The term growth refers to the group of bacteria than a single one and because of one single cell do not get larger in size, "growth" means, more exactly, the reproduction of cells.
The process of bacteria reproduction is called fission, the result of fission being two new cells.
The most important factors which influence the process of bacteria growing are: temperature, pressure, pH and moisture.
As about Vitamins and Bacteria:
Bacteria can be used for the production of vitamins as B1(thiamine), B2 (riboflavine), B6 (pirydoxine), B12, biotin, pantothenic acid, folic acid. Majority of vitamins are synthetically made, for economic reasons, but part of them are made biosynthetically, that's means, partly from bacteria and partly from synthetics.
B12 is a vitamin that is obtained in nature exclusively from bacteria, several producing strains being: Bacillus megaterium, Streptomyces olivaceus etc.
As about K vitamin, majority being synthesized by bacteria in the large intestine. Dietary sources include the photosynthetic (green) parts of plants.