What is a Microbial Culture?
A microbial culture is one of the diagnostic methods of microbiology and is used to determine the cause of an infectious disease by allowing the microorganism to multiply in certain specific media under controlled laboratory conditions. This helps to determine the type as well as severity of the infection by doing a colony count of the organism in the specimen.
Types of Specimen for Microbial Culture
Microbial infections may be caused by bacteria, viruses, fungi and parasites and the specimen to be collected for culture depends on the signs and symptoms of the disease. The correct amount of specimen should ideally be collected before starting treatment with an antimicrobial agent, like an antibiotic, and once extracted, it should be transported immediately to the laboratory for culturing. Special precautions should be taken to avoid contamination of the specimen by organisms in the surrounding skin, mucus membrane and air, during collection. Sensitivity tests may also be done to determine the relative susceptibility of individual microorganisms to certain drugs.
The types of specimen that may be used for culture are :
- Blood (Refer to Blood Culture)
- Sputum (Refer to Sputum Culture)
- Cerebrospinal fluid (CSF)
- Wound exudates
- Respiratory secretions
- Throat swab
- Urethral discharge
- Vaginal or cervical discharge
- Skin scrapings
- Exudates or pus from the ears and eyes
- Bronchoscopic culture
What is a sputum culture?
The material which is coughed up from the lungs and then spat out or expectorated is called sputum or sometimes referred to as phlegm. A sputum culture is done to identify the microorganism causing lower respiratory tract infections such as pneumonia and tuberculosis. A fever with a chronic cough , along with blood or pus-like material in the sputum, is usually an indication for undertaking a sputum culture.
Collection of Sputum Specimen
The sputum should be collected in a sterile container, preferably early in the morning before eating or drinking anything. The mouth should be rinsed with water to rinse out bacteria from the mouth and dilute the saliva which may contaminate the specimen. With a forceful cough, the sputum should be spat out into the sterile container immediately, avoiding prolonged collection in the mouth cavity. Three consecutive samples may have to be collected if testing for tuberculosis.
A special stain called the acid-fast stain may be done in the laboratory to identify the tuberculous bacilli. Different types of microorganisms may be identified using gram stain. A fungal culture may be done if a fungal infection is suspected and a viral culture is done to detect viral infection such as pneumonia. Due to the prevalence of bacterial respiratory tract infections, most sputum samples are first tested for bacteria.
Sputum Culture Report
The initial report indicating the presence of any bacteria may be available on the same day. The final report will take one to three days and this will include identification of the specific type and quantity of bacteria as well as the antibiotics most effective against it. Culture for tuberculosis may take two to four weeks.
Reports may take several weeks.
It may take several days to several weeks to get the report depending on the type of virus present.
What is a Blood Culture?
This test is advisable when there are symptoms of blood infection such as septicemia or bacteremia and blood culture is done to identify the bacteria or other microorganism. A blood microbial culture will be requested by your doctor depending on your case history and clinical findings upon physical examination.
Collection of Blood Specimen
About 10 ml (0.3 fl oz) of blood is withdrawn with a needle and syringe from a vein, either on the inside of the elbow or back of the hand, and collected in two vials. It is sent to the laboratory where it is placed on a petri dish with agar. This is the ideal medium for bacterial growth and after incubation, it will indicate clear areas of bacterial growth if there was bacteria present in the blood specimen.
A blood culture is done mainly for bacteria and if any growth is seen, further tests are done to identify the type of bacteria. A gram stain may be done using special types of stain to identify different organisms. Certain microorganisms pick up and retain the purple color and are called gram-positive organisms, while other organisms lose the purple stain and retain the red stain and are called gram-negative. No special preparation is needed before a blood culture and there is not much discomfort except for the needle prick.
A normal value means that no organism could be grown in the laboratory and hence there is no infection in the blood, while a positive result will show the growth and multiplication of organisms in the specimen. Treatment can then be commenced according to the micro-organism identified and the sensitivity or action of various drugs on it. Three blood cultures may need to be done to confirm results because of variable bacterial infection in the blood.
Blood Culture Report
The initial or preliminary report will indicate if any bacteria have been found and results should be available within a day, depending on the country or laboratory. The final report will show the type of bacteria and its sensitivity to antibiotics, and this may be available after five to seven days.
Article reviewed by Dr. Greg. Last updated on September 10, 2012