What is a culture and sensitivity test?
Also known as sensitivity testing, C & S, or antimicrobial susceptibility, a culture and sensitivity is a diagnostic test to determine which antibiotic will be the most effective against a microorganism (bacteria or fungi).
The veterinarian obtains a sample from the infected site which may include:
- Throat swabs
- Discharge (sores, eyes, vagina, anus, lesions or wounds etc.)
- Spinal fluid
The sample is sent to a laboratory where it is smeared over a petri dish on a medium (agar). Agar promotes the growth of bacteria or fungi. The petri dish is incubated to allow the microbes to grow and form a lawn.
Choosing the most effective antimicrobial drugs against bacteria or fungi is important to ensure the pathogens can be quickly killed on or in the host.
Once the bacteria or fungi have been cultured, discs made from filter paper that have been impregnated with antimicrobial drugs are placed on the bacterial or fungal lawn.
When bacteria or fungi are sensitive to a particular antimicrobial, a plaque or zone of inhibition will form around the disc, where the bacteria or fungus have been killed. This helps to determine which antimicrobials are most effective.
The laboratory will issue a report to the veterinarian with a list of antibiotics and one of the following next to each drug:
- R (resistant) – The bacteria or fungi can grow and is resistant to a particular microbial.
- S (sensitive) – This means the bacteria or fungi can’t grow if a particular microbial is present.
- I (intermediate) – The microbial is partially effective; however, a higher dose is necessary.