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Antibiogram – Definition, Procedure and Significance

An antibiogram is a laboratory test that shows which antibiotics are effective against a specific pathogen. It helps doctors select the most targeted antibiotic therapy.

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Things worth knowing about "Antibiogram"

An antibiogram is a laboratory test that shows which antibiotics are effective against a specific pathogen. It helps doctors select the most targeted antibiotic therapy.

What Is an Antibiogram?

An antibiogram is a microbiological test procedure that determines the susceptibility of an isolated pathogen – usually bacteria – to various antibiotics. The results indicate which antibiotics are effective (susceptible), moderately effective (intermediate), or ineffective (resistant) against the identified organism. The antibiogram is an essential tool in modern infectious disease medicine and forms the basis of rational, targeted antibiotic therapy.

When Is an Antibiogram Performed?

An antibiogram is requested whenever a bacterial infection is suspected or confirmed in a patient and antibiotic treatment is necessary. Common indications include:

Urinary tract infections, especially when resistant pathogens are suspected
Pneumonia (lung infection)
Wound infections and abscesses
Bloodstream infections (sepsis)
Meningitis (inflammation of the meninges)
Infections in immunocompromised patients
Suspected multidrug-resistant organisms (e.g., MRSA)

How Does an Antibiogram Work?

An antibiogram is performed in several steps within a microbiology laboratory:

1. Sample Collection

First, a sample is collected from the site of infection. Depending on the location of the infection, this may be urine, blood, sputum, a swab, or wound secretion. Samples must be collected under sterile conditions to avoid contamination.

2. Pathogen Culture and Identification

In the laboratory, the sample is cultivated on special growth media to allow the bacteria to multiply. The pathogen is then identified using biochemical or molecular methods.

3. Susceptibility Testing

The isolated pathogen is tested for its susceptibility to various antibiotics. The most common methods include:

Agar diffusion test (Kirby-Bauer test): Filter paper discs impregnated with different antibiotics are placed on an agar plate inoculated with the bacteria. Around effective antibiotics, a clear zone without bacterial growth (inhibition zone) forms. The larger the inhibition zone, the more effective the antibiotic.
Broth microdilution test: This method determines the minimum inhibitory concentration (MIC) of an antibiotic – the lowest concentration at which bacterial growth is inhibited.
Gradient diffusion test (E-test): A test strip with an antibiotic concentration gradient is placed on the inoculated agar plate. The MIC can be read directly at the point where the inhibition zone intersects the strip.

4. Interpretation and Reporting

Results are evaluated against established breakpoints defined by expert bodies such as EUCAST (European Committee on Antimicrobial Susceptibility Testing). Pathogens are classified into three categories:

S – Susceptible: The antibiotic is effective and can be used for treatment.
I – Intermediate: Efficacy is possible but uncertain; may be usable in specific situations or at higher doses.
R – Resistant: The antibiotic is ineffective; treatment with it is likely to fail.

Significance for Antibiotic Therapy

The antibiogram enables targeted antibiotic therapy rather than the use of broad-spectrum antibiotics that kill many different types of bacteria. This offers several advantages:

Higher treatment efficacy by selecting the most appropriate antibiotic
Reduced side effects by avoiding unnecessary antibiotic exposure
Protection of the natural gut flora and other microbiomes in the body
Slowing the development and spread of antibiotic resistance

Since producing an antibiogram typically takes 24 to 72 hours, physicians often begin with empirical broad-spectrum therapy in severe infections. Once the antibiogram results are available, treatment is narrowed to a more targeted antibiotic – a process known as de-escalation.

Antibiotic Resistance and Its Importance

Multidrug-resistant organisms that are resistant to many or nearly all available antibiotics represent a growing challenge in modern medicine. Well-known examples include MRSA (methicillin-resistant Staphylococcus aureus), VRE (vancomycin-resistant enterococci), and multidrug-resistant gram-negative bacteria. In these cases, the antibiogram is especially critical, as it is often the only way to identify remaining effective treatment options.

References

European Committee on Antimicrobial Susceptibility Testing (EUCAST): Breakpoint tables for interpretation of MICs and zone diameters. Version 14.0, 2024. Available at: www.eucast.org
World Health Organization (WHO): Global Action Plan on Antimicrobial Resistance. Geneva, 2015. Available at: www.who.int
Murray P.R., Rosenthal K.S., Pfaller M.A.: Medical Microbiology. 9th edition. Elsevier, Philadelphia, 2020.

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