Different Mediums for a Culture in Microbiology

How do you tell a fox from a pig? Study the characteristics of each. That is how to tell the difference between types of bacteria. They each have identifiable characteristics that can be studied to determine the bacteria species. Culture characteristics of bacteria include size, pigmentation and smell. The culture media used in the observation will affect the characteristics of the bacteria. Some mediums are nutritive and will encourage growth, while some mediums will restrict and inhibit bacteria growth.

Mannitol Salt Agar

• Mannitol salt agar is a selective medium. It has a high salt content, which makes it a perfect growth environment for the Staphylococcus bacteria. Staphylococcus thrives in protein-rich foods with a high salt content. Other organisms grow weakly in saline. Mannitol salt agar or tryptic soy broth are mediums used for Staphylococcus bacteria growth.

Phenylethyl Alcohol Agar

• Phenylethyl alcohol agar is used to cultivate gram positive organisms. Gram positive organisms have the ability to lyse or split a blood cell giving it access to the cell's nutrients and allowing the bacteria to grow. The phenylethyl alcohol inhibits growth of gram-negative organisms by interfering with DNA synthesis or replication of the organism.

Hektoen Enteric Agar

• Hektoen Enteric Agar isolates members of the species salmonella and shigella from other Enterobacteriaceae. This group of organisms causes primary infections to the human gastrointestinal tract. Bile salts inhibit the growth of gram-positive bacteria. Lactose and sucrose encourage their growth. Enterics, organisms that grow within intestines, produce yellow-to-salmon colored colonies.

MacConkey Agar

• MacConkey Agar isolates enterics based on their ability to ferment lactose. Bile salts inhibit the growth of gram-positives in this media. The bacteria micrococcus luteus does not grow in the presence of bile salts. Lactose provides a source of fermentable carbohydrate, producing an acidic environment which will appear pink on MAC.