Microbiology and the zone of inhibition

Antibiotics are crucial drugs designed to kill bacteria. They achieve this by precisely targeting bacterial cells and eliminating them without harming human cells.

Antibiotics can function in various ways by targeting bacteria-specific structures, such as the cell wall and ribosomes, or by interfering with bacterial DNA. For example, penicillin disrupts the bacterial cell wall, causing it to malfunction and ultimately leading to the death of the bacterial cell.

Antibiotic testing can be useful in both clinical and research contexts. This sort of testing will assist clinicians in determining which antibiotics are most suitable for treating a patient and the appropriate dosage level.

In research, this test is useful for creating novel antibiotics and determining how effective the candidates are against certain target bacterial strains.

The most commonly used analysis method is disc diffusion, which is a quick and cost-effective way to assess antimicrobial activity against a target organism. This method was progressively developed during the 20th century, but by the 1950s, it was clear that it needed to be standardized.

This standardization can be attributed to W. Kirby and A. Bauer, whose invention was recognized by the World Health Organization in 1961 and was appropriately named the Kirby-Bauer method.

Microbiology and the zone of inhibition

Image Credit: Synoptics Ltd

The process entails infusing small paper discs with a particular antibiotic before placing them on an agar plate that has been completely distributed with a bacterium of interest using a sterile swab. Common methods of arranging the discs are:

  • Six arranged in a ring formation on a 90 mm petri dish
  • In a grid formation on square dishes

Different antibiotic discs can be utilized on a single plate, saving time, money, and resources. Once the discs are put on the plate, the antibiotic starts to diffuse across the agar.

As the antibiotic diffuses deeper across the agar, its concentration decreases. The plates are subsequently incubated for around 18 to 24 hours, depending on the bacteria present, the antibiotic employed, and the laboratory protocols.

Once the dishes have finished incubation, the plates can be examined. The “zone of inhibition” refers to the region around each antibiotic disc where the antibiotic inhibits bacterial growth. The concentration decreases as the antibiotic diffuses further from the disc.

At a certain point, the antibiotic will become so weak that it can no longer hinder the bacterium’s growth, resulting in a visible barrier at the edge of the zone. The diameter of the zone indicates how sensitive the bacterium is to the antibiotic.

The larger the zone, the more powerful the antibiotic is against that particular bacterium. If there is no zone surrounding the antibiotic, it indicates that the bacterium is resistant to the antibiotic.

It is possible to compare the zone sizes to a standardized chart to establish if the bacteria are sensitive, resistant, or have intermediate sensitivity to the bacterium.

Most laboratories currently measure inhibition zones by manually assessing the zones using calipers or rulers.

This approach can be time-consuming and imprecise, especially when the standardized charts for comparing zone sizes are extremely specific, with narrow margins between bacteria that are sensitive or not sensitive to antibiotics.

Synbiosis can provide clients with products that make measuring the inhibition zone quick, straightforward, and precise, with repeatable results.

The ProcScan is the best choice for measuring the inhibition zone because its 600 dpi scanning technology provides crisp and high-quality images. Combined with the ProtoCOL software, it can count multiple zones simultaneously with great precision, measuring each zone to within 50 µm within seconds.

Both the ProtoCOL 3 and the ChromaZona can measure inhibition zones, providing the customer with a selection of devices to meet their specific requirements and budget.

Zones can also be manually altered by a user after the initial measurement with x500 zoom on the picture, giving a user total control over where the inhibition zone ends.

Microbiology and the zone of inhibition

Image Credit: Synoptics Ltd

Synoptics also offers additional software modules, such as “Antibiotic Susceptibility Testing”, which incorporates EUCAST and CLSI guidelines, enabling a user to choose the antibiotic and bacterium during setup so that the results can be compared against these breakpoints and give a “Sensitive”, “Intermediate”, or “Resistant” result, making the process much more efficient and streamlined.

These devices comply with 21 CFR part 11 and have audit trails for a variety of fields, including user log-ins and manual plate edits.

The data of the inhibition zones can be converted into customizable reports in PDF, Excel, or CSV format using the ProtoCOL software to meet your specific requirements. They can also be integrated into a LIMS to provide easily accessible results.

About Synoptics Ltd

Synoptics is a prominent Cambridge-based company that has been at the forefront of manufacturing scientific-grade digital imaging systems, water purification systems, and vacuum ovens for over 30 years. The company is comprised of three divisions: Syngene, Synbiosis, and Fistreem, each specializing in specific areas of expertise. With a strong commitment to innovation and quality, Synoptics has established itself as a trusted provider of cutting-edge solutions in the scientific research and laboratory equipment industry.


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Last updated: Jun 20, 2024 at 4:32 AM

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