The achievements in the study of the etiology and the development of diagnostic methods and immunoprophylactic preparations of these infections were significant. Also bacteria, mycoplasma and L forms, the antibiosis and the antibacterial medicinals, as well as the agents causing protozoic invasions were studied. The beginning of the investigations in the field of infectious immunology were set by Prof. Vladimir Markov, Member of BAS. The industrial microbiology started with the investigations of Prof. Ignat Emanuilov, Member of BAS, on the microbiology of milk and dairy products, biochemistry and physiology of lactic acid bacteria and the application of bacterial enzymes (proteinases) in cheese making. Along with bacteria as producers of biologically active substances filamentous fungi were studied. About 1964 the investigations were grouped into two principal problems: I. Metabolism and biosynthesis of microorganisms and their regulation; II. Nature and regularities of the interactions between the microorganisms and the macroorganism.

In the field of microbial morphology since 1964 electron microscope studies of bacteria, filamentous fungi, yeasts and cells of the immune system of the macroorganism were performed. The investigations in the field of microbial biochemistry after 1973 aimed the elucidation at contemporary biochemical level of the main peculiarities in the physiology and biosynthetic capabilities of the microbial cells. The main object of these studies were some enzymes (ribonucleases, acid and alkaline phosphatases from yeasts, bacterial neuraminidase and luciferase), proteinase and nuclease inhibitors, microbiological transformation of steroids; protein-lipid interactions in bacterial membrane in regard to the secretion of extracellular enzymes. As a result of the systemic investigations carried out by Prof. Simeon Galabov and his collaborators the relationship between antigenic structure and biological properties of bacterial endotoxins were clarified by the use of different methods for their detoxication. Data were obtained about the medical application of endotoxins, based on their antigenicity (diagnostic kits), immunogenicity (vaccines), interferon-inducing activity and tumor necrotic action. The research work in the field of microbial genetics was developed by Prof. Kaltcho Markov, Corresponding Member of BAS. The investigations were connected with the problem of prophage interference, genetic transfer in Bacillus thuringiensis and Streptococci from group A, hybridization of Actinomyces sp. in order to increase their productivity, construction of gene maps of Streptomycetes – producers of antibiotics, molecular identification of microorganisms by the use of PCR, as well as genomic differentiation by M13 DNA fingerprinting and ribotyping. The Institute of Microbiology has always played a leading role in applied microbiology. Microbial producers of a wide spectrum of biologically active substances were isolated: enzymes, aminoacids proteins, carotinoids, various antibiotics with wide spectrum of activity. In details were studied the physiology and biochemistry of microbial producers, including extremophilic bacteria (thermophilic and alkalophilic) and also the properties of the synthesized products. A wide spectrum of methods for immobilization of enzymes and microbial cells, producers of biologically active substances, were elaborated. The investigations in infectious bacteriology after 1962 were pointed on the changes provoked in the pathogen during the course of infectious and immunizatory process (Prof. Alexander Toshkov, Corresponding Member of BAS, and collaborators) and later researches were focused on the factors and mechanisms of bacterial virulence and bacteria-host interactions using as a model Yersinia infection (Prof. Dimitar Veljanov and collaborators). At the beginning of 60th years started investigations in the field of the experimental chemotherapy of viral infections. Search for viral inhibitors from Bulgarian medicinal plants (tested mainly on influenza viruses) was extended later with systemic studies on the creation and characterization of some new effective antivirals (vs. influenza, toga and other virus groups), on the combined effects of viral inhibitors (efficient against herpes and picornaviruses) and their mechanisms of action. After the pioneer for the Bulgarian immunology works carried out by Prof. Vladimir Markov followed the investigations on cell immunity and L-forms of bacteria under the guidance of Prof. Valchan Valchanov and Prof. Assen Toshkov. A wide spectrum immunopharmacological screening was performed aiming the selection of new active substances.