Screening an Archetypal Collection of Microorganisms for the Presence of Unexplored Antimicrobial Compounds

Background WHO estimates that more than 700.000 people die every year as a result of drug-resistant infections. To tackle this problem and change the current trend, it is necessary to design advanced strategies for drug discovery and to promote early-stage research activities finalized at the development of new drugs (World Health Organization 2015). New information In this study, we present the results of the preliminary screening of a library of microorganisms, collected from different environmental settings. Approximately 300 strains of the culture collection were tested on solid medium for inhibition of growth of three tester species, namely Bacillus subtilis, Escherichia coli and Staphylococcus aureus. The selection of the tester species was made according to the following criteria: • Staphylococcus aureus and Escherichia coli were chosen as representative of Gram+ and Gram- bacteria, respectively. Furthermore, these organisms are relevant for the global public health because the number of antibiotic resistant strains responsible for invasive diseases is steadily increasing (Centers for Disease Control and Prevention 2014, Lowy 2003); • Bacillus subtilis, instead, was selected as Gram+ tester microorganism commonly found in soil samples. Thus, this well characterized bacterium can be used to gain insight on the effect of metabolites produced by microorganisms of the culture collection described in this study. One of the active strains, MES18, was classified as a Bacillus spp. by means of smallsubunit rRNA gene sequencing. To identify the compound(s) responsible for this inhibitory activity, MES18 cells were grown in liquid medium at 30°C and samples were taken at different time points over a period of 12 days. The supernatants obtained from the fermentation media were subjected to fractionation by chromatography on reversed-phase column and all the eluted compounds were assayed for their ability to repress the growth of tester strains. This approach allowed us to identify the fractions containing the bioactive compound(s) and to establish that the production of these secondary metabolites reached a maximum during the idiophase of the cell culture, when cell growth and replication decline. Further analyses to identify the physical-chemical features of the compound(s) produced by this strain using HPLC coupled to mass-spectrometry are currently ongoing.