Plant growth promoting microorganisms (PGPM) are an important group of microbial inoculants, which exist in rhizosphere and have the ability to inhabit the root of the plants and improve their development. Their positive influence is achieved through solubilization of phosphorus, nitrogen fixation, production of plant nutrients and phytohormones, protection from pathogens and recovery from stressful environmental conditions. This is the main reason for the increasing usage of many PGPMs which formulations are commonly known as microbial fertilizers. Microbial fertilizers represent an attractive replacement for chemical fertilizers that are polluting the environment. They are used to increase the crop yield in an eco-friendly way while relying on sustainable agriculture principles. The biggest problem nowadays is the very poor quality of such products, which results in the lack of confidence and makes commercialization much more difficult. In order to increase production and hence the commercialization of microbial fertilizers, desired quality and stability should be achieved. For this reason, many researches are done in this particular field. In order to develop an optimal product, it is important to know and understand the process, including the physiology of bacteria and plants, mass multiplication technological processes as well as the existing formulation and the specific effect on the desired plant. For this purpose, the aim of this review is to indicate the significance of microbial fertilizers and their beneficial effects on the plants, as well as to give a brief survey of the different aspects of production processes with a special emphasis on mass multiplication.

Bacillus sp.-based microbial formulations have found wide application in many fields: from pharmacy and medicine to environmental protection and agriculture due to the ability of this species to produce various metabolites and to form endospores. Recently, these products have gained popularity as biopesticidal and phytostimulatory agents, which are a ?green? alternative to overused agrochemicals. In order to obtain a high-quality and long-lasting product with desired characteristics, it is necessary to optimize the production process at each stage, which implies coordinating the microbial species, the type and the conditions of microbial cultivation along with formulation technologies. This paper provides a concise overview of the most important findings in this area, regarding characteristics of microbial formulations and specific criteria that need to be met when such a product is formulated. It should serve as a beginning point for everyone starting new research, not just in the field of biofertilization and biological control of plant diseases, but generally in the field of biochemical engineering.