Automated blood culture systems are the primary choice for the detection of pathogens from blood specimens. Instead of manually reading broth-based conventional methods, laboratories use continuously monitored, automated blood culture systems for the processing of blood culture specimens nowadays. These systems have increased the yield of blood cultures, reduced the time to organism recovery, and diminished the laboratory technologist’s hands-on time.
Several automated blood culture systems are available commercially. Some of the most popular continuous-monitoring blood culture systems are:
- BACTEC (Becton Dickinson and Company),
- BacT/ALERT (bioMérieux), and
- VersaTREK (Thermo Scientific)
The BD BACTEC™ Automated Blood Culture System utilizes fluorescent technology to detect organisms’ growth in the blood culture bottles.
The sample to be tested is inoculated into one or more vials and inserted into the BD Bactec fluorescent series instrument for incubation and periodic reading. Each vial contains a chemical sensor that can detect an increase in carbon dioxide (CO₂) produced by the growth of microorganisms.
The instrument monitors the sensor every ten minutes for an increase in its fluorescence, which is proportional to the amount of CO₂ present. A positive reading indicates the presumptive presence of viable microorganisms in the vial.
Composition of Bactec Blood Culture vials
All the BACTEC culture vials contain soybean-casein digest broth, yeast, amino acids, sugar, vitamins, and sodium polyanetholsulfonate. The resin containing vials contains nonionic adsorbing resins and cationic-exchange resins. Sodium polyanetholsulfonate (liquiod) is an anticoagulant that prevents bacteria’s killing by innate cellular and humoral factors. Resin neutralizes the effect of antibiotics present in the blood and increases the chances of recovery of the pathogen.
The composition of BACTEC™ Plus Aerobic/F culture vials is as follows:
|List of Ingredients||BD BACTEC™ Plus Aerobic/F|
|Processed Water||30 mL w/v|
|Soybean-Casein Digest Broth||3.0%|
|Sodium Polyanetholsulfonate (SPS)||0.05%|
|Nonionic Adsorbing Resin||13.4%|
|Cationic Exchange Resin||0.9%|
Types of Bactec Blood Culture Vials
Depending on the type of suspected pathogen, several broth formulas are available, including media for the recovery of aerobic and anaerobic bacteria, mycobacteria, and fungi. No single type is optimal for all microorganisms. BACTEC Plus Aerobic/F, Plus Anaerobic/F, and Peds Plus/F bottles contain resins.
|Name of the medium||Use|
|BD BACTEC™ Plus Aerobic medium||All-purpose medium capable of supporting the growth of aerobic and facultative organisms.|
|BD BACTEC™ Plus Anaerobic medium||All-purpose medium for common obligate anaerobic and facultative organisms.|
|BD BACTEC™ Peds Plus media||Accommodate small volume and optimally detect common pediatric and non-pediatric organisms|
|BD BACTEC™ Lytic Anaerobic medium||Increase the detection and recovery of anaerobes. Contains detergent to lyse RBCs and WBCs.|
|BD BACTEC™ Standard Aerobic medium||Recovery of bacteria and yeast from the blood.|
|BD BACTEC™ Standard Anaerobic medium||Recovery of anaerobic organisms from the blood.|
Key points for Lab diagnosis of Blood Stream Infections
- Draw blood for culture before initiating antimicrobial therapy
- Volume of blood collection: Because the number of bacteria per milliliter of blood is usually low, it is important to take a reasonable quantity of blood: 20 ml per venepuncture for adults, 2–5 ml for children/infants, and 1–2 ml for infants and neonates.
- Use a 2–3 bottle blood culture set for adults, at least one aerobic and one anaerobic; use 1–2 aerobic bottles for children.
- Timing of blood collection: Whenever possible, blood should be taken before antibiotics are administered. The best time is when the patient is expected to have chills or a temperature spike. It is recommended that two or preferably three blood cultures be obtained, separated by intervals of approximately 1 hour.
Note: Overall volume of blood cultured is more critical to increase organism yield than the timing.
- Disinfect the venipuncture site with 2% tincture of iodine, 10% polyvidone iodine, 70% alcohol, or 0.5% chlorhexidine in 70% alcohol chlorhexidine. (chlorhexidine is NOT recommended for children <2 months old).
- Catheter-drawn blood cultures have a higher risk of contamination (false positives).
- Use of Anticoagulant: Anticoagulant such as sodium polyanethol sulfonate (SPS) recommended if the blood is not added to sufficient volume of broth (immediately). SPS also inhibits the antibacterial effect of serum and phagocytes. If the blood is immediately added to a sufficient volume (50 ml) of broth and thoroughly mixed to prevent clotting, no anticoagulant is needed.
- Do not submit catheter tips for culture without an accompanying blood culture obtained by venipuncture.
- Never refrigerate blood prior to incubation.
- A true infection of bloodstream is suspected in the following conditions:
- if the same organism grows in two bottles of the same blood specimen;
- if the same organism grows in cultures from more than one specimen;
- if growth is rapid (within 48 hours);
- if different isolates of one species show the same biotypes and antimicrobial- susceptibility profiles.
- Remove the flip-off cap from the BD BACTEC vial top and inspect the vial for cracks, contamination, excessive cloudiness in the medium, and bulging or indented septum. DO NOT USE if any defect is noted.
- Before inoculating, swab the septum with alcohol (iodine is NOT recommended).
- Aseptically inject or draw 8–10 mL of specimen per vial directly. If the sample volume is decreased, recovery will not be as great as with larger volumes (Find more in Blood Culture: Indications, Timing, and Volume)
- Place inoculated vials in the BD BACTEC fluorescent series instrument as soon as possible for incubation and monitoring.
- Vials entered into the instrument will be automatically tested every ten minutes for the duration of the testing protocol period. Positive vials will be determined by the BD BACTEC fluorescent series instrument and identified as such (see the appropriate BD BACTEC Fluorescent Series Instrument User’s Manual).
- Remove the positive vials, subculture in an appropriate solid culture medium (Chocolate Agar, Blood Agar, and MacConeky Agar), and Gram-stain. Gram stain will provide a necessary preliminary report that can help physicians make appropriate decisions while waiting for the culture report.
- If the growth is observed in the solid culture medium, note the colony morphology, do the necessary testing to identify the isolate, and perform an antimicrobial susceptibility test.
Working Principle of BD Bactec Automated Blood Culture System
When microorganisms are present in the test sample inoculated into the BD BACTEC vial, they metabolize nutrients in the culture medium, releasing CO₂ into the medium. A dye in the sensor at the bottom of the culture bottles will react with CO₂. This modulates the amount of light absorbed by a fluorescent material in the sensor. The instrument’s photo-detectors measure the fluorescence level, corresponding to the amount of CO₂ released by the organisms.
Analysis of the rate and amount of CO₂ increase enables the BD BACTEC fluorescent series instrument to determine if the vial is positive, i.e., that the test sample contains viable organisms.
References and Further Reading