. Common sample materials include:

Blood (when bloodstream infection is suspected)
Urine (for urinary tract infections)
Wound swabs
Sputum (respiratory secretions)
Cerebrospinal fluid (for meningitis)

Pathogen culture and isolation

The sample is cultured in a microbiology laboratory on special nutrient media. This allows the pathogens present to be grown and identified.

Antibiogram (antimicrobial susceptibility test)

The core component of resistance testing is the antibiogram. The isolated pathogen is exposed to a range of antibiotics. Two common methods are:

Disk diffusion test (Kirby-Bauer method): Antibiotic-impregnated paper disks are placed on an agar plate containing the pathogen. The larger the inhibition zone around a disk, the more susceptible the pathogen is to that antibiotic.
Broth microdilution test: The pathogen is incubated in liquid nutrient media with increasing antibiotic concentrations. This determines the minimum inhibitory concentration (MIC) – the lowest concentration that inhibits pathogen growth.

Interpretation of results

Results are categorized into three groups:

Susceptible (S): The antibiotic is effective against the pathogen.
Intermediate (I): Effectiveness may be sufficient under certain conditions (e.g., higher dosage).
Resistant (R): The antibiotic is not effective against the pathogen.

Modern methods of resistance testing

In addition to classical culture methods, molecular biology techniques are increasingly used to detect resistance rapidly and precisely:

PCR (polymerase chain reaction): Detects specific resistance genes directly in the sample material without prior pathogen culture.
Next-generation sequencing (NGS): A comprehensive gene-sequencing approach that can identify all resistance genes present in a pathogen simultaneously.
MALDI-TOF mass spectrometry: Enables rapid pathogen identification and can be combined with other tests for resistance detection.

Clinical significance and applications

Resistance testing is indispensable across many medical fields:

Infectious diseases: For targeted treatment of bacterial infections such as pneumonia, urinary tract infections, or sepsis.
Hospital hygiene: For detection and control of multidrug-resistant organisms such as MRSA (methicillin-resistant Staphylococcus aureus) or ESBL-producing bacteria.
Tuberculosis therapy: To identify resistance to anti-tuberculosis drugs and adjust treatment accordingly.
HIV therapy: To detect resistance to antiretroviral medications.

Duration and limitations of the procedure

A standard antibiogram typically takes 24 to 72 hours, as pathogen culture requires time. Molecular rapid-testing methods can deliver results within a few hours. Limitations include the fact that not all pathogens can be cultured, and laboratory conditions do not always perfectly reflect the in vivo environment of the human body.

References

World Health Organization (WHO): Antimicrobial Resistance. Available at: www.who.int/news-room/fact-sheets/detail/antimicrobial-resistance (accessed 2024).
European Committee on Antimicrobial Susceptibility Testing (EUCAST): Clinical Breakpoints and Dosing of Antibiotics. Available at: www.eucast.org (accessed 2024).
Murray P.R., Rosenthal K.S., Pfaller M.A.: Medical Microbiology. 9th edition. Elsevier, 2021.

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