Bacteria are unicellular prokaryotic microorganisms, which means they do not have nuclear wall or a proper nucleus that is characteristic of eukaryotic cells. Not belonging to the animal or vegetal reigns, they are included in the Monera reign (or inferior protists), which comprehends microorganisms having various type of energy supplying methods, as archaebacteria, eubacteria, cyanobacteria and blue algae.

Prokaryotic organisms have only one circular and double-stranded chromosome, making them the smallest self-sufficient organisms on Earth, able to reproduce and perform all the functions they need to survive. They usually measure about 1 micron, but their dimensions may vary on the species. Unlike eukaryotic cells, they have a cell wall, constituted of peptidoglycan conferring resistance and stiffness, which is the target of various antimicrobial agents (beta-lactams, glycopeptides, fosfomycines etc.). Bacteria do not have mitochondria or chloroplasts but perform similar biochemical reactions with enzymes situated on the cytoplasmatic membrane. They do not have endoplasmic reticulum, Golgi’s apparatus or lysosomes, but use mesosomes for respiration, energy production, DNA synthesis, mitosis, sporulation and other crucial functions.

Bacterial species can live isolated or in the can share resources living in the biofilm, which constitutes a community of different species immersed in an exopolysaccharide matrix. Examples of biofilms are well known in different habitat and on various surfaces, such as teeth, monuments, surgical implants, food, water pipes, and for bacteria represents a protection from the immune system, viruses and antimicrobials.

In nature, bacteria are widespread in each environment, colonizing a multitude of surfaces of animals and plants and every other place where nutritive substances are available along with favourable conditions of temperature and humidity. The set of all bacteria that live on mucous membranes, skin, intestines, airways, urinary tract ect. constitutes the microbiota of each organism, which not only performs functions of fundamental importance, but also contributes to its well-being. Numerous scientific studies confirm that microbiota alterations are related to various diseases and that they can be risk factors for some serious conditions. Under physiological conditions, the immune system can usually cope with most bacteria in the environment and only the one with a high pathogenic potential can cause disease. However, in stressful conditions or in the presence of comorbidities that reduce the effectiveness of the immune system, bacteria that are generally considered commensal can infect humans and animals, producing serious damage.

Bacteria can be grown in vitro using liquid or solid media, which allow to evaluate their morphology or to obtain high concentrations of bacterial cells that can be useful for various medical, industrial and biological applications. Strains worth to be preserved can be stored on solid substrates at 4°C for a few months, while for longer deposits they must be stored in cryo (-80°C, -152°C or -196°C in liquid nitrogen) or freeze dried.