In microbiology, there are many different techniques used to identify microbes. Differential media typically provide general information regarding identity.
* Bacterial Identification *
Tools like differential media and stains help microbiologists to narrow down the microbial suspects, but don’t necessarily provide the specific identity of the organism. For example, the Gram stain divides all bacteria into one of two categories, Gram+ or Gram-. This is a very general type of classification. Other techniques, such as the API-20E test strip, are very specific and are used to identify bacteria to the species level.
* Using Media to Identify Bacteria *
Sometimes selective growth media and differential media (such as Blood Agar, MacConkeys and Mannitol Salt) are used to provide clues about a microbe’s identity. Media aren’t used to examine individual bacteria, but rather to focus on the colony (millions of bacteria having arisen through the binary fission of a single progenitor) and the bacteria’s telltale metabolic capabilities that are a result of its specific genetic makeup.
* What is Media? *
A growth medium (plural: media) is a mixture of nutrients, moisture and other chemicals that bacteria need for growth in a laboratory environment. Media can be solid, such as Jell-O-like agar that is poured into the bottom half of a Petri dish, or media can be liquid to allow for bacterial growth in test tubes.
Differential Growth Media
A differential medium distinguishes between different types of bacteria based on some characteristic of the bacteria that is growing on it. Typically there is a color change that results from certain bacterial metabolic products reacting with substances or chemicals that have been added to the media.
* Blood Agar (BAP) *
This high-nutrient growth medium is made with 5% sheep’s blood. Blood Agar is not selective, and will grow a wide range of bacteria. This medium is, however, differential.
Some bacteria produce hemolysins, enzymes that can break down red blood cells (hemo = bood and lyse = to cut). Depending on the type of hemolysin, the red blood cells are either completely broken down (beta-hemolysis), or partially broken down (alpha-hemolysis).
Streptococcus pyogenes is a beta-hemolytic pathogenic bacterium and the causative agent of Strep throat. When plated on Blood Agar, S. pyogenes completely breaks down the red blood cells, leaving the areas of agar around its colonies clear and colorless.
Most normal throat flora either produce alpha-hemolysis, partially breaking down the red blood cells and creating a bruised appearance to the agar, or produce gamma-hemolysis (which means no hemolysis at all), leaving the agar is unaffected.
* MacConkeys & Mannitol Salt Agars *
In addition to being selective, MacConkey’s and Mannitol Salt are also differential. Each has a nutrient additive, lactose (sugar) and mannitol (alcohol) respectively, as well as pH indicators that react when these food products are fermented by bacteria.
If the Gram- organisms growing on MacConkey’s eat lactose, they produce acidic metabolites (wastes) that trigger the neutral red pH indicator. This causes the bacterial colonies themselves to turn mauve or pink in color.
So if there are colonies of bacteria growing on MacConkey’s, it’s understood that they are Gram-. If those colonies are colorless, they are not lactose fermenters. If the colonies have a pinkish appearance, they are lactose fermenters.
Bacteria growing on Mannitol Salt Agar are most likely Staphylococcus. But this media is also differential, containing mannitol and a pH indicator. If the colonies growing on Mannitol Salt Agar are a type of Staph that ferments mannitol, they produce acidic metabolic wastes that turn the originally orange-pink medium a bright yellow.
Pathogenic Staph, like S. aureus, are mannitol fermenters. So if something is growing on Mannitol Salt, one can be reasonably sure that it is Staph, if the media turns yellow, this color change signals the presence of pathogenic Staphylococcus.
* Sources *
Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.