Towards the definition of minimal inhibitory concentrations for Halobacterium salinarum

Archaea are prokaryotic organisms thriving mostly at extreme environments and sharing characteristics with both bacteria and eukaryotes. Within the Archaea domain, the Halobacteriaceae family comprises species that are mainly found in solar salterns, marine environments and salted food products. A major representative of this group is Halobacterium salinarum. Recently, these microorganisms have raised an increasing scientific interest, since they have been found in both food and human intestinal mucosa. Very little is known about their antibiotic resistances, and it is thus important to establish microbiological breakpoints for Halobacteriaceae, as done in the past for bacteria. At present only few works have studied antibiotic resistance in halophilic Archaea, but the Minimal Inhibitory Concentrations (MICs) proposed by these studies can’t be considered acceptable since they didn’t use at least 50 different strains for each species, as the European Committee on Antimicrobial Susceptibility (EUCAST) requires. Fifty-two different strains of Halobacterium salinarum were isolated and characterised by RAPD (Random Amplification of Polymorphic DNA) and 16S rRNA analyses within a study on the microbial ecology of animal casings. We assessed the microbiological breakpoints of these isolates and other three collection strains against anisomycin, ciprofloxacin, clindamycin, erythromycin, novobiocin, rifampicin and trimethoprim. While for rifampicin, ciprofloxacin and trimethoprim the MICs previously found have been confirmed, for the other antibiotics we obtained for clindamycin, novobiocin and anisomycin MICs lower than the reported ones, while for erythromycin they were higher. In particular, we observed that H. salinarum is mainly susceptible to antibiotics that inhibit DNA replication rather than those which interact with protein synthesis. High variability among strains was mainly found with novobiocin. This work provides new insights about MICs for H. salinarum, a strain that is often found along the food chain, and which can be a first model for the assessment of antibiotic resistance profiles in Archaea.