bacterial culturing - clinical microbiology culture media, differential and selective media

bacterial culturing is

the practice of growing bacteria in the lab

the nutrient bacteria need are provided by

the culture medium

bacteria can be grow in

liquid culture or on media made solid by the addition of agar (which give culture media a cell-like consistency)

other conditions important for bacterial growth

temperature

gas composition

these are regulated by the incubator

a culture that contains only a single type of microorganism is called a

pure culture

nutritive media

support the growth of most bacteria without giving any particular species an advantage

enrichment media

provide specific nutrient that selectively support the growth of particular bacteria over others

differential media

contain factors that allow colonies of one type of bacteria to be distinguished from another growing on the same agar plate based on their appearance

selective media

contain inhibitory agents, such an antimicrobial drugs, bile salts, and acids, that allow only the selected bacteria to grow

one medium can fall into

several categories

because differential and selective media can be used to distinguish bacteria, they are particularly useful in

diagnostic microbiology

selective media

contain agents that allow selected bacteria to grow, but inhibit the growth of others

inhibitory ages could be antimicrobial drugs, dyes, or alcohols

Differential media

distinguishes different types of bacteria based on the appearance of the culture

differential media contain

one or more factors that cause bacteria with certain metabolic or culture characteristics to look different from other bacteria growing on the same agar plate

differential media is also sometimes known as

indicator medium

in response to components of the media, the bacterial colonies on differential media may

change color or affect the appearance of the medium through the production of extracellular enzymes

blood agar

an enriched medium that is DIFFERENTIAL for hemolysis

in incomplete, or a-hemolysis, hemolysis the blood agar around the colonies changes to a

green color as hemoglobin is converted to methemoglobin

in complete, or b-hemolysis, the blood agar around the colonies changes to

colorless

in non-hemolytic, or g-hemolytic samples

no changes to the agar occur

hemolysis can be used to

help identify bacteria

in some species, such a E. coli, hemolytic activity

varies and can be associated with virulence

MacConkey agar is selective for

gram-negative organisms due to bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria

MacConkey agar is differential for

lactose metabolism

Lactose fermenting bacteria lower pH of the medium, in response

the pH indicator neutral red durns the colonies and medium pink

non-lactose fermenting organisms

remain white or colorless and the medium changes color to yellow