The incorrect statement among the provided options is:

Option C: Bio-reactors are used to produce small scale bacterial cultures.

Explanation:

Bio-reactors, also known as fermenters, are primarily utilized for scaling up microbial processes to industrial scales. They are designed to support the growth of a high density of cells and microbes under controlled conditions to produce large volumes of biochemical products such as enzymes, pharmaceuticals, and other biologically active compounds. While bio-reactors can technically be used for small-scale laboratory experiments, their main application and design focus is on large-scale production.

Other Options:

Option A: This statement is correct. Bio-reactors are specifically designed to provide optimal conditions such as temperature, pH, oxygen supply, and nutrients for the growth of microorganisms or cells, which helps in achieving a higher yield of the desired product.

Option B: This statement is also true. The most common type of bio-reactors used in industrial processes are of the stirred type, which have mechanical agitators to evenly distribute nutrients and maintain uniform conditions within the tank.

Option D: True, bio-reactors often include several essential systems: an agitator system for mixing, an oxygen delivery system to provide sufficient respiration for aerobic organisms, and a foam control system to prevent the formation of excessive foam, which could interfere with the operation of the reactor.

Thus, the incorrect statement about the typical scale and focus of bio-reactor use is Option C.