Selective Bacterial Growth 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 suspended in test tubes. Media aren’t used to examine individual bacteria, but rather to grow bacterial colonies; millions of bacteria having arisen through the binary fission of a single progenitor.

* Types of Bacterial Growth Media *

There are generalized media, like Tryptic Soy Agar (TSY), that will grow many different types of microbes. This all-purpose medium is the type of agar most often used to culture bacteria.

There are also specialized media that grow only certain microbes (selective media) and/or that display a color change which provides information about a microbe’s identity (differential media). These selective and differential growth media are useful in identifying general classes of bacteria.

* Selective Growth Media *

Think about what the word selective means. A club is selective if it only allows certain people to become members. A growth medium is selective if it allows only certain types of bacteria to grow, while inhibiting the growth of others. To make a medium selective, specific chemicals are added that are inhibitory to some groups of bacteria.

* MacConkey’s Agar (MAC) *

MacConkey’s agar, a transparent light mauve-colored medium, is selective due to the presence of crystal violet and bile salts. These additives inhibit the growth of Gram-positive bacteria. So if something is growing on MacConkey’s agar, it can only be Gram-negative bacteria. MacConkey’s is also a differential medium that provides information on the general type of Gram- bacteria present.

* Mannitol Salt Agar (MSA) *

This transparent orange-pink medium is selective due to a very high salt content; so high that it creates a hypertonic environment that most bacteria cannot survive in. Members of the genus Staphylococcus are specially adapted to live in salty environments.

Staphylococcus epidermidis (Staph epi) is part of the normal flora on the surface of our skin, a highly saline environment. Staphylococcus aureus (a virulent strain of which causes MRSA) is a pathogenic type of Staph; S. epi’s evil cousin. Both species grow well on Mannitol Salt Agar. MSA is also differential, producing a color change when pathogenic Staphylococci are present.

* Sources *

Schauer, Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.