Dr. Radhika Raghupathi,

MBBS, MD (Microbiology)
Graduated from Bangalore Medical College in the year 1989.
Post graduation in medical microbiology from Chennai.
Her interests include infectious diseases, anaerobic cultures, and pathogenesis of infections.
She has taught a number of students including nursing students during her tenure at St John’s Medical College Bangalore.

Know your Lab tests:

Culture and Sensitivity Tests What is a Culture and Sensitivity test?When a disease is suspected to be due to an infection, it is important to know which antibiotics (drugs that kill the disease causing organisms) will be effective against the particular pathogen (i.e., disease-causing agent).

For this:

The species (and strain) of bacteria (or other pathogen) must be identified The drugs most effective at inhibiting their growth must be determined. The only reliable way this can be done is a culture and sensitivity test. What is the basis of the test?The test examines for the presence of bacteria, in the hope of identifying it and also attempts to point out to the drugs which are likely to be effective against the infection in life.From where can the material for culture & sensitivity be taken?Cultures may be taken from any infected (or potentially infected) tissue or fluid,

for example:

Urine Sputum Blood Wound fluid or pus Body fluids-CSF (cerebrospinal fluid from spinal cord), Peritoneal fluid (from abdomen), pleural effusion (chest fluid) etc.How the test is performed? A cotton-wool swab, which is like an ear-bud, is used to collect a small amount of fluid from a wound or surface. The swab is transported in a special medium that encourages growth of bacteria.At the bed side, some of this sample may also be transferred onto a glass slide for microscopy and for a preliminary look at the offending organism.For culture, the swab is smeared on a culture medium filled glass plate systematically.
The glass dish may have to be incubated for body temperature conditions. The bacteria begin to grow on the culture plates as small colonies which have different appearances and characteristics based on their species. For the (antibiotic) sensitivity test, small round pieces of special tissue paper containing various antibiotics is placed in another glass dish and the bacteria are inoculated on the dish. Those antibiotics that can kill the bacteria will not allow growth around them in the culture medium.
This way, effective drugs can be identified.It may take 2 to 3 days to actually grow the relevant bacterium in the laboratoryHow long does it take for a culture sensitivity report?About 48-72 hours. What are the precautions to be taken? Avoiding contamination-The specimens must be collected with appropriate precautions in order to avoid contaminating the specimen with other organisms present in atmosphere, on body surface etc.

Proper transport of the specimen Ideally the patient should have a culture sensitivity done before starting any antibiotic therapy in order to avoid interference with results.

1.     Petri dish with medium and bacterial colony growth

2.     Culture and sensitivity plate: Note halos of various diameters around the antibiotic strip dots. The halos are areas wher bacterial growth is inhibited by the antibiotic.Larger tha halo, more effective is the drug against bacteria. No halo means that the bacteria are resistant to that drug.Contributed by: Dr. Radhika Raghupathi MBBS, MD (Microbiology)