Short-term fire-induced changes to the soil microbial community are usually closely associated to fire severity, which essentially consists in the fire-induced loss or decomposition of organic matter above ground and below ground. Many functional processes and soil properties, including plant recolonization and soil microorganism activity, depend on fire severity. Seven days after burning, we evaluated the impact of two fire severities (low and high) on basic soil properties and the microbial communities in an outdoor experimental controlled system composed of six forest soil monoliths. The magnitude of change in microbial community was far greater than the change in physical and chemical soil properties. Total N was the only selected soil property that significantly varied depending on fire severity. The severely burned soils experienced significant changes in overall microbial biomass composition and phylogenetic composition of bacterial communities in comparison with control plots. Immediately after the fire, in fact, phyla and genera such as Acidobacteria-Gp4 or Bacteroidetes-Ohtaekwangia were much more abundant in the control monoliths. On the other hand, Firmicutes or Proteobacteria (e.g. Firmicutes Paenibacillus, Proteobacteria Phenylobacterium) were relatively more abundant in the monoliths burned with high severity in comparison with the low severity burned ones. Overall, the effect of fire on soil microbial communities was greater in the high severity burned monoliths than in the low severity burned ones. We concluded that in Mediterranean forest ecosystems, fire significantly alters soil bacterial composition depending on its severity.