2.1.1 Processing blood cultures
Blood cultures are inoculated directly from the patient into sterile liquid media. These media may be locally made, or commercial culture bottles can be purchased. The bottles are incubated in the laboratory, observed and sub-cultured. They are plated out onto agar plates for further work to determine if the blood culture is positive. Sub-culturing on a mix of media types is necessary to isolate most of the organisms likely to cause bloodstream infections.
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Which types of organisms would you expect to isolate when sub-culturing onto blood agar in atmospheric oxygen, MacConkey in atmospheric oxygen, and chocolate agar in CO2.
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With these culture media and conditions you should be able to isolate the main pathogens causing bloodstream infection. The chocolate agar and the CO2 will allow
fastidious organisms like Streptococcus pneumoniae and Neisseria species to grow. Most other organisms will grow well on blood agar in atmospheric conditions. Colonies can be taken from this for the next stages of testing. MacConkey is used to help in the initial identification of Gram-negative organisms (see Section 4.3).
There are also a number of automated systems in common use, such as BD BACTECTM, or BacT/ALERT® (Biomerieux) which use pre-prepared bottles (Figure 2). These can monitor the samples for growth and indicate when it is time to do the sub-culture. Although the equipment is expensive, the risk of contaminants being introduced is reduced and turnaround times are usually faster too, as the subculture can be set up as soon as the equipment indicates positive growth, instead of once daily as is usual for manual blood cultures.
2 Basic tests used to isolate and identify bacterial pathogens
