Bacteria in the Media?!
What comes to mind when you hear the word "media"? Do you think of humorous television shows, intriguing news blogs, or celebrity photos posted online? Maybe you think of various art supplies, such as paint or watercolor. Well, you may be surprised to learn that "media" also refers to food for microbes! Yes, despite the fact that microbes are so tiny, they are living organisms and they need nutrients to fuel their reproduction. While all microbes need fuel, let's focus on bacteria for this article. Bacteria need various amounts of chemical elements, as well as growth factors. They need "building blocks" in the greatest amount: carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur, which you can remember by the acronym CHNOPS. In smaller quantities, microbes need mineral nutrients (such as) and micronutrients (such as magnesium or calcium). Growth factors are compounds such as vitamins or amino acids; different bacteria require different growth factors.
Creating media (singular medium) in the lab is a great way to give bacteria their much-needed nutrients. By now you may be wondering what do these media even look like? We can make solid agar gelatin, used in plates or tubes. We can also make a liquid broth and place it in tubes. We then place our bacteria on the plate or in the tube, allow it to incubate at a given temperature, and let the bacteria multiply to large numbers. At first, we won't see the bacteria in our medium. After 24-48 hours, we should see colonies (clumps of bacterial cells) on our plate or solid tube. We may see turbidity (cloudiness from bacteria) in our broth tube.
We can create media from many different recipes. For what purpose might we use different recipes to feed our bacteria? It depends on the experiment and what we are trying to learn. Nutrient, MacConkey, Mannitol salt, and sheep blood are some examples of agar used for plates. Selective media (such as MacConkey agar) select for certain types of bacteria, by including chemicals that don't let the other types grow. This is useful if we want to study a certain type of bacteria. As a loose analogy, imagine that you took a large group of friends out to eat. Unfortunately, half of them were allergic to a menu ingredient, so they had to leave (of course this situation is good when studying bacteria, but terrible in the case of people!).
General purpose media, such as nutrient agar, provide nutrition for a wide range of organisms. Remember your group of friends? Now you brought them to a diverse buffet with all kinds of food that accommodates their needs: everyone can eat and everyone stays. Or should I say.... you all found a happy medium? This kind of media is useful when you collect an environmental sample, such as a soil sample, and you'd like to cultivate any bacteria present. You probably don't know which bacteria are in that soil, so you'd have no idea what specific nutrients they need or what would prevent their growth. Therefore, you can play it safe with general purpose media to give any bacteria a chance to grow.
Differential media allow us to differentiate the bacteria's species or capabilities, based on color changes or other visual changes. Media can be both differential and selective, such as Mannitol salt agar. They may be differential but not selective, such as sheep blood agar. Both of these differential agars are useful in medical diagnostic tests! Mannitol salt agar selects for Staphylococcus species by adding a lot of salt that other bacteria would not appreciate; S. aureus, the most dangerous species, will turn a distinctive yellow. If you had a patient with a skin infection, you could swab their skin and rub the swab on a Mannitol salt agar plate. This would help narrow down which bacteria are on their skin, and what could be causing their infection! Sheep blood agar differentiates between different types of hemolysis, or how the bacteria break down blood cells. As the agar contains blood, we see hemolysis in action! Alpha hemolysis will produce a partial clear zone with some discoloration. Beta hemolysis will result in a wide zone of clearance. If the bacteria do not break down blood cells (aka gamma hemolysis), a clear zone will not be present. This plate is a useful beginning step in determining whether a patient has strep throat, which is caused by Streptococcus pyogenes; bacteria of the Streptococcus species are known for hemolysis. You would have to perform further tests to identify the exact species.
