The bacterial group feature many differences across species. These differences allow for the identification and isolation between species and related families. They differ in their shape, colonial morphology, size, cellular structure, metabolic energy sources and more. 
 	The use of selective and differential media is one way in which wanted cultures can be obtained, and then observed. Using specific environmental conditions within the media, the growth of specific bacterial strains can be facilitated whilst others can have their growth inhibited. One variable that can be altered is temperature, and can be used to select growth between bacteria living in an animal host or ones that live in cold external environments from between each other for example. Other times the use of chemicals may be needed where they result in different outcomes between strains.
 	Selective media is the use of an agent to inhibit the growth of unwanted bacterial. Differential media uses an agent, such as a pH indicator to differentiate between bacterial species within a single plate. When a pH indicator is used for example, another chemical is added and depending on how the strains metabolise it or not, can alter the pH of the environment around the colony. It results in a colour change needed for quick and simple observation and identification. 
 	MacConkey agar is both a differential and selective media, composed of bile salts and lactose with a neutral red pH indicator in a nutrient base. The bile salts inhibit the growth of non-enteric bacteria, or bacteria not of the intestines, giving the media its selective component. The lactose is there to test for the ability to metabolize lactose, resulting in lactic acid and a red/pink colour via the pH indicator around the Lac+ colonies whilst the other colonies appear beige due to the deamination of amino acids producing ammonia instead of relying on lactose fermentation and are considered Lac-. 
 	Whilst selective and differential media inhibit and cause visible differences between colonies, there are more sorts of media for example enriched media. Enriched media enhance the growth of a wanted organism, for example horse blood agar improves the growth of bacteria that are found in high blood concentration areas such as the upper respiratory tract. It is also differential as the bacteria’s ability to haemolyse, or break down the red blood cells is also present.
 
Aim:
To understand and observe the characteristics of selective, enriched and differential media as well as use selective and differential media to culture specific bacteria.

Materials and methods:
As per practical manual page 70.
MIC 2011, Class Notes, 2005
Results:
A mixture of Bacillus megaterium, Escherichia coli and Providencia rettgeri was streak plated onto a nutrient agar plate and a MacConkey agar plate. 
Table 1: The difference between nutrient agar and MacConkey agar plates on the growth of the three species within the mixed culture.
Bacteria	NA	MAC
Bacillus megaterium	Growth, circular, white, opaque, Gram + rods	No growth
Escherichia coli	Growth, circular, white, clear, Gram - rods	Red/pink colour around colonies
Providencia rettgeri	Growth, white, glossy, 
Gram - rods	Beige colour around colonies

Discussion: