This work aimed to determine the influence of the inoculation of autochthonous cellulolytic bacteria on the composting process without any modifications of physical or chemical parameters. Bacteria with cellulolytic abilities were isolated from composted material containing food and plant leftovers and identified as Bacillus licheniformis, Bacillus altitudinis, and Lysinibacillus xylanilyticus. The experimental composter containing garden and household wastes was inoculated with bio-vaccine prepared as a mixture of isolated cellulolytic bacterial strains and composted for the next 96 days parallelly to the control composter without the inoculation. During the experiment, changes in temperature, humidity, the content of the humic acids (HAs), organic carbon, nitrogen, and C:N ratio were determined. As the particular microbial groups play a key role in the composting process, the biodiversity of the microorganisms present in the composter as well as the number of psychrophilic, mesophilic, and sporeforming microorganisms, Actinomycetes, and fungi were analyzed. The changes in the abundance of particular bacterial groups were convergent with temperature changes in the temperature of composting material. The composting material inoculated with autochthonous microorganisms was characterized by higher HA content and lower biodiversity. The inoculation with autochthonous microorganisms positively influenced the composting material in the corners for the entire process and in the middle of the container for 61 days. Thus, the effect of inoculation depended on the localization of the process inside the container subjected to biopreparation.