Sometimes, it’s not the high end technology that saves lives, but the cheaper low end stuff. One of the ways microbiologists identify different bacteria is by growing them on ‘defined’ growth media. We have built up a vast knowledge of which growth conditions will support particular bacteria (and what those bacteria will look like on those media).
One of those media is blood agar plates.
A biological sample, such as infected blood, sputum (the gross stuff you hack up), or other disgusting bodily fluid will be cultured on an agar plate that contains a variety of nutrients (the agar, derived from seaweed, is what solidifies the plate into a hard gel–it feels like mushy rubber). Blood agar plates are like regular nutrient plates, except that they’re composed of five percent blood.
What these plates measure is the ability to produce haemolysin, a toxin that destroys blood cells (and, yes, that’s as bad for you as it sounds). If a bacterial colony (a clump of cells visible to the human eye) produces haemolysin, then a ‘halo’ occurs around the colony and the plate, in that region, is no longer opaque, like so:
The two plates have blood cells infected with different types of bacteria. The plate on the left shows a positive staphylococcus infection. The plate on the right shows a positive streptococcus infection and with the halo effect shows specifically a beta-hemolytic group A (click to embiggen).
In developing countries, human blood is used
VAMPIREZ!!!! ZOMG!!! because it’s cheaper than using wool sheep or horse blood. Unfortunately, human blood agar isn’t a very good diagnostic: lots of bacteria, even though they are good at causing sepsis infections (growing in human blood), don’t grow on or lyse human blood cells, making accurate diagnosis difficult. In some cases, streptococcal infections are misdiagnosed as malaria, partly because malarial tests aren’t always effective, and the bacterial cultures don’t indicate a bacterial infection because the cells can’t grow (I have former colleagues funded to work on this problem). Instead of treating with an antibacterial, patients are then treated with an antimalarial, which does nothing to stop the infection.
A recent PLoS paper found that using hair sheep blood, which is cheap and widely available, is just as good as horse or wool sheep blood:
The results of all studies showed that blood agar prepared from citrated hair sheep blood is suitable for microbiological tests used in routine identification and susceptibility profiling of human pathogens. The validation of citrated hair sheep blood eliminates the labor-intensive and equipment-requiring process of manual defibrination. Use of hair sheep blood, in lieu of human blood currently used by many developing world laboratories and as an alternative to cost-prohibitive commercial sheep blood, offers the opportunity to dramatically improve the safety and accuracy of laboratory diagnosis of pathogenic bacteria in resource-poor countries.
This is a good low-tech solution to a very important problem. It also highlights the importance of Open Access: anyone with an internet connection can find out exactly how to make this media–no expensive journal subscription is needed.
Cited article: Yeh E., Pinsky, B.A., Banaei, N., & E.J., Baron. 2009. Hair Sheep Blood, Citrated or Defibrinated, Fulfills All Requirements of Blood Agar for Diagnostic Microbiology Laboratory Tests. PLoS One. 4(7): e6141. doi: 10.1371/journal.pone.0006141.