Bacteria will grow on practically any source of organic food which provides carbon compounds to be respired for energy, and nitrogen compounds to be incorporated into proteins for growth. These substances are normally provided dissolved in water. However, in nature, bacteria can break down solid and insoluble substances by releasing enzymes into the substrate in which they are growing. These substances are thus broken down or digested to simpler substances and the process is called extracellular digestion because it takes place outside the bacterial cells.
The two normal media used in bacteriology are a clear soup-like liquid
nutrient broth, usually in tubes, and nutrient agar, which
is set into a jelly by the addition of a seaweed extract called agar, and
when melted poured into glass or plastic Petri dishes - also known as
"plates".
A standard carbon source is glucose, and nitrogen is often provided by peptones (partially digested proteins), or inorganic salts. Minerals and vitamins may also be provided, according to the growth requirements of the bacteria. Combinations of chemicals (buffers) may be used to keep the pH stable. Measured amounts of the concentrates are added to water, and dissolved to reconstitute the media.
Sometimes, substances are mixed into media, in order to suppress growth of other types of bacteria. There are many such selective media.
These media must then be sterilised by heating in an autoclave (like a pressure cooker) at 121°C (pressure 1 bar or 15 lb/sq. in.) for 15 minutes, which kills all living organisms, including spores.
All apparatus used from this point onwards must be sterilised by heat (glassware - 160 °C for 2 hrs) or exposure to radiation.
Aseptic techniques must be used to reduce the likelihood of bacterial contamination. This usually involves disinfection of working areas, minimising possible access by bacteria from the air to exposed media, and use of flames to kill bacteria which might enter vessels as they are opened.
Then the Petri dishes containing agar or tubes containing broth are incubated, i.e. put in a special apparatus at a fixed temperature (usually 37°C - human body temperature, for possible pathogens - or 25°C for bacteria from the environment). In schools, lower incubation temperatures are used in order to discourage the growth of potential pathogens.
When growing bacteria, it is usual to invert the Petri dishes, so as to prevent condensation droplets from falling onto the surface of the agar. Petri dishes are often "sealed" at this stage to prevent people who handle them from contamination by bacteria, which will multiply greatly. It is normal to use 2 strips of adhesive tape from base to lid rather than attempt seal the circular edge of the Petri dish. This is to guard against the possibility of anaerobic organisms growing due to lack of air. However, it must be borne in mind that any drips from a partially sealed Petri dish are potential sources of infection.
Liquid media such as broth become cloudy if bacteria are
present. This could be the result of only one bacterial cell originally entering
the medium, then dividing repeatedly to produce millions! 
Bacteria on agar "plates" become visible as distinct circular colonies;
each colony should represent an individual bacterial cell (or group) which
has divided repeatedly but, being kept in one place, the resulting cells
have accumulated to form a visible patch.
After use, bacterial cultures, etc. must be sterilised by the use of heat,
before disposal.
