Bacillus subtilis extracellular protease production incurs a context-dependent cost

Agro Active Potential of Bacillus subtilis PE7 against Didymella bryoniae (Auersw.), the Causal Agent of Gummy Stem Blight of Cucumis melo

Microbial agents such as the Bacillus species are recognized for their role as biocontrol agents against various phytopathogens through the production of diverse bioactive compounds. This study evaluates the effectiveness of Bacillus subtilis PE7 in inhibiting the growth of Didymella bryoniae, the pathogen responsible for gummy stem blight (GSB) in cucurbits. Dual culture assays demonstrate significant antifungal activity of strain PE7 against D. bryoniae. Volatile organic compounds (VOCs) produced by strain PE7 effectively impede mycelial formation in D. bryoniae, resulting in a high inhibition rate. Light microscopy revealed that D. bryoniae hyphae exposed to VOCs exhibited abnormal morphology, including swelling and excessive branching. Supplementing a potato dextrose agar (PDA) medium with a 30% B. subtilis PE7 culture filtrate significantly decreased mycelial growth. Moreover, combining a 30% culture filtrate with half the recommended concentration of a chemical fungicide yielded a more potent antifungal effect than using the full fungicide concentration alone, inducing dense mycelial formation and irregular hyphal morphology in D. bryoniae. Strain PE7 was highly resilient and was able to survive in fungicide solutions. Additionally, B. subtilis PE7 enhanced the nutrient content, growth, and development of melon plants while mitigating the severity of GSB compared to fungicide and fertilizer treatments. These findings highlight B. subtilis PE7 as a promising biocontrol candidate for integrated disease management in crop production.

Reciprocal sharing of extracellular proteases and extracellular matrix molecules facilitates Bacillus subtilis biofilm formation

Extracellular proteases are a class of public good that support growth of Bacillus subtilis when nutrients are in a polymeric form. Bacillus subtilis biofilm matrix molecules are another class of public good that are needed for biofilm formation and are prone to exploitation. In this study, we investigated the role of extracellular proteases in B.  subtilis biofilm formation and explored interactions between different public good producer strains across various conditions. We confirmed that extracellular proteases support biofilm formation even when glutamic acid provides a freely available nitrogen source. Removal of AprE from the NCIB 3610 secretome adversely affects colony biofilm architecture, while sole induction of WprA activity into an otherwise extracellular protease-free strain is sufficient to promote wrinkle development within the colony biofilm. We found that changing the nutrient source used to support growth affected B. subtilis biofilm structure, hydrophobicity and architecture. We propose that the different phenotypes observed may be due to increased protease dependency for growth when a polymorphic protein presents the sole nitrogen source. We however cannot exclude that the phenotypic changes are due to alternative matrix molecules being made. Co-culture of biofilm matrix and extracellular protease mutants can rescue biofilm structure, yet reliance on extracellular proteases for growth influences population coexistence dynamics. Our findings highlight the intricate interplay between these two classes of public goods, providing insights into microbial social dynamics during biofilm formation across different ecological niches.

Biological Characterization and Whole-Genome Analysis of Bacillus subtilis MG-1 Isolated from Mink Fecal Samples

Bacillus subtilis is an important part of the gut microbiota and a commonly used probiotic. In the present study, to assess the biological characteristics and probiotic properties of B. subtilis derived from mink, we isolated B. subtilis MG-1 isolate from mink fecal samples, characterized its biological characteristics, optimized the hydrolysis of casein by its crude extract, and comprehensively analyzed its potential as a probiotic in combination with whole-genome sequencing. Biological characteristics indicate that, under low-pH conditions (pH 2), B. subtilis MG-1 can still maintain a survival rate of 64.75%; under the conditions of intestinal fluid, gastric acid, and a temperature of 70 °C, the survival rate was increased by 3, 1.15 and 1.17 times compared with the control group, respectively. This shows that it can tolerate severe environments. The results of hydrolyzed casein in vitro showed that the crude bacterial extract of isolate MG-1 exhibited casein hydrolyzing activity (21.56 U/mL); the enzyme activity increased to 32.04 U/mL under optimized reaction conditions. The complete genome sequencing of B. subtilis MG-1 was performed using the PacBio third-generation sequencing platform. Gene annotation analysis results revealed that B. subtilis MG-1 was enriched in several Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathways, and most genes were related to Brite hierarchy pathways (1485-35.31%) and metabolism pathways (1395-33.17%). The egg-NOG annotation revealed that most genes were related to energy production and conversion (185-4.10%), amino acid transport and metabolism (288-6.38%), carbohydrate transport and metabolism (269-5.96%), transcription (294-6.52%), and cell wall/membrane/envelope biogenesis (231-5.12%). Gene Ontology (GO) annotation elucidated that most genes were related to biological processes (8230-45.62%), cellular processes (3582-19.86%), and molecular processes (6228-34.52%). Moreover, the genome of B. subtilis MG-1 was predicted to possess 77 transporter-related genes. This study demonstrates that B. subtilis MG-1 has potential for use as a probiotic, and further studies should be performed to develop it as a probiotic additive in animal feed to promote animal health.

Serine peptidase Vpr forms enzymatically active fibrils outside Bacillus bacteria revealed by cryo-EM

Bacteria develop a variety of extracellular fibrous structures crucial for their survival, such as flagella and pili. In this study, we use cryo-EM to identify protein fibrils surrounding lab-cultured Bacillus amyloiquefaciens and discover an unreported fibril species in addition to the flagellar fibrils. These previously unknown fibrils are composed of Vpr, an extracellular serine peptidase. We find that Vpr assembles into fibrils in an enzymatically active form, potentially representing a strategy of enriching Vpr activities around bacterial cells. Vpr fibrils are also observed under other culture conditions and around other Bacillus bacteria, such as Bacillus subtilis, which may suggest a general mechanism across all Bacillus bacterial groups. Taken together, our study reveals fibrils outside the bacterial cell and sheds light on the physiological role of these extracellular fibrils.

TerC proteins function during protein secretion to metalate exoenzymes

Cytosolic metalloenzymes acquire metals from buffered intracellular pools. How exported metalloenzymes are appropriately metalated is less clear. We provide evidence that TerC family proteins function in metalation of enzymes during export through the general secretion (Sec-dependent) pathway. Bacillus subtilis strains lacking MeeF(YceF) and MeeY(YkoY) have a reduced capacity for protein export and a greatly reduced level of manganese (Mn) in the secreted proteome. MeeF and MeeY copurify with proteins of the general secretory pathway, and in their absence the FtsH membrane protease is essential for viability. MeeF and MeeY are also required for efficient function of the Mn2+-dependent lipoteichoic acid synthase (LtaS), a membrane-localized enzyme with an extracytoplasmic active site. Thus, MeeF and MeeY, representative of the widely conserved TerC family of membrane transporters, function in the co-translocational metalation of Mn2+-dependent membrane and extracellular enzymes.