How to Determine Which Bacteria Agar to Use

Assess the risks involved with different versions of agar. Neomycin agar is known to breed Streptococcus bacteria, meaning you could get sick very easily if you mishandle it. You should also avoid Thayer-Martin agar, which is specifically designed to grow Neisseria Gonorhhoeae, the bacteria that causes gonorrhea. You'd be better off using Luria Bertani (LB) agar, or a variation known as Miller's LB agar. These agars provide a ready habitat for benign bacteria, meaning they are ideal for high school-level research. Nutrient agar is also safe, but it can grow fungi as well and not all bacteria thrive in it as readily as they do in LB agar. Any other agars discussed in this article are not suitable for students below the collegiate or graduate level.

Consider what type of bacteria you want to breed. Some agars are designed with a specific bacteria in mind. MacConkey agar will only grow gram-negative bacteria and is useful if you are studying the metabolic activity of E. Coli bacteria, which create an acidic environment when they process sugars. A pH marker will indicate the metabolic rate of these organisms. If you are trying to grow Haemophilus or Neisseria, which require specific conditions to grow, you may want to go with chocolate agar, which is enriched with sheep's blood and provides a suitable environment for those species.

Determine what mode of inoculation you will be using, as some agars are designed for a particular inoculate. Xylose lysine deoxycholate (XLD) agar inhibits gram-positive bacteria and encourages gram-negative bacterial growth. Microbiologists use XLD agar when working with stool samples because colonies of lactose fermenters turn yellow in XLD agar. If you are worried about fungal contamination, Sambouraud agar inhibits fungal growth, though it also contains gentamicin, an antibiotic used to treat gram-negative infection, which may inhibit some bacterial growth.