.......MEDIA

Bile Esculin Agar

Bile esculin agar is a medium used to identify group D streptococci. This group of bacteria have the ability to grow in the presence of bile, an emulsifying agent produced in the liver. Group D streptococci also have the ability to hydrolyze esculin. This hydrolysis of esculin turns the medium black and denotes a positive test. Other bacteria capable of growing in the presence of bile do not turn the medium black. A variation of this medium uses sodium azide to inhibit the growth of all other Gram-positive bacteria and Gram-negative bacteria.

HEKTOEN ENTERIC(HE) AGAR

Hektoen enteric agar is a medium designed for the isolation and recovery of fecal specimens belonging to the Enterbacteriaceae family, especially Salmonella and Shigella. It can differentiate between bacteria that ferment lactose and those that don't. Acid produced from fermenting lactose imparts a yellow-orange color to the medium due to the presence of a pH indicator. Non-lactose fermenters do not significantly change the color of the medium. HE agar can also detect the production of hydrogen sulfide gas, which turns parts of the medium black.

MACCONKEY AGAR

MacConkey agar is probably the most popular solid differential medium in the world. It is mainly used in identification of lactose fermenting, Gram-negative enteric pathogens and for inhibiting growth of Gram-positive organisms. Bacterial colonies that can ferment lactose turn the medium red. This red color is due to the pH indicators response to the acidic environment created by fermenting lactose. Organisms that do not ferment lactose do not cause a color change.

A common medium used for the isolation of pathogenic staphylococci is the Mannitol Salts Agar. The high salt concentration of this medium inhibits the growth of most other organisms. Pathogenic staphylococci not only grow on the medium, but they also produce acid from it. This acid production turns the pH indicator from red to yellow. Non-pathogenic staphylococci can grow on the medium but produce no acid from it.

CHOPPED MEAT GLUCOSE MEDIUM

Chopped meat broth is used for culturing anaerobic organisms

TRIPLE SUGAR IRON (TSI) AGAR

Triple sugar iron (TSI) agar is a medium used in the identification of Gram-negative enteric rods. The medium measures a bacterium's ability to utilize three sugars, glucose, sucrose and lactose, the concentrations of which are 0.1%, 1.0%, and 1.0%, respectively. A pH indicator incuded in the medium can detect acid production from fermentation of these carbohydrates. A yellow color change indicates acid in the medium while no color change indicates an alkaline surrounding. Inoculation of the tube is a two step procedure. First, a loop of bacteria is spread across the surface of the agar. Second, a needle of bacteria is inserted (stabbed) into the bottom (butt) of the tube. Carbohydrate uilization can be determined through analysis of the extent of acid production. Acid production limited to only the butt of the tube is indicative of glucose utilization. This is because the concentration of glucose is lower than that of other sugars, thus the acid production is not very extensive. Acid production in the slant and butt indicates sucrose or lactose fermentation because of the relatively high concentrations of these sugars, thus leading to extensive acid production. TSI agar can also detect reduction of sodium thiosulfate to hydrogen sulfide. Hydrogen sulfide production will turn parts of the agar black. Production of other gases is marked by cracks in the agar as well as an air gap at the bottom of the test tube

TRYPTIC SOY BROTH/AGAR

Tryptic soy agar and broth is a basic medium used for culturing many kinds of microorganisms. Tryptic soy agar is mainly used as an initial growth medium for the following purposes:

observe colony morphology
develop a pure culture
achieve sufficient growth for further biochemical testing
culture storage
Tryptic soy broth is used mostly to generate a large supply of bacteria for certain bichemical tests. It can also be used in the determination of bacterial numbers.

CHOCOLATE AGAR

Chocolate agar is a nutrient medium which is used in culturing fastidious organisms such as Haemophilus species and Neisseria. It is comprised of sheep blood that provides the X and V factors necessary for Haemophilus growth. Chocolate agar, however, does not reveal hemolysis data, so species differentiation among the members of Haemophilus must be performed in another manner.

EOSIN METHYLENE BLUE (EMB) AGAR

EMB agar is a differential medium used in identification and isolation of Gram-negative enteric rods. EMB agar also inhibits the growth of Gram-positive organisms. The differential basis of this medium involves two indicator dyes, eosin and methylene blue, that distinguish between lactose fermenting and non-lactose fermenting organisms. Lactose fermenters form colonies with dark centers and clear borders while the non-lactose fermenters form completely coloroless colonies.