Culture is a microbiological test the purpose of which is to detect and identify microorganisms in blood, urine, feces or other biological material. Inoculation is a multi-stage process and is only performed in justified cases. Check what are the indications for culture and what the test is about.
Culturetomicrobiological testperformed to detect and identify pathogens in samples of biological material.
Due to the type of pathogens tested, thebacteriological culture(bacteriological test) andmycological culture(mycological test) are distinguished.
Due to the type of biological material, there arecultures of blood , urine, feces, semen, sputum or vaginal discharge. Biological material can also be cerebrospinal fluid, swabs from the throat, ear, nose, oral mucosa, and even from the conjunctival sacs of the eyes, from pustules and other skin lesions, as well as from wounds, fistulas or abscesses, etc.
Culture - indications for the test
Culture is performed when an infection is suspected, i.e. when the patient develops symptoms. For example, indications for urine culture will be symptoms of a urinary tract infection such as low back pain or frequent and painful urination.
In turn, blood infection can be suspected when fever and chills, rapid breathing and heart rate, drop in blood pressure, impaired consciousness or oliguria appear. However, stool culture is performed in a patient who has diarrhea, abdominal cramps and / or blood or mucus in the stool.
Cultivation of urine and vaginal discharge should be performed in pregnant women - even in the absence of symptoms of infection - to exclude the presence of microorganisms that may pose a threat to the baby.
Cultivation - collection of biological material
First, you must properly collect a sample of biological material. For blood cultures, at least two blood samples must be taken from two different veins, into two test tubes containing two types of culture medium (capable of detecting both aerobic and anaerobic bacteria).
In turn, the biological material from the vagina is collected with a special swab - one swab is taken from thethe atrium of the vagina and the other around the anus. Then the swabs are also placed on a special substrate (the so-called transport substrate). On the other hand, urine culture is carried out on the first sample, urine after the night (then the number of bacteria is the highest), which is collected in a sterile container using the so-called mid stream.
ImportantTell your doctor about all medications you are taking (especially antibiotics and drugs that affect blood clotting), medical conditions, and possible carriers of blood borne pathogens (e.g. HBV, HCV, HIV) prior to inoculation.
Cultivation - what is the test?
STAGE 1 - SEEDING
The first stage is the seeding of biological material, i.e. applying a sample of the material to the so-called culture medium (microbiological). It is a mixture of carefully selected nutrients that allow the growth and multiplication of microorganisms. Blood-enriched growth media are the most commonly used as they provide optimal conditions for the growth of most species of pathogens.
The inoculation can be done in several ways: on a liquid medium, on an agar slant, but most often it is performed on Petri dishes (it is a laboratory vessel, a cylindrical stand with a wide, flat bottom), i.e. a bit of the tested material is spread in a zigzag pattern or radially over the entire surface of the substrate, or they are divided into sectors and the material is spread over them.
inoculation is usually done under a laminar chamber, which allows for keeping aseptic conditions. The chamber protects against the ingress of bacteria or fungal spores, which are constantly floating in the air outside the chamber.
This is the case, for example, in the case of urine culture. In the case of blood culture, this step is omitted as the media is introduced into sterile bottles or test tubes into which blood is directly collected. The same applies to swabs, which are placed on a special medium immediately after collection and immediately intended for breeding.
STAGE 2 - BREEDING
After applying the sample to the substrate, it is placed in an incubator, where the temperature corresponds to the temperature of the human body. These are the conditions that lead to the multiplication of microorganisms. Stimulation of the growth of the desired microorganisms can also be obtained by selecting the appropriate pH and oxygenation. Cultivation of pathogens lasts from 24-48 hours (some species of bacteria) to even several dozen days (some fungi).
STEP 3 - INSULATION
The goal of this stage is to isolate a specific species of pathogen. In the case of isolating microorganismsa bacterial suspension of a specific density is made, which will be used to identify and determine the drug susceptibility of the strain.
STEP 4 - IDENTIFICATION
Currently, biochemical, manual or automatic tests are used to identify microorganisms. An example of a biochemical test is the API test. It consists in introducing the bacterial suspension to each of the microtubes included in the set. The next stage is the cultivation of microorganisms at the right temperature. The biochemical reactions that take place during this time will cause color changes, either spontaneously or after the addition of the indicator reagent. A code book or a special computer program is used for reading.
The microbilogical diagnostics is supplemented with a microscopic specimen which enables the observation of pathogen cells after their prior staining.
STEP 5 - ANTIBIOGRAM
After the inoculation is completed, an antibiogram is performed to assess the susceptibility of the pathogen to the drug. The microbiologist places discs with antibiotics of specific concentrations on a suitable substrate with the bacterial suspension applied (the so-called antibiogram). Antibiogram readings are made by measuring the size of the zones of inhibition of growth around the discs containing the antibiotic.
Based on the results of the microbiological examination and the antibiogram, the doctor decides about the appropriate treatment.
