Surfactin production enhances the level of cardiolipin in the cytoplasmic membrane of Bacillus subtilis

Biosurfactants: Secondary Metabolites Involved in the Process of Bioremediation and Biofilm Removal

The search for environmentally friendly methods to remove persistent substances such as organic pollutants and sessile communities such as biofilms that severely affect the environment and human health resulted in biosurfactant discovery. Owing to their low level of toxicity and high biodegradability, biosurfactants are increasingly preferred to be used for removal of pollutants from nature. These amphipathic molecules can be synthesized inexpensively, employing cheap substrates such as agricultural and industrial wastes. Recent progress has been made in identifying various biosurfactants that can be used to remove organic pollutants and harmful microbial aggregates, as well as novel microbial strains that produce these surface-active molecules to survive in a hydrocarbon-rich environment. This review focuses on the identification and understanding the role of biosurfactants and the microorganisms involved in the removal of biofilms and remediation of xenobiotics and various types of hydrocarbons such as crude oil, aromatic hydrocarbons, n-alkanes, aliphatic hydrocarbons, asphaltenes, naphthenes, and other petroleum products. This property of biosurfactant is very important as biofilms are of great concern due to their impact on the environment, public health, and industries worldwide. This work also includes several advanced molecular methods that can be used to enhance the production of biosurfactants by the microorganisms studied.

Membrane lytic activity of antibacterial ionenes, critical role of phosphatidylcholine (PC) and cardiolipin (CL)

Understanding the mechanism by which an antibacterial agent interacts with a model membrane provides vital information for better design of future antibiotics. In this study, we investigated two antibacterial polymers, hydrophilic C0-T-p and hydrophobic C8-T-p ionenes, known for their potent antimicrobial activity and ability to disrupt the integrity of lipid bilayers. Our hypothesize is that the composition of a lipid bilayer alters the mechanism of ionenes action, potentially providing an explanation for the observed differences in their bioactivity and selectivity. Calcein release experiments utilizing a range of liposomes to examine the impact of (i) cardiolipin (CL) to phosphatidylglycerol (PG) ratio, (ii) overall vesicle charge, and (iii) phosphatidylethanolamine (PE) to phosphatidylcholine (PC) ratio on the activity of ionenes were performed. Additionally, polymer-bilayer interactions were also investigated through vesicle fusion assay and the black lipid membrane (BLM) technique The activity of C0-T-p is strongly influenced by the amount of cardiolipin, while the activity of C8-T-p primarily depends on the overall vesicle charge. Consequently, C0-T-p acts through interactions with CL, whereas C8-T-p modifies the bulk properties of the membrane in a less-specific manner. Moreover, the presence of a small amount of PC in the membrane makes the vesicle resistant to permeabilization by tested molecules. Intriguingly, more hydrophilic C0-T-p retains higher membrane activity compared to the hydrophobic C8-T-p. However, both ionenes induce vesicle fusion and increase lipid bilayer ion permeability.

Added Value of Biophysics to Study Lipid-Driven Biological Processes: The Case of Surfactins, a Class of Natural Amphiphile Molecules

The role of membrane lipids is increasingly claimed to explain biological activities of natural amphiphile molecules. To decipher this role, biophysical studies with biomimetic membrane models are often helpful to obtain insights at the molecular and atomic levels. In this review, the added value of biophysics to study lipid-driven biological processes is illustrated using the case of surfactins, a class of natural lipopeptides produced by Bacillus sp. showing a broad range of biological activities. The mechanism of interaction of surfactins with biomimetic models showed to be dependent on the surfactins-to-lipid ratio with action as membrane disturber without membrane lysis at low and intermediate ratios and a membrane permeabilizing effect at higher ratios. These two mechanisms are relevant to explain surfactins' biological activities occurring without membrane lysis, such as their antiviral and plant immunity-eliciting activities, and the one involving cell lysis, such as their antibacterial and hemolytic activities. In both biological and biophysical studies, influence of surfactin structure and membrane lipids on the mechanisms was observed with a similar trend. Hence, biomimetic models represent interesting tools to elucidate the biological mechanisms targeting membrane lipids and can contribute to the development of new molecules for pharmaceutical or agronomic applications.

Effects of homemade biosurfactant from Bacillus methylotrophicus on bioremediation efficiency of a clay soil contaminated with diesel oil

Despite constant progress in the understanding of the mechanisms related to the effects of biosurfactants in the bioremediation processes of oily residues, the possibility of antagonist effects on microbial growth and the production in situ of these compounds must be elucidated. The aims of this work were a) to evaluate the effects of the addition of a homemade biosurfactant of Bacillus methylotrophicus on the microbial count in soil in order to determine the possibility of inhibitory effects, and b) to accomplish biostimulation using media prepared with whey and bioaugmentation with B. methylotrophicus, analyzing the effects on the bioremediation of diesel oil and evidencing the in situ production of biosurfactants through effects on surface tension. The homemade bacterial biosurfactant did not present inhibitory effects acting as a biostimulant until 4000 mg biosurfactant/kg of soil. The biostimulation and bioaugmentation presented similar better results (p > 0.05) with the degradation of oil (~60%) than natural attenuation due to the low quantities of biostimulants added. For bioaugmentated and biostimulated soils, a decrease of surface tension between 30 and 60 days was observed, indicating the production of tensoactives in the soil, which was not observed in natural attenuation or a control treatment.

Integrated Proteomics and Lipidomics Reveal That the Swarming Motility of Paenibacillus polymyxa Is Characterized by Phospholipid Modification, Surfactant Deployment, and Flagellar Specialization Relative to Swimming Motility

Paenibacillus polymyxa is a Gram-positive bacterium commonly found associated with plant roots. P. polymyxa can exhibit two forms of flagellar motility: swimming in liquid culture and swarming on a surface. Here, swimming cells were compared to swarming cells using an integrated proteomic and lipidomic approach, yielding information about how lipid modifications and protein/enzyme pathways are tailored for these specific phenotypes. Observed differences in both phospholipid composition and metabolism between the two conditions suggest membrane remodeling in response to the surrounding environment. Key enzymes involved in glycerophospholipid metabolism were abundant in swimming bacteria, while enzymes associated with glycerol-3-phosphate metabolism were more abundant in swarming bacteria. Several glycoside hydrolases were either unique to or more abundant during swarming. This likely reflects the degradation of their own exopolysaccharides to both enhance swarming and supply the necessary chemical energy to compensate for increased flagellar synthesis. The observed upregulation of biosynthetic gene clusters (polyketides, lantibiotics, and surfactin) in swarming bacteria suggest the importance of signaling, antimicrobial activity, and surfactin production during this mode of motility – the latter of which is confirmed via RT-PCR.

Linearmycins are lytic membrane-targeting antibiotics

The linearmycin family of polyketides was originally classified as antifungal metabolites. However, in addition to antifungal activity, we previously found that linearmycins cause cellular lysis and colony degradation of the Gram-positive bacterium Bacillus subtilis. We recently showed that Streptomyces sp. strain Mg1 incorporates linearmycins into extracellular vesicles, which are capable of lysing B. subtilis. However, the mechanism of linearmycin-induced lysis was hitherto unexplored. Therefore, we sought to determine how linearmycin-laden vesicles cause lysis. In this study, we found that linearmycins inhibited the growth of all Gram-positive bacteria that we tested, but lysis was limited to some Bacillus species. Next, we found that linearmycin-induced lysis occurred even when cellular metabolism and growth were inhibited, which suggested that linearmycins possess the intrinsic capacity to lyse cells, unlike cell-wall targeting antibiotics. We showed that linearmycin exposure caused changes consistent with rapid depolarization of the B. subtilis cytoplasmic membrane, which was correlated with a loss of viability. Finally, using liposomes as in vitro membrane models, we demonstrated that linearmycins are capable of disrupting lipid bilayers without any other cellular components. Taken together, our results strongly indicate that the cytoplasmic membrane is the direct antibacterial target of linearmycins.

Bacillus subtilis alters the proportion of major membrane phospholipids in response to surfactin exposure

Surfactin, an anionic lipopeptide produced by Bacillus subtilis, is an antimicrobial that targets the cytoplasmic membrane. Nowadays it appears increasingly apparent that the mechanism of resistance against these types of antibiotics consists of target site modification. This prompted us to investigate whether the surfactin non-producing strain B. subtilis 168 changes its membrane composition in response to a sublethal surfactin concentration. Here we show that the exposure of B. subtilis to surfactin at concentrations of 350 and 650 μg/ml (designated as SF350 and SF650, respectively) leads to a concentration-dependent growth arrest followed by regrowth with an altered growth rate. Analysis of the membrane lipid composition revealed modifications both in the polar head group and the fatty acid region. The presence of either surfactin concentration resulted in a reduction in the content of the major membrane phospholipid phosphatidylglycerol (PG) and increase in phosphatidylethanolamine (PE), which was accompanied by elevated levels of phosphatidic acid (PA) in SF350 cultures. The fatty acid analysis of SF350 cells showed a marked increase in non-branched high-melting fatty acids, which lowered the fluidity of the membrane interior measured as the steady-state fluorescence anisotropy of DPH. The liposome leakage of carboxyfluorescein-loaded vesicles resembling the phospholipid composition of surfactin-adapted cells showed that the susceptibility to surfactin-induced leakage is strongly reduced when the PG/PE ratio decreases and/or PA is included in the target bilayer. We concluded that the modifications of the phospholipid content of B. subtilis cells might provide a self-tolerance of the membrane active surfactin.

Lipid composition in a strain of Bacillus subtilis, a producer of iturin A lipopeptides that are active against uropathogenic bacteria

Abstract

Urinary tract infections are a common disease in humans. Therefore, new methods are needed to destroy biofilms that are formed by uropathogens. Iturin A lipopeptides (LPs) C14 and C15 are potent biosurfactants synthetized by the Bacillus subtilis I′1a strain. The biological activity of extracted LPs was confirmed by examining extracts from I′1a cultures against uropathogenic bacteria that had been isolated from biofilms on urinary catheters. Compared with cultures of DSM 3257, which produce surfactin at a relatively low level, the extract obtained from strain I′1a exhibited a greater inhibitory effect against both planktonic and sessile forms of Escherichia coli, Serratia marcescens, Enterobacter cloacae, Proteus mirabilis, Citrobacter freundii and Enterococcus faecalis. Moreover, cyclic LP biosurfactants may disturb the integrity of cytoplasmic membranes; therefore, we investigated the effects of synthetized LPs on fatty acids and phospholipids of B. subtilis. LPs and lipids were analyzed using GC–MS, LC–MS/MS and MALDI-TOF/TOF techniques. Compared with B. subtilis DSM 3257, membranes of the I′1a strain were characterized by an increased amount of anteiso fatty acids and a ten-fold higher ratio of phosphatidylglycerol (PG)-to-phosphatidylethanolamine (PE). Interestingly, in cultures of B. subtilis DSM 3257 supplemented with LP extracts of the I′1a strain, the PG-to-PE ratio was fourfold higher, and the amount of anteiso fatty acids was also increased.

Graphical Abstract

Not so simple, not so subtle: the interspecies competition between Bacillus simplex and Bacillus subtilis and its impact on the evolution of biofilms

Bacillus subtilis biofilms have a fundamental role in shaping the soil ecosystem. During this process, they unavoidably interact with neighbour bacterial species. We studied the interspecies interactions between biofilms of the soil-residing bacteria B. subtilis and related Bacillus species. We found that proximity between the biofilms triggered recruitment of motile B. subtilis cells, which engulfed the competing Bacillus simplex colony. Upon interaction, B. subtilis secreted surfactin and cannibalism toxins, at concentrations that were inert to B. subtilis itself, which eliminated the B. simplex colony, as well as colonies of Bacillus toyonensis. Surfactin toxicity was correlated with the presence of short carbon-tail length isomers, and synergistic with the cannibalism toxins. Importantly, during biofilm development and interspecies interactions a subpopulation in B. subtilis biofilm lost its native plasmid, leading to increased virulence against the competing Bacillus species. Overall, these findings indicate that genetic programs and traits that have little effect on biofilm development when each species is grown in isolation have a dramatic impact when different bacterial species interact.

Effects of membrane lipid composition and antibacterial drugs on the rigidity of Escherichia coli: Different contributions of various bacterial substructures

The rigidity/stiffness is an important biomechanical property of bacteria and potentially correlated with many bacterial activities. While the rigidity or fluidity of the bacterial membrane has been extensively studied, the contributions of different bacterial substructures to the bacterial rigidity are less investigated. Here, we utilized four Escherichia coli (E. coli) strains with different membrane lipid compositions and three antibacterial drugs (EDTA, lysozyme, and streptomycin) to specifically alter bacterial substructures. By using atomic force microscopy (AFM), we found that the average height and Young's modulus of phosphatidylethanolamine (PE)-deficient E. coli strains were larger than those of PE(+) strains and that EDTA, EDTA plus lysozyme instead of lysozyme alone, and streptomycin all caused significant decreases in height and Young's modulus of the four E. coli strains. Our data imply that membrane lipid composition, the integrated outer membrane, the cell wall, and the cytoplasmic content are all responsible for bacterial rigidity but to different extents.

Therapeutic cyclic lipopeptides mining from microbes: latest strides and hurdles

Infectious diseases impose serious public health burdens and often have devastating consequences. The cyclic lipopeptides elaborated by bacteria Bacillus, Paenibacillus, Pseudomonas, Streptomyces, Serratia, Propionibacterium and fungus Fusarium are very crucial in restraining the pathogens. Composed of a peptide and a fatty acyl moiety these amphiphilic metabolites exhibit broad spectrum antimicrobial effects. Among the plethora of cyclic lipopeptides, only selective few have emerged as robust antibiotics. For their functional vigor, polymyxin, daptomycin, surfactin, iturin, fengysin, paenibacterin and pseudofactin have been integrated in mainstream healthcare. Daptomycin has been a significant part of antimicrobial arsenal since the past decade. As the magnitude of drug resistance rises in unprecedented manner, the urgency of prospecting novel cyclic lipopeptides is being perceived. Intense research has revealed the implication of these bioactive compounds stretching beyond antibacterial and antifungal. Anticancer, immunomodulatory, prosthetic parts disinfection and vaccine adjuvancy are some of the validated prospects. This review discusses the emerging applications, mechanisms governing the biological actions, role of genomics in refining structure and function, semi-synthetic analog discovery, novel strain isolation, setbacks etc. Though its beyond the scope of the current topic, for holistic purpose, the role of lipopeptides in bioremediation and crop biotechnology has been briefly outlined. This updated critique is expected to galvanize innovations and diversify therapeutic recruitment of microbial lipopeptides.