Bacitracin sensitivity test, Taxos A - Introduction, Purpose, Principle, Procedure, Result, Interpretation

Introduction to Bacitracin (sensitivity test)

Bacitracin, a key component in the bacitracin sensitivity test/Taxos A, is a polypeptide antibiotic naturally produced by the bacteria Bacillus licheniformis. It works by interfering with the cell wall and peptidoglycan synthesis of the gram-positive bacteria, hence narrow-spectrum.

Despite its valuable properties, it is only used as a topical antibiotic because if used internally, it is nephrotoxic and can cause kidney damage. Even in cases of topic use, there have been reports of hypersensitivity, and allergic or anaphylactic reactions.

History of Bacitracin (sensitivity test)

Bacitracin, a key component in the bacitracin sensitivity test/Taxos A, was isolated by the bacteriologist Balbina Johnson at the Columbia University College of Physicians and Surgeons. FDA approved the antibiotic in 1948.

Bacitracin sensitivity test (Taxos A)

Purpose of Bacitracin sensitivity test/Taxos A

Aside from its use in medicine, Bacitracin has also been widely used to differentiate several micro-organisms i.e. to distinguish Streptococcus pyogenes from other β-hemolytic streptococci.

S. pyogenes is sensitive to bacitracin while other species of the genus are resistant to it.

Another purpose of the bacitracin sensitivity test/Taxos A is to identify Staphylococcus spp (resistant) from Micrococcus spp.(sensitive) and Haemophilus influenza (resistant) from other respiratory microflora.

Principle of Bacitracin sensitivity test/Taxos A

Bacitracin inhibits bacterial cell wall synthesis by interfering with the dephosphorylation of the lipids C₅₅-isoprenyl pyrophosphate and bactoprenol pyrophosphate. These lipids have the function of transporting the building blocks of the peptidoglycan bacterial cell wall outside the inner cell membrane.

Disk impregnated with 0.04 units of bacitracin is used for the bacitracin sensitivity test/Taxos A.

Procedure of Bacitracin sensitivity test/Taxos A

The steps/procedure of the bacitracin sensitivity test/Taxos A is as follows:

1. Around 3-4 wells, isolated colonies of the test colonies are picked up by inoculating loop.

2. The isolate is streaked on trypticase soy agar (TSA) + 5% sheep blood agar plate. The streaked culture must be spread evenly like a lawn.

3. With the help of sterile forceps, the bacitracin (0.04 units, 6mm) disc is placed onto the streaked surface and pressed on the disk gently to adhere the disc to the agar plate.

4. The culture is incubated at 35°C-37°C for 18-24 hours for staphylococci and in 5-10% CO₂ for streptococci and Haemophilus influenza.

5. ZOI is observed around the bacitracin disc.

Result of Bacitracin sensitivity test/Taxos A

The result of the bacitracin sensitivity test/Taxos A can be interpreted as:

Positive

If the zone of inhibition (ZOI) is equal to or greater than 10mm, then the organism is susceptible.

Negative

If the zone of inhibition (ZOI) is less than 10mm, then the organism is resistant.

* S. pyogenes is susceptible

Staphylococcus spp are resistant

Haemophilus influenza is resistant

Micrococcus spp. are sensitive