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this lab bench displays the different means of growing bacterial cultures. in the foreground, the culture is in the early stages of exponential growth while the flask in the background is much older, with the cells inside forming endospores. on the left are two large flasks containing lb growth media. on the right, an lb - agar plate streaked with bacteria is displayed, in front of two stacks of additional uninoculated plates; on the left are lb - agar plates (yellow), and on the right, macconkey agar plates (pink). in the image, the lb agar is a general media while the macconkey agar is classified as selective and differential media. compare and contrast these two types of agar and explain how the growth of common bacteria such as the gram negative, salmonella and the gram positive, staphylococcus on these plates would be different.

Explanation:

LB agar is a general - purpose medium that supports the growth of most bacteria. MacConkey agar contains bile salts and crystal violet which inhibit the growth of gram - positive bacteria, making it selective for gram - negative bacteria. It also contains lactose and a pH indicator. Gram - negative lactose fermenters produce acid, turning the colonies pink, while non - lactose fermenters remain colorless, making it differential. Salmonella, a gram - negative non - lactose fermenter, would form colorless colonies on MacConkey agar. Staphylococcus, a gram - positive bacterium, would be inhibited on MacConkey agar but would grow well on LB agar. On LB agar, both Salmonella and Staphylococcus could grow, but with different colony morphologies.

Answer:

LB agar is a general medium allowing growth of most bacteria. MacConkey agar is selective for gram - negative bacteria due to inhibitory components and differential as it differentiates lactose fermenting and non - lactose fermenting gram - negative bacteria. Staphylococcus (gram - positive) is inhibited on MacConkey and grows on LB. Salmonella (gram - negative non - lactose fermenter) forms colorless colonies on MacConkey and grows on LB.