Re: growing bacteria in petri dishes All you want is a control to show that what is growing on your plate is due to whatever you swabbed and not due to something in the air. So a control would simply be an empty plate incubated along with the other plates. Or simply a plate that you streaked with a clean swab.

Petri dishes need to be incubated upside-down to lessen contamination risks from airborne particles landing on them and to prevent the accumulation of water condensation that could disturb or compromise a culture.

Nutrient agar is a basic culture medium commonly used for the culture of non-fastidious microorganisms, and for quality control and checking purity prior to biochemical or serological testing. Nutrient agar has been used for the cultivation and enumeration of many bacteria that are not particularly fastidious.

Agar, which is a polysaccharide derived from red seaweed (Rhodophyceae) is preferred because it is an inert, non-nutritive substance. The agar provides a solid growth surface for the bacteria, upon which bacteria reproduce until the distinctive lumps of cells that we call colonies form.

ERT

Escherichia coli colonies grow with a metallic sheen with a dark center. Aerobacter aerogenes colonies have a brown center, and non-lactose-fermenting gram-negative bacteria appear pink. EMB agar is also used to differentiate the organisms in the colon-typhoid-dysentery group.