Competitive dominance of antibiotic-producing marine bacteria in mixed cultures

An Analysis of Biosynthesis Gene Clusters and Bioactivity of Marine Bacterial Symbionts

Symbiotic marine bacteria have a pivotal role in drug discovery due to the synthesis of diverse biologically potential compounds. The marine bacterial phyla proteobacteria, actinobacteria and firmicutes are commonly associated with marine macro organisms and frequently reported as dominant bioactive compound producers. They can produce biologically active compounds that possess antimicrobial, antiviral, antitumor, antibiofilm and antifouling properties. Synthesis of these bioactive compounds is controlled by a set of genes of their genomes that is known as biosynthesis gene clusters (BGCs). The development in the field of biotechnology and bioinformatics has uncovered the potential BGCs of the bacterial genome and its functions. Now-a-days researchers have focused their attention on the identification of potential BGCs for the discovery of novel bioactive compounds using advanced technology. This review highlights the marine bacterial symbionts and their BGCs which are responsible for the synthesis of bioactive compounds.

The effect of bacteria on planula-larvae settlement and metamorphosis in the octocoral Rhytisma fulvum fulvum

While increasing evidence supports a key role of bacteria in coral larvae settlement and development, the relative importance of environmentally-acquired versus vertically-transferred bacterial population is not clear. Here we have attempted to elucidate the role of post-brooding-acquired bacteria on the development of planula-larvae of the octocoral Rhytisma f. fulvum, in an in vitro cultivation system employing different types of filtered (FSW) and autoclaved (ASW) seawater and with the addition of native bacteria. A good development of larvae was obtained in polystyrene 6-well cell culture plates in the absence of natural reef substrata, achieving a 60–80% of larvae entering metamorphosis after 32 days, even in bacteria-free seawater, indicating that the bacteria acquired during the brooding period are sufficient to support planulae development. No significant difference in planulae attachment and development was observed when using 0.45 μm or 0.22 μm FSW, although autoclaving the 0.45 μm FSW negatively affected larval development, indicating the presence of beneficial bacteria. Autoclaving the different FSW homogenized the development of the larvae among the different treatments. The addition of bacterial strains isolated from the different FSW did not cause any significant effect on planulae development, although some specific strains of the genus Alteromonas seem to be beneficial for larvae development. Light was beneficial for planulae development after day 20, although no Symbiodinium cells could be observed, indicating either that light acts as a positive cue for larval development or the presence of beneficial phototrophic bacteria in the coral microbiome. The feasibility of obtaining advanced metamorphosed larvae in sterilized water provides an invaluable tool for studying the physiological role of the bacterial symbionts in the coral holobiont and the specificity of bacteria-coral interactions.

Effects of dietary supplementation with fenugreek seeds, alone or in combination with probiotics, on gilthead seabream (Sparus aurata L.) skin mucosal immunity

Despite increasing interest in modulating the immune response of fish, providing a combination of probiotics and herbal immunostimulants in aquafeed has rarely has been studied. The effects on gilthead seabream (Sparus aurata L.) of the dietary administration of fenugreek (Trigonella foenum graecum) seeds alone (FE), or combined with one of the following probiotic strains: Bacillus licheniformis (FEBL), Lactobacillus plantarum (FELP) or Bacillus subtilis (FEBS) were evaluated. Fish were fed a control or one of the supplemented diets for 3 weeks. After 2 and 3 weeks of the feeding trial, the abundance of terminal carbohydrates, IgM levels, enzymatic activities (proteases, alkaline phosphatase, esterase and ceruloplasmin) and bactericidal activity were determined in skin mucus. Our results demonstrated that the dietary administration of FE in combination with L. plantarum, particularly, increased carbohydrate abundance, the activity of certain enzymes such as ceruloplasmin, and bactericidal activity against the pathogenic bacterium Photobacterium damselae and the non-pathogenic bacterium B. subtilis in skin mucus at the end of the trial. The carbohydrates most affected by the FELP diet were mannose/glucose, N-acetyl-d-galactosamine and N-acetyl-β-d-glucosamine. Interestingly, IgM levels were significantly higher in fish fed the FELP and FEBS diets whilst protease activity generally increased in all supplemented diets, which could suggests that the main effect in this activity was to the result of FE supplementation although that fact cannot be confirmed because the effects of probiotics addition alone were not studied. These results suggest that the combined dietary administration of fenugreek and L. plantarum will best enhance the skin mucosal immunity response of gilthead seabream.

Antagonistic interactions mediated by marine bacteria: the role of small molecules

Marine bacteria are known to produce a wide variety of structurally diverse and biologically active secondary metabolites. Considerably less is known about the ecological functions of these compounds, in part due to methodological challenges associated with this field of research. Here, we review the antagonistic activities mediated by marine bacteria with a focus on activities linked to structurally defined secondary metabolites. Bacterial antagonism has been documented against other marine bacteria as well as eukaryotes, and includes antibiosis, the inhibition of quorum sensing, larval settlement deterrence, and defense against predation. These compounds likely play important ecological roles that ultimately affect ecosystem structure and function, however, much remains to be learned before these roles can be fully appreciated. Recent technological advances coupled with a better understanding of the diverse processes mediated by secondary metabolites provide new opportunities to expand our understanding of the chemical ecology of bacterial antagonism in the marine environment.

Enzymatic resistance to the lipopeptide surfactin as identified through imaging mass spectrometry of bacterial competition

Many species of bacteria secrete natural products that inhibit the growth or development of competing species. In turn, competitors may develop or acquire resistance to antagonistic molecules. Few studies have investigated the interplay of these countervailing forces in direct competition between two species. We have used an imaging mass spectrometry (IMS) approach to track metabolites exchanged between Bacillus subtilis and Streptomyces sp. Mg1 cultured together. Surfactin is a cyclic lipopeptide produced by B. subtilis that inhibits the formation of aerial hyphae by streptomycetes. IMS analysis exposed an addition of 18 mass units to surfactin in the agar proximal to Streptomyces sp. Mg1 but not other streptomycetes tested. The spatially resolved change in the mass of surfactin indicated hydrolysis of the molecule. We observed that the aerial growth of Streptomyces sp. Mg1 was resistant to inhibition by surfactin, which suggests that hydrolysis was a mechanism of resistance. To identify possible enzymes from Streptomyces sp. Mg1 with surfactin hydrolase activity, we isolated secreted proteins and identified candidates by mass spectrometry. We purified one candidate enzyme that hydrolyzed surfactin in vitro. We tested the role of this enzyme in surfactin resistance by deleting the corresponding gene from the S. Mg1 genome. We observed that aerial growth by the ΔsfhA mutant strain was now sensitive to surfactin. Our results identify an enzyme that hydrolyzes surfactin and confers resistance to aerial growth inhibition, which demonstrates the effective use of an IMS approach to track natural product modifications during interspecies competition.

Vibrio sp. DSM 14379 pigment production--a competitive advantage in the environment?

The ability to produce several antibacterial agents greatly increases the chance of producer's survival. In this study, red-pigmented Vibrio sp. DSM 14379 and Bacillus sp., both isolated from the same sampling volume from estuarine waters of the Northern Adriatic Sea, were grown in a co-culture. The antibacterial activity of the red pigment extract was tested on Bacillus sp. in microtiter plates. The MIC(50) for Bacillus sp. was estimated to be around 10⁻⁵ mg/L. The extract prepared form the nonpigmented mutant of Vibrio sp. had no antibacterial effect. The pigment production of Vibrio sp. was studied under different physicochemical conditions. There was no pigment production at high or low temperatures, high or low salt concentrations in peptone yeast extract (PYE) medium, low glucose concentration in mineral growth medium or high glucose concentration in PYE medium. This indicates that the red pigment production is a luxurious good that Vibrio sp. makes only under favorable conditions. The Malthusian fitness of Bacillus sp. in a co-culture with Vibrio sp. under optimal environmental conditions dropped from 4.0 to -7.6, which corresponds to three orders of magnitude decrease in the number of CFU relative to the monoculture. The nonpigmented mutant of Vibrio sp. in a co-culture with Bacillus sp. had a significant antibacterial activity. This result shows that studying antibacterial properties in isolation (i.e. pigment extract only) may not reveal full antibacterial potential of the bacterial strain. The red pigment is a redundant antibacterial agent of Vibrio sp.

Characterization of Streptomyces spp. isolated from the sea surface microlayer in the Trondheim Fjord, Norway

The water surface microlayer is still poorly explored, although it has been shown to contain a high density of metabolically active bacteria, often called bacterioneuston. Actinomycetes from the surface microlayer in the Trondheim fjord, Norway, have been isolated and characterized. A total of 217 isolates from two separate samples morphologically resembling the genus Streptomyces have been further investigated in this study. Antimicrobial assays showed that about 80% of the isolates exhibited antagonistic activity against non-filamentous fungus, Gram-negative, and Gram-positive bacteria. Based on the macroscopic analyses and inhibition patterns from the antimicrobial assays, the sub-grouping of isolates was performed. Partial 16S rDNAs from the candidates from each subgroup were sequenced and phylogenetic analysis performed. 7 isolates with identical 16S rDNA sequences were further studied for the presence of PKS type I genes. Sequencing and phylogenetic analysis of the PKS gene fragments revealed that horizontal gene transfer between closely related species might have taken place. Identification of unique PKS genes in these isolates implies that de-replication can not be performed based solely on the 16S rDNA sequences. The results obtained in this study suggest that streptomycetes from the neuston population may be an interesting source for discovery of new antimicrobial agents.

Occurrence and distribution antibiotic resistance of heterotrophic bacteria isolated from a marine beach

Antibiotic resistance of heterotrophic bacteria isolated from a sandy beach in Sopot, at the Southern Baltic Sea coast was determined. The levels of resistance of bacteria to various antibiotics differed considerably. Bacteria inhabiting the middle part of the beach and the dune were most resistant; the least resistant were bacteria isolated from the sea-beach contact zone. Generally, there were no significant differences in antibiotic resistance between pigmented and non-pigmented bacteria. Bacteria isolated from the surface layer of the sand were more resistant to the tested antibiotics than bacteria from the subsurface layers. The majority of bacterial strains were resistant to 3-8 antibiotics. Bacterial resistance to antibiotics was dependent on their chemical structure.

2-n-Pentyl-4-quinolinol produced by a marine Alteromonas sp. and its potential ecological and biogeochemical roles

Bacterium-bacterium interactions occur at intimate spatial scales on the order of micrometers, but our knowledge of interactions at this level is rudimentary. Antagonism is a potential interaction in such microenvironments. To study the ecological role of antibiosis, we developed a model system involving an antibiotic-producing isolate (SWAT5) derived from a marine particle and its dominant antibiotic product, 2-n-pentyl-4-quinolinol (PQ). This system was used to address questions about the significance of this antibiotic for microbial ecology and carbon cycling on particles. We characterized the chemical and inhibitory properties of PQ in relation to the mechanisms used by particle-associated bacteria in interacting with particles and with other attached bacteria. PQ was produced by SWAT5 only on surfaces. When SWAT5 was grown in polysaccharide matrices, PQ diffused within the matrices but not into the surrounding seawater. SWAT5 might thus be able to generate a localized zone of high antibiotic concentration on particles suspended or sinking through seawater. Target bacterial respiration was most sensitive to PQ (75 nM), while inhibition of DNA synthesis, protein synthesis, and bacterial motility required higher (micromolar) PQ levels. The presence of PQ altered the composition of the bacterial community that colonized and developed in a model particle system. PQ also inhibited Synechococcus and phytoplankton growth. Our results suggest that antibiosis may significantly influence community composition and activities of attached bacterial and thus regulate the biogeochemical fate of particulate organic matter in the ocean.

The natural functions of secondary metabolites

Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve: (i) as competitive weapons used against other bacteria, fungi, amoebae, plants, insects, and large animals; (ii) as metal transporting agents; (iii) as agents of symbiosis between microbes and plants, nematodes, insects, and higher animals; (iv) as sexual hormones; and (v) as differentiation effectors. Although antibiotics are not obligatory for sporulation, some secondary metabolites (including antibiotics) stimulate spore formation and inhibit or stimulate germination. Formation of secondary metabolites and spores are regulated by similar factors. This similarity could insure secondary metabolite production during sporulation. Thus the secondary metabolite can: (i) slow down germination of spores until a less competitive environment and more favorable conditions for growth exist; (ii) protect the dormant or initiated spore from consumption by amoebae; or (iii) cleanse the immediate environment of competing microorganisms during germination.

Probiotic bacteria as biological control agents in aquaculture

There is an urgent need in aquaculture to develop microbial control strategies, since disease outbreaks are recognized as important constraints to aquaculture production and trade and since the development of antibiotic resistance has become a matter of growing concern. One of the alternatives to antimicrobials in disease control could be the use of probiotic bacteria as microbial control agents. This review describes the state of the art of probiotic research in the culture of fish, crustaceans, mollusks, and live food, with an evaluation of the results obtained so far. A new definition of probiotics, also applicable to aquatic environments, is proposed, and a detailed description is given of their possible modes of action, i.e., production of compounds that are inhibitory toward pathogens, competition with harmful microorganisms for nutrients and energy, competition with deleterious species for adhesion sites, enhancement of the immune response of the animal, improvement of water quality, and interaction with phytoplankton. A rationale is proposed for the multistep and multidisciplinary process required for the development of effective and safe probiotics for commercial application in aquaculture. Finally, directions for further research are discussed.