Soil myxobacteria as a potential source of polyketide-peptide substances

Abstract

Myxobacteria, a group of antimicrobial producing bacteria, have been successfully cultured and characterized from 10 soil samples collected from different parts of Slovakia. A total of 79 myxobacteria belonging to 4 genera (Myxococcus, Corallococcus, Sorangium, and Polyangium) were isolated based aspects on of their life cycle. Twenty five of them were purified, fermented, and screened for antimicrobial activities against 11 test microorganisms. Results indicated that crude extracts showed more significant activities against Gram-positive than against Gram-negative bacteria or fungi. Based on a higher degree and broader range of antimicrobial production, the two most potential extracts (K9-5, V3-1) were selected for HPLC fractionation against Micrococcus luteus and Staphylococcus aureus and LC/MS analysis of potential antibiotic metabolites. The analysis resulted in the identification of polyketide-peptide antibiotics, namely corallopyronin A and B (K9-5) and myxalamid B and C (V3-1), which were responsible for important Gram-positive activity in the observed strains. A sequence similarity search through BLAST revealed that these strains showed the highest sequence similarity to Corallococcus coralloides (K9-5, NCBI accession number KX256198) and Myxococcus xanthus (V3-1, NCBI accession number KX256197). Although screening of myxobacteria is laborious, due to difficulties in isolating cultures, this research represented the first report covering the isolation and cultivation of this challenging bacterial group from Slovakian soils as well as the screening of their antimicrobial activity, cultural identification, and secondary metabolite identification.

Myxobacteria Are Able to Prey Broadly upon Clinically-Relevant Pathogens, Exhibiting a Prey Range Which Cannot Be Explained by Phylogeny

“…When samples gave more than one isolate, isolates were required to be morphologically distinct, to ensure the collection was non-redundant. As observed in other studies ( Zhang et al, 2013 ; Charousová et al, 2017 ), isolates were predominantly Corallococcus spp. (70%), and Myxococcus spp.…”

“…In the current study 113 myxobacterial strains were isolated, the majority of which were Corallococcus spp. and M. xanthus/virescens ; an isolation bias seen in other studies ( Zhang et al, 2013 ; Mohr et al, 2016 ; Charousová et al, 2017 ). We were unable to isolate members of the Nannocystineae sub-order, with all our isolates belonging to the Cystobacterineae, except for one Sorangium spp.…”

“…are described as efficient predators of Gram-negative, but with only variable activity against Gram-positive bacteria ( Gerth and Müller, 2005 ; Morgan et al, 2010 ; Xiao et al, 2011 ; Müller et al, 2014 ). Conversely, crude extracts of secondary metabolites from these organisms typically show greater activity against Gram-positive than against Gram-negative bacteria ( Morgan et al, 2010 ; Charousová et al, 2017 ). It would be interesting to test whether the antimicrobial activities manifested by the isolates described here are also exhibited by cell extracts made from those isolates.…”

“…Complex isolation and propagation techniques are required, which impede the isolation and characterization of novel myxobacteria. For instance, most isolates do not grow as smooth suspensions in liquid cultures, which particularly hampers efforts to study their secondary metabolites ( Weissman and Müller, 2009 ), and which in turn promotes a strategy of studying culture extracts and purified products ( Charousová et al, 2017 ).…”

“…Among the many aspects of myxobacterial predation, prey range remains particularly poorly understood. Similar isolates can exhibit very different patterns of prey susceptibility ( Morgan et al, 2010 ) and the range of organisms susceptible to the action of purified secondary metabolites can differ from that observed in co-culture experiments ( Kunze et al, 2008 ; Charousová et al, 2017 ). To date, studies employing co-culture predation assays have tended to either use environmental saprophytes as potential prey, or a small number of human pathogens ( Morgan et al, 2010 ; Evans et al, 2012 ; Seccareccia et al, 2015 ).…”