Transcriptional Downregulator HexS Controlling Prodigiosin and Serrawettin W1 Biosynthesis inSerratia marcescens

Serratia marcescens ATCC 274 increases production of the red pigment prodigiosin in response to Chi phage infection

Serratia marcescens is an opportunistic human pathogen that produces a vibrant red pigment called prodigiosin. Prodigiosin has implications in virulence of S. marcescens and promising clinical applications. We discovered that addition of the virulent flagellotropic bacteriophage χ (Chi) to a culture of S. marcescens stimulates a greater than fivefold overproduction of prodigiosin. Active phage infection is required for the effect, as a χ-resistant strain lacking flagella does not respond to phage presence. Via a reporter fusion assay, we have determined that the addition of a χ-induced S. marcescens cell lysate to an uninfected culture causes a threefold increase in transcription of the pig operon, containing genes essential for pigment biosynthesis. Replacement of the pig promoter with a constitutive promoter abolished the pigmentation increase, indicating that regulatory elements present in the pig promoter likely mediate the phenomenon. We hypothesize that S. marcescens detects the threat of phage-mediated cell death and reacts by producing prodigiosin as a stress response. Our findings are of clinical significance for two main reasons: (i) elucidating complex phage-host interactions is crucial for development of therapeutic phage treatments, and (ii) overproduction of prodigiosin in response to phage could be exploited for its biosynthesis and use as a pharmaceutical.

Increased Proteolytic Activity of Serratia marcescens Clinical Isolate HU1848 Is Associated with Higher eepR Expression

Serratia marcescens is a global opportunistic pathogen. In vitro cytotoxicity of this bacterium is mainly related to metalloprotease serralysin (PrtS) activity. Proteolytic capability varies among the different isolates. Here, we characterized protease production and transcriptional regulators at 37°C of two S. marcescens isolates from bronchial expectorations, HU1848 and SmUNAM836. As a reference strain the insect pathogen S. marcescens Db10 was included. Zymography of supernatant cultures revealed a single (SmUNAM836) or double proteolytic zones (HU1848 and Db10). Mass spectrometry confirmed the identity of PrtS and the serralysin-like protease SlpB from supernatant samples. Elevated proteolytic activity and prtS expression were evidenced in the HU1848 strain through azocasein degradation and qRT-PCR, respectively. Evaluation of transcriptional regulators revealed higher eepR expression in HU1848, whereas cpxR and hexS transcriptional levels were similar between studied strains. Higher eepR expression in HU1848 was further confirmed through an in vivo transcriptional assay. Moreover, two putative CpxR binding motifs were identified within the eepR regulatory region. EMSA validated the interaction of CpxR with both motifs. The evaluation of eepR transcription in a cpxR deletion strain indicated that CpxR negatively regulates eepR. Sequence conservation suggests that regulation of eepR by CpxR is common along S. marcescens species. Overall, our data incorporates CpxR to the complex regulatory mechanisms governing eepR expression and associates the increased proteolytic activity of the HU1848 strain with higher eepR transcription. Based on the global impact of EepR in secondary metabolites production, our work contributes to understanding virulence factors variances across S. marcescens isolates.

BarA/UvrY differentially regulates prodigiosin biosynthesis and swarming motility in Serratia marcescens FS14

BarA/UvrY, a two-component system and global regulator that controls expression of more than a hundred of genes involved in virulence, motility, biofilm formation, and central carbon metabolism under various stress conditions. In this study, we investigated the function of BarA/UvrY system in Serratia marcescens FS14. The disruption of barA or/and uvrY results in the yield increase of secondary metabolite prodigiosin. We further demonstrated that BarA/UvrY system represses prodigiosin production by inhibiting the transcription level of pig gene cluster with direct binding to the pigA promoter. In addition, deletion of barA or/and uvrY abolished the swarming motility of FS14, but not the swimming motility. We revealed that BarA/UvrY activates swarming through directly upregulating the expression of the biosurfactant synthesis gene swrW rather than flagella system. We also observed that BarA/UvrY positively regulates the resistance to H₂O₂ same as in Escherichia coli highlighting the importance of BarA/UvrY on hydrogen peroxide resistance. Our results demonstrated that the BarA/UvrY system differentially regulates the biosynthesis of the secondary metabolite prodigiosin and swarming motility in S. marcescens FS14. Comparison of our results with those observed for Serratia sp. 39006 suggests that BarA/UvrY's role in regulation of secondary metabolite production is different among Serratia species.

PsrA is a novel regulator contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in Serratia marcescens

Serratia marcescens is a Gram-negative bacterium of the Enterobacteriaceae family that can produce numbers of biologically active secondary metabolites. However, our understanding of the regulatory mechanisms behind secondary metabolites biosynthesis in S. marcescens remains limited. In this study, we identified an uncharacterized LysR family transcriptional regulator, encoding gene BVG90_12635, here we named psrA, that positively controlled prodigiosin synthesis in S. marcescens. This phenotype corresponded to PsrA positive control of transcriptional of the prodigiosin-associated pig operon by directly binding to a regulatory binding site (RBS) and an activating binding site (ABS) in the promoter region of the pig operon. We demonstrated that L-proline is an effector for the PsrA, which enhances the binding affinity of PsrA to its target promoters. Using transcriptomics and further experiments, we show that PsrA indirectly regulates pleiotropic phenotypes, including serrawettin W1 biosynthesis, extracellular polysaccharide production, biofilm formation, swarming motility and T6SS-mediated antibacterial activity in S. marcescens. Collectively, this study proposes that PsrA is a novel regulator that contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in S. marcescens and provides important clues for future studies exploring the function of the PsrA and PsrA-like proteins which are widely present in many other bacteria.

Fnr Negatively Regulates Prodigiosin Synthesis in Serratia sp. ATCC 39006 During Aerobic Fermentation

The well-known Crp/Fnr family regulator Fnr has long been recognized as an oxygen sensor to regulate multiple biological processes, including the switch between aerobic/anaerobic metabolism, nitrogen fixation, bioluminescence, infection, and virulence. In most cases, Fnr was found to be active under anaerobic conditions. However, its role in aerobic antibiotic metabolism has not yet been revealed. In this research, we report that in the model organism, Serratia sp. ATCC 39006, Fnr (Ser39006_013370) negatively regulates prodigiosin production by binding to the spacer between the −10 and −35 region in the promoter of prodigiosin biosynthetic gene cluster under aerobic conditions. Fnr was also shown to modulate the anti-bacterial activity and motility by regulating pathway-specific regulatory genes, indicating that Fnr acts as a global regulator in Serratia sp. ATCC 39006. For the first time, we describe that Fnr regulates antibiotic synthesis in the presence of oxygen, which expands the known physiological functions of Fnr and benefits the further investigation of this important transcriptional regulator.

Identification of Essential Genes Associated With Prodigiosin Production in Serratia marcescens FZSF02

Prodigiosin is a promising secondary metabolite produced mainly by Serratia strains. To study the global regulatory mechanism of prodigiosin biosynthesis, a mutagenesis library containing 23,000 mutant clones was constructed with the EZ-Tn5 transposon, and 114 clones in the library showed altered prodigiosin production ability. For 37 of the 114 clones, transposon insertion occurred on the prodigiosin biosynthetic cluster genes; transposon inserted genes of the 77 clones belonged to 33 different outside prodigiosin biosynthetic cluster genes. These 33 genes can be divided into transcription-regulating genes, membrane protein-encoding genes, and metabolism enzyme-encoding genes. Most of the genes were newly reported to be involved in prodigiosin production. Transcriptional levels of the pigA gene were significantly downregulated in 22 mutants with different inserted genes, which was in accordance with the phenotype of decreased prodigiosin production. Functional confirmation of the mutant genes involved in the pyrimidine nucleotide biosynthesis pathway was carried out by adding orotate and uridylate (UMP) into the medium. Gene complementation confirmed the regulatory function of the EnvZ/OmpR two-component regulatory system genes envZ and ompR in prodigiosin production.

The Effect of Secondary Metabolites Produced by Serratia marcescens on Aedes aegypti and Its Microbiota

Serratia marcescens is a bacterial species widely found in the environment, which very efficiently colonizes mosquitoes. In this study, we isolated a red-pigmented S. marcescens strain from our mosquito colony (called S. marcescens VA). This red pigmentation is caused by the production of prodigiosin, a molecule with antibacterial properties. To investigate the role of prodigiosin on mosquito-S. marcescens interactions, we produced two white mutants of S. marcescens VA by random mutagenesis. Whole genome sequencing and chemical analyses suggest that one mutant has a nonsense mutation in the gene encoding prodigiosin synthase, while the other one is deficient in the production of several types of secondary metabolites including prodigiosin and serratamolide. We used our mutants to investigate how S. marcescens secondary metabolites affect the mosquito and its microbiota. Our in vitro tests indicated that S. marcescens VA inhibits the growth of several mosquito microbiota isolates using a combination of prodigiosin and other secondary metabolites, corroborating published data. This strain requires secondary metabolites other than prodigiosin for its proteolytic and hemolytic activities. In the mosquito, we observed that S. marcescens VA is highly virulent to larvae in a prodigiosin-dependent manner, while its virulence on adults is lower and largely depends on other metabolites.

Associations between cellular levels of ATP and prodigiosin pigment throughout the growth cycle of Serratia marcescens

Serratia marcescens is a prolific producer of the red, membrane-associated pigment prodigiosin. Earlier work has established both a positive role for prodigiosin in ATP production during the population lag phase and a negative role during high-rate, low cell density growth. This study uses the growth rate and growth phase modulation afforded by chemostat culture to extend prodigiosin functional analysis to the high-density and stationary phases. Cellular levels of prodigiosin were positively associated with cellular levels of ATP during high-density growth, and artificial pigment induction during this phase increased cellular ATP levels. Following peak high-density ATP per cell, the early stationary phase enabled significant population growth, while prodigiosin levels remained high and ATP declined. During the late stationary phase, ATP per cell was positively associated with prodigiosin per cell, while both declined during continued growth. These results provide correlational evidence for the multiple effects of prodigiosin pigment on ATP production throughout the growth cycle. Earlier work and the data presented here enable the formulation of a working model for the oscillating relationships between cellular levels of ATP and prodigiosin during batch culture.

Isolation and identification of two Serratia marcescens strains from silkworm, Bombyx mori

Bacterial septicemia commonly occurs and usually cause huge losses in sericulture industry. Here, two pathogenic bacterial strains were isolated from dead silkworm and named as ZJ-1 and ZJ-2. Phenotypic and genotypic analysis results revealed that both of these two strains are closely related to Serratia marcescens (S. marcescens). The morphological as well as physiological and biochemical characteristics of ZJ-1 were accordant with S. marcescens mentioned in Bergey's manual of determinative bacteriology, whereas ZJ-2 showed some discrepancies such as the utilization of malonate and starch, fermentation of maltose and sucrose, and tests of urease, etc. Surprisingly, ZJ-2 could produce red pigment at high temperature (37°) but ZJ-1 could not. Besides, by analyzing the lethal concentration 50 (LC₅₀) of ZJ-1 and ZJ-2, it was found that the virulence of ZJ-2 was lower than that of ZJ-1. These results revealed that ZJ-1 and ZJ-2 were two different strains of S. marcescens and that ZJ-2 was expected to be a safe (low-toxicity) and efficient strain for the production of red pigment. Nonetheless, further research in molecular level is needed to understand the regulation mechanism of pigment production and infection of ZJ-2.

RpoS is a pleiotropic regulator of motility, biofilm formation, exoenzymes, siderophore and prodigiosin production, and trade-off during prolonged stationary phase in Serratia marcescens

Prodigiosin is an important secondary metabolite produced by Serratia marcescens. It can help strains resist stresses from other microorganisms and environmental factors to achieve self-preservation. Prodigiosin is also a promising secondary metabolite due to its pharmacological characteristics. However, pigmentless S. marcescens mutants always emerge after prolonged starvation, which might be a way for the bacteria to adapt to starvation conditions, but it could be a major problem in the industrial application of S. marcescens. To identify the molecular mechanisms of loss of prodigiosin production, two mutants were isolated after 16 days of prolonged incubation of wild-type (WT) S. marcescens 1912768R; one mutant (named 1912768WR) exhibited reduced production of prodigiosin, and a second mutant (named 1912768W) was totally defective. Comparative genomic analysis revealed that the two mutants had either mutations or deletions in rpoS. Knockout of rpoS in S. marcescens 1912768R had pleiotropic effects. Complementation of rpoS in the ΔrpoS mutant further confirmed that RpoS was a positive regulator of prodigiosin production and that its regulatory role in prodigiosin biosynthesis was opposite that in Serratia sp. ATCC 39006, which had a different type of pig cluster; further, rpoS from Serratia sp. ATCC 39006 and other strains complemented the prodigiosin defect of the ΔrpoS mutant, suggesting that the pig promoters are more important than the genes in the regulation of prodigiosin production. Deletion of rpoS strongly impaired the resistance of S. marcescens to stresses but increased membrane permeability for nutritional competence; competition assays in rich and minimum media showed that the ΔrpoS mutant outcompeted its isogenic WT strain. All these data support the idea that RpoS is pleiotropic and that the loss of prodigiosin biosynthesis in S. marcescens 1912768R during prolonged incubation is due to a mutation in rpoS, which appears to be a self-preservation and nutritional competence (SPANC) trade-off.

Complete Genome Sequence of Temperature-Dependent Pigment-Producing Serratia marcescens ATCC 274

Serratia marcescens ATCC 274 produces the red pigment prodigiosin and the biosurfactant serrawettin W1 at 30°C but not at 37°C. A complete, high-quality genome sequence of S. marcescens ATCC 274 was obtained and found to comprise a single 5,148,533-bp circular genome with 4,799 genes.

Serratia marcescens ATCC 274 produces the red pigment prodigiosin and the biosurfactant serrawettin W1 at 30°C but not at 37°C. A complete, high-quality genome sequence of S. marcescens ATCC 274 was obtained and found to comprise a single 5,148,533-bp circular genome with 4,799 genes.

Loss of Serine-Type D-Ala-D-Ala Carboxypeptidase DacA Enhances Prodigiosin Production in Serratia marcescens

Serratia marcescens, a gram-negative bacterium, found in a wide range of ecological niches can produce several high-value products, including prodigiosin, althiomycin, and serratamolide. Among them, prodigiosin has attracted attention due to its immunosuppressive, antimicrobial, and anticancer properties. However, the regulatory mechanisms behind prodigiosin synthesis in Serratia marcescens remains limited. Here, a transposon mutant library was constructed to identify the genes related to prodigiosin synthesis, and BVG90_02415 gene encoding a peptidoglycan synthesizing enzyme D-Ala-D-Ala carboxypeptidase DacA was found to negatively regulates prodigiosin synthesis. Quantitative measurements revealed that disruption of dacA increased prodigiosin production 1.46-fold that of the wild-type strain JNB5-1 in fermentation medium. By comparing differences in cell growth, pigA gene expression level, cell morphology, membrane permeability, and intracellular prodigiosin concentration between wild-type strain JNB5-1 and dacA mutant SK4-72, results revealed that the mechanism for hyper-producing of prodigiosin by the dacA mutant was probably that dacA disruption enhanced prodigiosin leakage, which in turn alleviated feedback inhibition of prodigiosin and increased expression of pig gene cluster. Collectively, this work provides a novel insight into regulatory mechanisms of prodigiosin synthesis and uncovers new roles of DacA protein in regulating cell growth, cell morphology, and membrane permeability in Serratia marcescens. Finally, this study offers a new strategy for improving production of high-value compounds in Serratia marcescens.

A New Suite of Allelic-Exchange Vectors for the Scarless Modification of Proteobacterial Genomes

Despite the advent of new techniques for genetic engineering of bacteria, allelic exchange through homologous recombination remains an important tool for genetic analysis. Currently, sacB-based vector systems are often used for allelic exchange, but counterselection escape, which prevents isolation of cells with the desired mutation, occasionally limits their utility. To circumvent this, we engineered a series of "pTOX" allelic-exchange vectors. Each plasmid encodes one of a set of inducible toxins, chosen for their potential utility in a wide range of medically important proteobacteria. A codon-optimized rhaS transcriptional activator with a strong synthetic ribosome-binding site enables tight toxin induction even in organisms lacking an endogenous rhamnose regulon. Expression of the gene encoding blue AmilCP or magenta TsPurple nonfluorescent chromoprotein facilitates monitoring of successful single- and double-crossover events using these vectors. The versatility of these vectors was demonstrated by deleting genes in Serratia marcescens, Escherichia coli O157:H7, Enterobacter cloacae, and Shigella flexneri Finally, pTOX was used to characterize the impact of disruption of all combinations of the 3 paralogous S. marcescens peptidoglycan amidohydrolases on chromosomal ampC β-lactamase activity and the corresponding β-lactam antibiotic resistance. Mutation of multiple amidohydrolases was necessary for high-level ampC derepression and β-lactam resistance. These data suggest why β-lactam resistance may emerge during treatment less frequently in S. marcescens than in other AmpC-producing pathogens, like E. cloacae Collectively, our findings suggest that the pTOX vectors should be broadly useful for genetic engineering of Gram-negative bacteria.IMPORTANCE Targeted modification of bacterial genomes is critical for genetic analysis of microorganisms. Allelic exchange is a technique that relies on homologous recombination to replace native loci with engineered sequences. However, current allelic-exchange vectors often enable only weak selection for successful homologous recombination. We developed a suite of new allelic-exchange vectors, pTOX, which were validated in several medically important proteobacteria. They encode visible nonfluorescent chromoproteins that enable easy identification of colonies bearing integrated vectors and permit stringent selection for the second step of homologous recombination. We demonstrate the utility of these vectors by using them to investigate the effect of inactivation of Serratia marcescens peptidoglycan amidohydrolases on β-lactam antibiotic resistance.

Thermoregulation of Prodigiosin Biosynthesis by Serratia marcescens is Controlled at the Transcriptional Level and Requires HexS

Several biotypes of the Gram-negative bacterium Serratia marcescens produce the tri-pyrole pigment and secondary metabolite prodigiosin. The biological activities of this pigment have therapeutic potential. For over half a century it has been known that biosynthesis of prodi giosin is inhibited when bacteria are grown at elevated temperatures, yet the fundamental mechanism underlying this thermoregulation has not been characterized. In this study, chromosomal and plasmid-borne luxCDABE transcriptional reporters revealed reduced transcription of the prodigiosin biosynthetic operon at 37°C compared to 30°C indicating transcriptional control of pigment production. Moreover, induced expression of the prodigiosin biosynthetic operon at 37°C was able to produce pigmented colonies and cultures demonstrating that physiological conditions at 37°C allow prodigiosin production and indicating that post-transcriptional control is not a major contributor to the thermoregulation of prodigiosin pigmentation. Genetic experiments support the model that the HexS transcription factor is a key contributor to thermoregulation of pigmentation, whereas CRP plays a minor role, and a clear role for EepR and PigP was not observed. Together, these data indicate that thermoregulation of prodigiosin production at elevated temperatures is controlled largely, if not exclusively, at the transcriptional level.

Stable Isotope Labeling of Prodiginines and Serratamolides Produced by Serratia marcescens Directly on Agar and Simultaneous Visualization by Matrix-Assisted Laser Desorption/Ionization Imaging High-Resolution Mass Spectrometry

Matrix-assisted laser desorption/ionization imaging high-resolution mass spectrometry (MALDI-imaging-HRMS) is an important technique for visualizing the spatial distribution of compounds directly on the surface of organisms such as microorganisms, insects, plants, animals, and human tissues. However, MALDI-imaging-HRMS and the stable isotope labeling approach have never been combined for the detection and simultaneous visualization of labeled and unlabeled compounds, their analogues and derivatives, as well as their precursors. Herein, we present a methodology that labels microbial secondary metabolites directly on agar with stable isotopes and allows concurrent spatial distribution analyses by MALDI-imaging-HRMS. Using a thin film of labeled agar supplemented with [1-¹³C]-l-proline, [methyl-D₃]-l-methionine, ¹⁵NH₄Cl, or [¹⁵N]-l-serine overlaid on unlabeled agar, we demonstrate the incorporation of labeled precursors into prodiginines and serratamolides produced by an endophytic bacterium, Serratia marcescens, by MALDI-imaging-HRMS and HPLC-HRMS. Further, we show the incorporation of CD₃ into prodigiosin as well as its characteristic fragments directly by MALDI-imaging-HRMS². Our methodology has several advantages over currently existing techniques. First, both labeled and unlabeled compounds can be visualized simultaneously in high spatial resolution along with their labeled and unlabeled precursors. Second, by using a thin film of labeled agar, we utilize minimum amounts of expensive labeled compounds (1-3 mg) ensuring a cost-effective method for investigating biosynthetic pathways. Finally, our method allows in situ visualization and identification of target and nontarget compounds without the need of isolating the compounds. This is important for compounds that are produced by microorganisms in low, physiologically, or ecologically relevant concentrations.

Natural biocide cocktails: Combinatorial antibiotic effects of prodigiosin and biosurfactants

Bacterial secondary metabolites are naturally produced to prevail amongst competitors in a shared habitat and thus represent a valuable source for antibiotic discovery. The transformation of newly discovered antibiotic compounds into effective drugs often requires additional surfactant components for drug formulation. Nature may also provide blueprints in this respect: A cocktail of two compounds consisting of the antibacterial red pigment prodigiosin and the biosurfactant serrawettin W1 is naturally produced by the bacterium Serratia marcescens, which occurs in highly competitive habitats including soil. We show here a combinatorial antibacterial effect of these compounds, but also of prodigiosin mixed with other (bio)surfactants, against the soil-dwelling bacterium Corynebacterium glutamicum taken as a model target bacterium. Prodigiosin exerted a combinatorial inhibitory effect with all tested surfactants in a disk diffusion assay which was especially pronounced in combination with N-myristoyltyrosine. Minimal inhibitory and bactericidal concentrations (MIC and MBC) of the individual compounds were 2.56 μg/mL prodigiosin and 32 μg/mL N-myristoyltyrosine, and the MIC of prodigiosin was decreased by 3 orders of magnitude to 0.005 μg/mL in the presence of 16 μg/mL N-myristoyltyrosine, indicative of synergistic interaction. Investigation of bacterial survival revealed similar combinatorial effects; moreover, antagonistic effects were observed at higher compound concentrations. Finally, the investigation of microcolony formation under combined application of concentrations just below the MBC revealed heterogeneity of responses with cell death or delayed growth. In summary, this study describes the combinatorial antibacterial effects of microbial biomolecules, which may have ecological relevance by inhibiting cohabiting species, but shall furthermore inspire drug development in the combat of infectious disease.

Prodigiosin pigment of Serratia marcescens is associated with increased biomass production

Serratia marcescens is a gram-negative, facultatively-anaerobic bacterium and opportunistic pathogen which produces the red pigment prodigiosin. We employed both batch culture and chemostat growth methods to investigate prodigiosin function in the producing organism. Pigmentation correlated with an increased rate of ATP production during population lag phase. Results with a lacZ transcriptional fusion to the prodigiosin (pig) biosynthetic operon revealed that operon transcription is activated by low cellular levels of ATP at high cell density. Furthermore, these data enabled estimation of the ATP per cell minimum value at which the operon is induced. Pigmented cells were found to accumulate ATP more rapidly and to multiply more quickly than non-pigmented cells during the high density growth phase. Finally, results with both batch and chemostat culture revealed that pigmented cells grow to approximately twice the biomass yield as non-pigmented S. marcescens bacteria. Prodigiosin production may, therefore, provide a growth advantage at ambient temperatures.

Spatial-temporal profiling of prodiginines and serratamolides produced by endophytic Serratia marcescens harbored in Maytenus serrata

An endophytic bacterium, Serratia marcescens MSRBB2, isolated from inner bark of a Cameroonian Maytenus serrata plant, was subjected to the OSMAC (One Strain Many Compounds) approach and metabolic profiling using HPLC-HRMSⁿ. We identified 7 prodiginines along with 26 serratamolides. Their biosynthetic pathways were elucidated by feeding with labeled precursors in combination with HRMSⁿ. Dual-culture confrontation/restriction assays of the bacterial endophyte were devised with coexisting fungal endophytes (Pestalotiopsis virgatula, Aspergillus caesiellus and Pichia spp.) as well as with unrelated, non-endophytic fungi belonging to the same genera. The assays were combined with scanning electron microscopy (SEM) as well as matrix-assisted laser desorption ionization imaging high-resolution mass spectrometry (MALDI-imaging-HRMS) for visualizing, both in high spatial and temporal resolution, the distribution and interplay of the compounds during microbial interactions. We demonstrated the effect of prodigiosin produced by endophytic S. marcescens MSRBB2 as an allelochemical that specifically inhibits coexisting endophytic fungi. Our results provide new insights into the physiological and ecological relevance of prodiginines and serratamolides within the context of allelopathy and chemical defense interaction occurring between coexisting endophytes harbored in M. serrata.

An IgaA/UmoB Family Protein from Serratia marcescens Regulates Motility, Capsular Polysaccharide Biosynthesis, and Secondary Metabolite Production

Secondary metabolites are an important source of pharmaceuticals and key modulators of microbe-microbe interactions. The bacterium Serratia marcescens is part of the Enterobacteriaceae family of eubacteria and produces a number of biologically active secondary metabolites. In this study, we screened for novel regulators of secondary metabolites synthesized by a clinical isolate of S. marcescens and found mutations in a gene for an uncharacterized UmoB/IgaA family member here named gumB Mutation of gumB conferred a severe loss of the secondary metabolites prodigiosin and serratamolide. The gumB mutation conferred pleiotropic phenotypes, including altered biofilm formation, highly increased capsular polysaccharide production, and loss of swimming and swarming motility. These phenotypes corresponded to transcriptional changes in fimA, wecA, and flhD Unlike other UmoB/IgaA family members, gumB was found to be not essential for growth in S. marcescens, yet igaA from Salmonella enterica, yrfF from Escherichia coli, and an uncharacterized predicted ortholog from Klebsiella pneumoniae complemented the gumB mutant secondary metabolite defects, suggesting highly conserved function. These data support the idea that UmoB/IgaA family proteins are functionally conserved and extend the known regulatory influence of UmoB/IgaA family proteins to the control of competition-associated secondary metabolites and biofilm formation.IMPORTANCE IgaA/UmoB family proteins are found in members of the Enterobacteriaceae family of bacteria, which are of environmental and public health importance. IgaA/UmoB family proteins are thought to be inner membrane proteins that report extracellular stresses to intracellular signaling pathways that respond to environmental challenge. This study introduces a new member of the IgaA/UmoB family and demonstrates a high degree of functional similarity between IgaA/UmoB family proteins. Moreover, this study extends the phenomena controlled by IgaA/UmoB family proteins to include the biosynthesis of antimicrobial secondary metabolites.

Suppressor analysis of eepR mutant defects reveals coordinate regulation of secondary metabolites and serralysin biosynthesis by EepR and HexS

The EepR transcription factor positively regulates secondary metabolites and tissue-damaging metalloproteases. To gain insight into mechanisms by which EepR regulates pigment and co-regulated factors, genetic suppressor analysis was performed. Suppressor mutations that restored pigment to the non-pigmented ∆eepR mutant mapped to the hexS ORF. Mutation of hexS also restored haemolysis, swarming motility and protease production to the eepR mutant. HexS is a known direct and negative regulator of secondary metabolites in Serratia marcescens and is a LysR family regulator and an orthologue of LrhA. Here, we demonstrate that HexS directly controls eepR and the serralysin gene prtS. EepR was shown to directly regulate eepR expression but indirectly regulate hexS expression. Together, these data indicate that EepR and HexS oppose each other in controlling stationary phase-associated molecules and enzymes.

Analysis of the genomic sequences and metabolites of Serratia surfactantfaciens sp. nov. YD25T that simultaneously produces prodigiosin and serrawettin W2

Background

Gram-negative bacteria of the genus Serratia are potential producers of many useful secondary metabolites, such as prodigiosin and serrawettins, which have potential applications in environmental bioremediation or in the pharmaceutical industry. Several Serratia strains produce prodigiosin and serrawettin W1 as the main bioactive compounds, and the biosynthetic pathways are co-regulated by quorum sensing (QS). In contrast, the Serratia strain, which can simultaneously produce prodigiosin and serrawettin W2, has not been reported. This study focused on analyzing the genomic sequence of Serratia sp. strain YD25^T isolated from rhizosphere soil under continuously planted burley tobacco collected from Yongding, Fujian province, China, which is unique in producing both prodigiosin and serrawettin W2.

Results

A hybrid polyketide synthases (PKS)-non-ribosomal peptide synthetases (NRPS) gene cluster putatively involved in biosynthesis of antimicrobial serrawettin W2 was identified in the genome of YD25^T, and its biosynthesis pathway was proposed. We found potent antimicrobial activity of serrawettin W2 purified from YD25^T against various pathogenic bacteria and fungi as well as antitumor activity against Hela cells. Subsequently, comparative genomic analyses were performed among a total of 133 Serratia species. The prodigiosin biosynthesis gene cluster in YD25^T belongs to the type I pig cluster, which is the main form of pig-encoding genes existing in most of the pigmented Serratia species. In addition, a complete autoinducer-2 (AI-2) system (including luxS, lsrBACDEF, lsrGK, and lsrR) as a conserved bacterial operator is found in the genome of Serratia sp. strain YD25^T. Phylogenetic analysis based on concatenated Lsr and LuxS proteins revealed that YD25^T formed an independent branch and was clearly distant from the strains that solely produce either prodigiosin or serrawettin W2. The Fe (III) ion reduction assay confirmed that strain YD25^T could produce an AI-2 signal molecule. Phylogenetic analysis using the genomic sequence of YD25^T combined with phylogenetic and phenotypic analyses support this strain as a member of a novel and previously uncharacterized Serratia species.

Conclusion

Genomic sequence and metabolite analysis of Serratia surfactantfaciens YD25^T indicate that this strain can be further explored for the production of useful metabolites. Unveiling the genomic sequence of S. surfactantfaciens YD25^T benefits the usage of this unique strain as a model system for studying the biosynthesis regulation of both prodigiosin and serrawettin W2 by the QS system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3171-7) contains supplementary material, which is available to authorized users.

Response Mechanisms in Serratia marcescens IBBPo15 During Organic Solvents Exposure

Serratia marcescens strain IBB_Po15 (KT315653) which possesses serratiopeptidase (ser) gene (KT894207) exhibited good solvent tolerance. During the exposure of S. marcescens IBB_Po15 cells to 5 % organic solvents, n-decane was less toxic for this bacterium, compared with n-hexane, cyclohexane, ethylbenzene, toluene, and styrene. The exposure of the S. marcescens IBB_Po15 cells to n-hexane, cyclohexane, ethylbenzene, toluene, and styrene induced the formation of large clusters, while in control and n-decane-exposed cells, only organization into small clusters was observed. The data obtained suggested that S. marcescens IBB_Po15 cells produced some secondary metabolites (i.e., surfactant serrawettin, red pigment prodigiosin) which are well known as valuable molecules due to their large applications. The exposure of the bacterial cells to organic solvents induced secondary metabolites profile modifications. However, S. marcescens IBB_Po15 possesses only alkB1, todM, rhlAB, pswP, mpr, and ser genes, the unspecific amplification of other fragments being acquired also when the primers for alkM1, xylM, ndoM, and C23DO genes were used. Modifications of DNA patterns were not depicted in S. marcescens IBB_Po15 cells exposed to organic solvents.

Exploitation of a "hockey-puck" phenotype to identify pilus and biofilm regulators in Serratia marcescens through genetic analysis

Pili are essential adhesive determinants for many bacterial pathogens. A suppressor mutation screen that takes advantage of a pilus-mediated self-aggregative "hockey-puck" colony phenotype was designed to identify novel regulators of type I pili in Serratia marcescens. Mutations that decreased pilus biosynthesis mapped to the fimABCD operon; to the genes alaT, fkpA, and oxyR; upstream of the flagellar master regulator operon flhDC; and to an uncharacterized gene encoding a predicted DUF1401 domain. Biofilm formation and pilus-dependent agglutination assays were used to characterize the relative importance of the identified genes in pilus biosynthesis. Additional mutagenic or complementation analysis was used to verify the role of candidate genes in pilus biosynthesis. Presented data support a model that CRP negatively regulates pilus biosynthesis through increased expression of flhDC and decreased expression of oxyR. Further studies are warranted to determine the mechanism by which these genes mediate pilus biosynthesis or function.

Carbon-Starvation Induces Cross-Resistance to Thermal, Acid, and Oxidative Stress in Serratia marcescens

The broad host-range pathogen Serratia marcescens survives in diverse host and non-host environments, often enduring conditions in which the concentration of essential nutrients is growth-limiting. In such environments, carbon and energy source starvation (carbon-starvation) is one of the most common forms of stress encountered by S. marcescens. Related members of the family Enterobacteriaceae are known to undergo substantial changes in gene expression and physiology in response to the specific stress of carbon-starvation, enabling non-spore-forming cells to survive periods of prolonged starvation and exposure to other forms of stress (i.e., starvation-induced cross-resistance). To determine if carbon-starvation also results in elevated levels of cross-resistance in S. marcescens, both log-phase and carbon-starved cultures, depleted of glucose before the onset of high cell-density stationary-phase, were grown in minimal media at either 30 °C or 37 °C and were then challenged for resistance to high temperature (50 °C), low pH (pH 2.8), and oxidative stress (15 mM H₂O₂). In general, carbon-starved cells exhibited a higher level of resistance to thermal stress, acid stress, and oxidative stress compared to log-phase cells. The extent of carbon-starvation-induced cross-resistance was dependent on incubation temperature and on the particular strain of S. marcescens. In addition, strain- and temperature-dependent variations in long-term starvation survival were also observed. The enhanced stress-resistance of starved S. marcescens cells could be an important factor in their survival and persistence in many non-host environments and within certain host microenvironments where the availability of carbon sources is suboptimal for growth.

A LysR-Type Transcriptional Regulator, RovM, Senses Nutritional Cues Suggesting that It Is Involved in Metabolic Adaptation of Yersinia pestis to the Flea Gut

Yersinia pestis has evolved as a clonal variant of Yersinia pseudotuberculosis to cause flea-borne biofilm–mediated transmission of the bubonic plague. The LysR-type transcriptional regulator, RovM, is highly induced only during Y. pestis infection of the flea host. RovM homologs in other pathogens regulate biofilm formation, nutrient sensing, and virulence; including in Y. pseudotuberculosis, where RovM represses the major virulence factor, RovA. Here the role that RovM plays during flea infection was investigated using a Y. pestis KIM6+ strain deleted of rovM, ΔrovM. The ΔrovM mutant strain was not affected in characteristic biofilm gut blockage, growth, or survival during single infection of fleas. Nonetheless, during a co-infection of fleas, the ΔrovM mutant exhibited a significant competitive fitness defect relative to the wild type strain. This competitive fitness defect was restored as a fitness advantage relative to the wild type in a ΔrovM mutant complemented in trans to over-express rovM. Consistent with this, Y. pestis strains, producing elevated transcriptional levels of rovM, displayed higher growth rates, and differential ability to form biofilm in response to specific nutrients in comparison to the wild type. In addition, we demonstrated that rovA was not repressed by RovM in fleas, but that elevated transcriptional levels of rovM in vitro correlated with repression of rovA under specific nutritional conditions. Collectively, these findings suggest that RovM likely senses specific nutrient cues in the flea gut environment, and accordingly directs metabolic adaptation to enhance flea gut colonization by Y. pestis.

EepR Mediates Secreted-Protein Production, Desiccation Survival, and Proliferation in a Corneal Infection Model

Serratia marcescens is a soil- and water-derived bacterium that secretes several host-directed factors and causes hospital infections and community-acquired ocular infections. The putative two-component regulatory system composed of EepR and EepS regulates hemolysis and swarming motility through transcriptional control of the swrW gene and pigment production through control of the pigA-pigN operon. Here, we identify and characterize a role for EepR in regulation of exoenzyme production, stress survival, cytotoxicity to human epithelial cells, and virulence. Genetic analysis supports the model that EepR is in a common pathway with the widely conserved cyclic-AMP receptor protein that regulates protease production. Together, these data introduce a novel regulator of host-pathogen interactions and secreted-protein production.

Properties and applications of undecylprodigiosin and other bacterial prodigiosins

The growing demand to fulfill the needs of present-day medicine in terms of novel effective molecules has lead to reexamining some of the old and known bacterial secondary metabolites. Bacterial prodigiosins (prodiginines) have a long history of being re markable multipurpose compounds, best examined for their anticancer and antimalarial activities. Production of prodigiosin in the most common producer strain Serratia marcescens has been described in great detail. However, few reports have discussed the ecophysiological roles of these molecules in the producing strains, as well as their antibiotic and UV-protective properties. This review describes recent advances in the production process, biosynthesis, properties, and applications of bacterial prodigiosins. Special emphasis is put on undecylprodigiosin which has generally been a less studied member of the prodigiosin family. In addition, it has been suggested that proteins involved in undecylprodigiosin synthesis, RedG and RedH, could be a useful addition to the biocatalytic toolbox being able to mediate regio- and stereoselective oxidative cyclization. Judging by the number of recent references (216 for the 2007-2013 period), it has become clear that undecylprodigiosin and other bacterial prodigiosins still hold surprises in terms of valuable properties and applicative potential to medical and other industrial fields and that they still deserve continuing research curiosity.

RNA-seq reveals the RNA binding proteins, Hfq and RsmA, play various roles in virulence, antibiotic production and genomic flux in Serratia sp. ATCC 39006

Background

Serratia sp. ATCC 39006 (S39006) is a Gram-negative enterobacterium that is virulent in plant and animal models. It produces a red-pigmented trypyrrole secondary metabolite, prodigiosin (Pig), and a carbapenem antibiotic (Car), as well as the exoenzymes, pectate lyase and cellulase. Secondary metabolite production in this strain is controlled by a complex regulatory network involving quorum sensing (QS). Hfq and RsmA (two RNA binding proteins and major post-transcriptional regulators of gene expression) play opposing roles in the regulation of several key phenotypes within S39006. Prodigiosin and carbapenem production was abolished, and virulence attenuated, in an S39006 ∆hfq mutant, while the converse was observed in an S39006 rsmA transposon insertion mutant.

Results

In order to define the complete regulon of Hfq and RsmA, deep sequencing of cDNA libraries (RNA-seq) was used to analyse the whole transcriptome of S39006 ∆hfq and rsmA::Tn mutants. Moreover, we investigated global changes in the proteome using an LC-MS/MS approach. Analysis of differential gene expression showed that Hfq and RsmA directly or indirectly regulate (at the level of RNA) 4% and 19% of the genome, respectively, with some correlation between RNA and protein expression. Pathways affected include those involved in antibiotic regulation, virulence, flagella synthesis, and surfactant production. Although Hfq and RsmA are reported to activate flagellum production in E. coli and an adherent-invasive E. coli hfq mutant was shown to have no flagella by electron microscopy, we found that flagellar production was increased in the S39006 rsmA and hfq mutants. Additionally, deletion of rsmA resulted in greater genomic flux with increased activity of two mobile genetic elements. This was confirmed by qPCR and analysis of rsmA culture supernatant revealed the presence of prophage DNA and phage particles. Finally, expression of a hypothetical protein containing DUF364 increased prodigiosin production and was controlled by a putative 5′ cis-acting regulatory RNA element.

Conclusion

Using a combination of transcriptomics and proteomics this study provides a systems-level understanding of Hfq and RsmA regulation and identifies similarities and differences in the regulons of two major regulators. Additionally our study indicates that RsmA regulates both core and variable genome regions and contributes to genome stability.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-14-822) contains supplementary material, which is available to authorized users.

Identification of a methicillin-resistant Staphylococcus aureus inhibitory compound isolated from Serratia marcescens

In this study, we identified an antimicrobial compound produced by the Gram-negative bacterium Serratia marcescens. Colonies of S. marcescens inhibited the growth of nine different methicillin-resistant Staphylococcus aureus (MRSA) isolates and several other tested Gram-positive bacterial species, but not Gram-negative bacteria. Genetic analysis revealed the requirement for the swrW gene which codes for a non-ribosomal peptide synthetase that generates the cyclodepsipeptide antibiotic serratamolide, also known as serrawettin W1. This is the first report describing the anti-MRSA properties of serratamolide.

A Serratia marcescens PigP homolog controls prodigiosin biosynthesis, swarming motility and hemolysis and is regulated by cAMP-CRP and HexS

Swarming motility and hemolysis are virulence-associated determinants for a wide array of pathogenic bacteria. The broad host-range opportunistic pathogen Serratia marcescens produces serratamolide, a small cyclic amino-lipid, that promotes swarming motility and hemolysis. Serratamolide is negatively regulated by the transcription factors HexS and CRP. Positive regulators of serratamolide production are unknown. Similar to serratamolide, the antibiotic pigment, prodigiosin, is regulated by temperature, growth phase, HexS, and CRP. Because of this co-regulation, we tested the hypothesis that a homolog of the PigP transcription factor of the atypical Serratia species ATCC 39006, which positively regulates prodigiosin biosynthesis, is also a positive regulator of serratamolide production in S. marcescens. Mutation of pigP in clinical, environmental, and laboratory strains of S. marcescens conferred pleiotropic phenotypes including the loss of swarming motility, hemolysis, and severely reduced prodigiosin and serratamolide synthesis. Transcriptional analysis and electrophoretic mobility shift assays place PigP in a regulatory pathway with upstream regulators CRP and HexS. The data from this study identifies a positive regulator of serratamolide production, describes novel roles for the PigP transcription factor, shows for the first time that PigP directly regulates the pigment biosynthetic operon, and identifies upstream regulators of pigP. This study suggests that PigP is important for the ability of S. marcescens to compete in the environment.

Biosurfactants in agriculture

Agricultural productivity to meet growing demands of human population is a matter of great concern for all countries. Use of green compounds to achieve the sustainable agriculture is the present necessity. This review highlights the enormous use of harsh surfactants in agricultural soil and agrochemical industries. Biosurfactants which are reported to be produced by bacteria, yeasts, and fungi can serve as green surfactants. Biosurfactants are considered to be less toxic and eco-friendly and thus several types of biosurfactants have the potential to be commercially produced for extensive applications in pharmaceutical, cosmetics, and food industries. The biosurfactants synthesized by environmental isolates also has promising role in the agricultural industry. Many rhizosphere and plant associated microbes produce biosurfactant; these biomolecules play vital role in motility, signaling, and biofilm formation, indicating that biosurfactant governs plant–microbe interaction. In agriculture, biosurfactants can be used for plant pathogen elimination and for increasing the bioavailability of nutrient for beneficial plant associated microbes. Biosurfactants can widely be applied for improving the agricultural soil quality by soil remediation. These biomolecules can replace the harsh surfactant presently being used in million dollar pesticide industries. Thus, exploring biosurfactants from environmental isolates for investigating their potential role in plant growth promotion and other related agricultural applications warrants details research. Conventional methods are followed for screening the microbial population for production of biosurfactant. However, molecular methods are fewer in reaching biosurfactants from diverse microbial population and there is need to explore novel biosurfactant from uncultured microbes in soil biosphere by using advanced methodologies like functional metagenomics.

The LysR Transcription Factor, HexS, Is Required for Glucose Inhibition of Prodigiosin Production by Serratia marcescens

Generation of many useful microbe-derived secondary metabolites, including the red pigment prodigiosin of the bacterium Serratia marcescens, is inhibited by glucose. In a previous report, a genetic approach was used to determine that glucose dehydrogenase activity (GDH) is required for inhibiting prodigiosin production and transcription of the prodigiosin biosynthetic operon (pigA-N). However, the transcription factor(s) that regulate this process were not characterized. Here we tested the hypothesis that HexS, a LysR-family transcription factor similar to LrhA of Escherichia coli, is required for inhibition of prodigiosin by growth in glucose. We observed that mutation of the hexS gene in S. marcescens allowed the precocious production of prodigiosin in glucose-rich medium conditions that completely inhibited prodigiosin production by the wild type. Unlike previously described mutants able to generate prodigiosin in glucose-rich medium, hexS mutants exhibited GDH activity and medium acidification similar to the wild type. Glucose inhibittion of pigA expression was shown to be dependent upon HexS, suggesting that HexS is a key transcription factor in secondary metabolite regulation in response to medium pH. These data give insight into the prodigiosin regulatory pathway and could be used to enhance the production of secondary metabolites.

Serratia marcescens quinoprotein glucose dehydrogenase activity mediates medium acidification and inhibition of prodigiosin production by glucose

Serratia marcescens is a model organism for the study of secondary metabolites. The biologically active pigment prodigiosin (2-methyl-3-pentyl-6-methoxyprodiginine), like many other secondary metabolites, is inhibited by growth in glucose-rich medium. Whereas previous studies indicated that this inhibitory effect was pH dependent and did not require cyclic AMP (cAMP), there is no information on the genes involved in mediating this phenomenon. Here we used transposon mutagenesis to identify genes involved in the inhibition of prodigiosin by glucose. Multiple genetic loci involved in quinoprotein glucose dehydrogenase (GDH) activity were found to be required for glucose inhibition of prodigiosin production, including pyrroloquinoline quinone and ubiquinone biosynthetic genes. Upon assessing whether the enzymatic products of GDH activity were involved in the inhibitory effect, we observed that d-glucono-1,5-lactone and d-gluconic acid, but not d-gluconate, were able to inhibit prodigiosin production. These data support a model in which the oxidation of d-glucose by quinoprotein GDH initiates a reduction in pH that inhibits prodigiosin production through transcriptional control of the prodigiosin biosynthetic operon, providing new insight into the genetic pathways that control prodigiosin production. Strains generated in this report may be useful in large-scale production of secondary metabolites.

Serratamolide is a hemolytic factor produced by Serratia marcescens

Serratia marcescens is a common contaminant of contact lens cases and lenses. Hemolytic factors of S. marcescens contribute to the virulence of this opportunistic bacterial pathogen. We took advantage of an observed hyper-hemolytic phenotype of crp mutants to investigate mechanisms of hemolysis. A genetic screen revealed that swrW is necessary for the hyper-hemolysis phenotype of crp mutants. The swrW gene is required for biosynthesis of the biosurfactant serratamolide, previously shown to be a broad-spectrum antibiotic and to contribute to swarming motility. Multicopy expression of swrW or mutation of the hexS transcription factor gene, a known inhibitor of swrW expression, led to an increase in hemolysis. Surfactant zones and expression from an swrW-transcriptional reporter were elevated in a crp mutant compared to the wild type. Purified serratamolide was hemolytic to sheep and murine red blood cells and cytotoxic to human airway and corneal limbal epithelial cells in vitro. The swrW gene was found in the majority of contact lens isolates tested. Genetic and biochemical analysis implicate the biosurfactant serratamolide as a hemolysin. This novel hemolysin may contribute to irritation and infections associated with contact lens use.

Development of natural anti-tumor drugs by microorganisms

Discoveries of tumor-resistant pharmacological drugs have mainly resulted from screening of natural products and their analogs. Some are also discovered incidentally when studying organisms. The great biodiversity of microorganisms raises the possibility of producing secondary metabolites (e.g., mevastatin, lovastatin, epothilone, salinosporamide A) to cope with adverse environments. Recently, natural plant pigments with anti-tumor activities such as β-carotene, lycopene, curcumin and anthocyanins have been proposed. However, many plants have a long life cycle. Therefore, pigments from microorganisms represent another option for the development of novel anti-tumor drugs. Prodigiosin (PG) is a natural red pigment produced by microorganisms, i.e., Serratia marcescens and other gram-negative bacteria. The anti-tumor potential of PG has been widely demonstrated. The families of PG (PGs), which share a common pyrrolylpyrromethene (PPM) skeleton, are produced by various bacteria. PGs are bioactive pigments and are known to exert immunosuppressive properties, in vitro apoptotic effects, and in vivo anti-tumor activities. Currently the most common strain used for producing PGs is S. marcescens. However, few reports have discussed PGs production. This review therefore describes the development of an anti-tumor drug, PG, that can be naturally produced by microorganisms, and evaluates the microbial production system, fermentation strategies, purification and identification processes. The application potential of PGs is also discussed.

The RssB/RssA two-component system regulates biosynthesis of the tripyrrole antibiotic, prodigiosin, in Serratia marcescens

Serratia marcescens CH-1 produces a red, cell-associated pigment, prodigiosin, synthesized by enzymes encoded in the pig operon. The underlying regulatory mechanism, especially its relationship with the RssAB two-component system signaling, remained uncharacterized. Here, we show that phosphorylated RssB (RssB-P) directly binds to the promoter region of the pig operon (pigA promoter), as observed using an electrophoretic mobility shift assay. Furthermore, we identify the RssB-P binding site located downstream of the -10 and -35 regions in pigA using a DNase I footprinting assay. A compilation of the RssB-P binding sites in flhDC, rssB and pigA promoter regions reveals the presence of a conserved core sequence, GAGATTTTAGCTAAATTAATBTTT (B=C, G, or T), which we believe is the RssB binding sequence. Site-specific mutation of conserved nucleotides within the conserved RssB binding sequence in the pigA promoter region leads to absence of retardation in the presence of RssB-P in vitro and elevated transcription of pigA in vivo. These data suggest that RssAB signaling negatively regulates prodigiosin production, and such inhibition is mediated through direct and specific repression of transcriptional activity of the pig operon.

The biosynthesis and regulation of bacterial prodiginines

The red-pigmented prodiginines are bioactive secondary metabolites produced by both Gram-negative and Gram-positive bacteria. Recently, these tripyrrole molecules have received renewed attention owing to reported immunosuppressive and anticancer properties. The enzymes involved in the biosynthetic pathways for the production of two of these molecules, prodigiosin and undecylprodigiosin, are now known. However, the biochemistry of some of the reactions is still poorly understood. The physiology and regulation of prodiginine production in Serratia and Streptomyces are now well understood, although the biological role of these pigments in the producer organisms remains unclear. However, research into the biology of pigment production will stimulate interest in the bioengineering of strains to synthesize useful prodiginine derivatives.