BLOOD CULTURE TECHNIQUE

 

Blood Culture Overview

Blood culture is a laboratory test used to determine if microorganisms have invaded the bloodstream, this condition is known as bacteremia. An infection can spread to the bloodstream and become a systemic infection known as septicemia. The clinical syndrome is characterized by fever, chills, malaise, and tachycardia.    

Bacteremia may be transient, intermittent, or continuous, reflecting several mechanisms by which bacteria enter the bloodstream.

Transient bacteremia occurs when organisms, often members of the normal community of microorganisms are introduced into the blood by minimal trauma to membranes.

Intermittent bacteremia occurs when bacteria from an infected site are periodically released into the blood from an extravascular abscess, spreading infections of body cavities like empyema, peritonitis, or septic arthritis.

Continuous bacteremia occurs when the infection is intravascular (Infective endocarditis, Intravenous catheter-associated bacteremia).

What Causes Blood Infection?

1.    Enteric fever

2.    Infections occur in cases of injury, dental surgery, vigorous chewing, and instrumentation of genitourinary tract/bowel.

3.    As a result of spillover effect

4.    Needle stick injury

Common agents

1.    Enterococci

2.    Staphylococcus aureus

3.    Staphylococcus sp. (Coagulase Negative Staphylococcus sp.)

4.    Streptococcus sp.

5.    Viridians streptococci

6.    Enterobacteriaceae

7.    Pseudomonas sp.

8.    Candida sp.

Terminologies

BACTERAEMIA: Presence of bacteria in the blood.

SEPTICAEMIA: (Sepsis) - Poisoning of blood by bacteria.

ENDOCARDITIS; Inflammation of the inner lining of the heart's chambers and valves.

ANEURYSM: An abnormal bulge or ballooning in the wall of a blood vessel.

THROMBOPHLEMITIS: inflammation of the wall of a vein with associated thrombosis, often occurring in the legs during pregnancy.

Purpose of the blood culture

To detect bloodstream infection, a patient’s blood must be obtained by sterile venipuncture and incubated in culture media. The bacterial growth can be detected in blood culture, the organisms is isolated, identified and tested for its susceptibility to various appropriate antimicrobial agent.

Skin Contamination

Skin contaminants occurs commonly in during the blood sample collection, the following bacterial are common skin contaminants,

• Coagulase negative Staphylococci sp.
• Micrococci
• Corynebacterium
• Bacillus

MEDIA AND ADDITIVES USED IN BLOOD CULTURE

Media:

BASIC BOTTLE: Nutrient broth + anticoagulant

Blood culture bottles: Trypticase soy broth, brain heart infusion broth, supplemented peptone, thioglycolate broth.

Specialized broth: Includes Columbia or Brucella broth.

Additives:

Hypertonic medium: Osmotic stabilizers, sucrose, mannitol, sorbose.

Isotonic medium: no osmotic stabilizers

Resin: Inactivates activity of antibiotics and enhance isolation of Staphylococcus- in patients on bacteriostatic drugs.

CONVENTIONAL BLOOD CULTURE

Sample collection

1.    Blood should be collected before medication.

2.    To increase the chance of isolation at least two specimens should be cultured.

3.    A strict aseptic technique should be used for sample collection.

4.    A sterile syringe and needle used to withdraw 5 ml from adult and 2-3 ml from child (1-12 yrs)

5.    Dispense the drawn blood into 50ml /20 ml of enriched culture media (i.e., Brain Heart Infusion, tryptic or trypticase soy, supplemented peptone and etc.) for aerobic culture method.

6.    Wipe the top of culture bottle or protective cover.

Procedure

1.    Blood culture bottles should be incubated 35°C for 6 to 18 hours and after incubation, examined visually for evidence of growth (i.e., hemolysis and turbidity).

2.    Then Gram Stain is performed from the Broth and Subculture on Blood agar incubated aerobically at 35°C for 48 hours.

3.    After 48 hours of incubation, the culture-negative bottles are then re-incubated for 5-7 days.  If the growth is detected in the bottles, organisms are isolated, identified, and tested for their susceptibility.

  

Result interpretation

Negative Culture: No growth in culture is based on 5-7 days of incubation of the specimen. 

Positive culture: The growth of organisms in culture and Isolate is identified and reported with antibiotic sensitivity.