This is Jennifer. On the forth week of my SIP, I was posted to the blood culture station.



Blood Culture

The BACTEC Fluorescent Series
If bacteria or fungi grow in a medium that provides the essential nutrients which enhances their growth, they may cause haemolysis of red blood cells if the medium is blood. They may also cause the medium to be turbid or producing gaseous by product such as carbon dioxide production.
The BACTEC Fluorescent Series detects the production of carbon dioxide by the increasing numbers of bacteria in the blood.

If bacteria or fungi are present in the blood or other body fluids, they will break down the nutrients in these medium and release carbon dioxide. The dye in the sensor will react with the carbon dioxide produced. This reaction affects the amount of light that is absorbed by a fluorescent material in the sensor. The instrument’s photo detector measures the amount of fluorescence which responds proportionally to the amount of carbon dioxide produced by bacteria or fungi.

The instrument begins continuous automated testing and the vials’ fluorescent sensors are activated by the LED (Light Emitting Diodes) behind the vials that laminated the racks. The instrument’s detector would start taking the readings after a warm up period. The vials will be read by the detector every ten minutes and the cycle for all the racks inside the incubator will be finished after every ten minutes. If there is a positive culture, the indicator light will light up in front of the instrument and also shown on the monitor and the alarm will sound. After the positive vial was detected, the medical technologist in charge will remove the vial from the incubator and send them for blood culture for the isolation and detect the identity of the organisms present in the blood or medium.

Steps for the positive culture to be lighted up or identified:
1. Carbon dioxide was released by bacteria
2. Carbon dioxide reacts with dye in the sensor
3. LED, modulated by dye, activates fluorescent material in sensor
4. Increasing amount of fluorescent is read by photo detector
5. Raw data from detector is sent to rack microprocessor
6. To where positivity analysis is performed
7. Positive vial lamp illuminates, audible alarm sounds, positive stations are displayed


Usually, when blood specimens or other body fluids are received such as stem cells, peritoneum fluid from renal, and dialysis fluid, it comes in two bottles where one is to detect for present of aerobic bacteria while another is for anaerobic bacteria. But in some cases if there is a need to detect for the present of fungi, a third bottle will be sent together with the two bottles.

For aerobic bacteria, the cap of the bottle is in blue colour while gold colour for anaerobic bacteria.
For fungi or mycobacterium, it is in red colour.
Below is a picture that shows the BACTEC blood culture bottles with different cap colours for different types of purposes:



Procedures
The BACTEC bottles or vials are inserted into the rackets of the incubator and the racks constantly incubated at 35 degree Celsius and are agitated throughout the 5 days.
For aerobic and anaerobic bacteria, the vials are read every 10minutes by the detector and the bottles are discarded after 5 days if no positive are flag on the bottles.
In some cases such as patients with Endocarditis or Brucella, the bottles are tied with red or blue ribbons before incubated and cannot be thrown away even if flagged negative.
For positive vials, immediately being removed from incubator and sent to for blood culture.