Several pseudomonads, associated with the cultivated mushrooms Agaricus bisporus or Pleurotus sp., are hemolytic

Ewingella americana: An Emerging Multifaceted Pathogen of Edible Mushrooms

Mycopathogenic bacteria play a pivotal role in the productivity of edible mushrooms grown under controlled conditions. In this study, we carried out a comprehensive farm survey and sampling (2018 to 2021) on button mushroom (Agaricus bisporus) farms in 15 provinces in Iran to monitor the status of bacterial pathogens infecting the crop. Mycopathogenic bacterial strains were isolated from pins, stems, and caps, as well as the casing layer on 38 mushroom farms. The bacterial strains incited symptoms on mushroom caps ranging from faint discoloration to dark brown and blotch of the inoculated surfaces. Among the bacterial strains inciting disease symptoms on bottom mushroom, 40 were identified as Ewingella americana based on biochemical assays and phylogeny of 16S rRNA and the gyrB gene. E. americana strains differed in their aggressiveness on mushroom caps and stipes, where the corresponding symptoms ranged from deep yellow to dark brown. In the phylogenetic analyses, all E. americana strains isolated in this study were clustered in a monophyletic clade closely related to the nonpathogenic and environmental strains of the species. BOX-PCR-based fingerprinting revealed intraspecific diversity. Using the cutoff level of 73 to 76% similarity, the strains formed six clusters. A chronological pattern was observed, where the strains isolated in 2018 were differentiated from those isolated in 2020 and 2021. Taken together, due to the multifaceted nature of the pathogen, such a widespread occurrence of E. americana on mushroom farms in Iran could be an emerging threat for the mushroom industry in the country.

Bioactive Lipodepsipeptides Produced by Bacteria and Fungi

Natural products are a vital source for agriculture, medicine, cosmetics and other fields. Lipodepsipeptides (LPDs) are a wide group of natural products distributed among living organisms such as bacteria, fungi, yeasts, virus, insects, plants and marine organisms. They are a group of compounds consisting of a lipid connected to a peptide, which are able to self-assemble into several different structures. They have shown different biological activities such as phytotoxic, antibiotic, antiviral, antiparasitic, antifungal, antibacterial, immunosuppressive, herbicidal, cytotoxic and hemolytic activities. Their biological activities seem to be due to their interactions with the plasma membrane (MP) because they are able to mimic the architecture of the native membranes interacting with their hydrophobic segment. LPDs also have surfactant properties. The review has been focused on the lipodepsipeptides isolated from fungal and bacterial sources, on their biological activity, on the structure–activity relationships of some selected LPD subgroups and on their potential application in agriculture and medicine. The chemical and biological characterization of lipodepsipeptides isolated in the last three decades and findings that resulted from SCI-FINDER research are reported. A critical evaluation of the most recent reviews dealing with the same argument has also been described.

Genomic Characterisation of Mushroom Pathogenic Pseudomonads and Their Interaction with Bacteriophages

Bacterial diseases of the edible white button mushroom Agaricus bisporus caused by Pseudomonas species cause a reduction in crop yield, resulting in considerable economic loss. We examined bacterial pathogens of mushrooms and bacteriophages that target them to understand the disease and opportunities for control. The Pseudomonastolaasii genome encoded a single type III protein secretion system (T3SS), but contained the largest number of non-ribosomal peptide synthase (NRPS) genes, multimodular enzymes that can play a role in pathogenicity, including a putative tolaasin-producing gene cluster, a toxin causing blotch disease symptom. However, Pseudomonasagarici encoded the lowest number of NRPS and three putative T3SS while non-pathogenic Pseudomonas sp. NS1 had intermediate numbers. Potential bacteriophage resistance mechanisms were identified in all three strains, but only P. agarici NCPPB 2472 was observed to have a single Type I-F CRISPR/Cas system predicted to be involved in phage resistance. Three novel bacteriophages, NV1, ϕNV3, and NV6, were isolated from environmental samples. Bacteriophage NV1 and ϕNV3 had a narrow host range for specific mushroom pathogens, whereas phage NV6 was able to infect both mushroom pathogens. ϕNV3 and NV6 genomes were almost identical and differentiated within their T7-like tail fiber protein, indicating this is likely the major host specificity determinant. Our findings provide the foundations for future comparative analyses to study mushroom disease and phage resistance.

Study of the Regulatory Role of N-Acyl Homoserine Lactones Mediated Quorum Sensing in the Biological Activity of Burkholderia gladioli pv. agaricicola Causing Soft Rot of Agaricus spp

Many Burkholderia spp. produce in vitro secondary metabolites with relevant biological activities and potential practical applications. Burkholderia gladioli pv. agaricicola (Bga) possess promising biological activities regulated by N-Acyl homoserine lactones (N.AHLs) based quorum sensing (QS) mechanism. In the current study, N.AHLs-deficient (ICMP11096glad-I) and N.AHLs-complemented (ICMP11096glad-IR) mutants were constructed in which the gene coding for AHL synthase was inactivated by allelic exchange in glad I mutant strain. The aims of this research were to (i) assess the antagonistic activity of the wild type (WT) and the glad-I mutant of Bga against Bacillus megaterium (G+ve) and Escherichia coli (G-ve), (ii) screen their hydrolytic enzymes and hemolytic substances, (iii) monitor the pathogenic effect against Agaricus bisporus, and finally (iv) analyze the bioactive secondary metabolites produced by WT and mutant strain using high performance liquid chromatography (HPLC). Results showed that N.AHLs-deficient mutant exhibited high reduction of antagonistic activity against the tested microorganisms and notable reduction of chitinolytic, proteolytic and glucanolytic activities and complete absence of hemolytic activity, and the glad-IR complemented mutant was able to regain the major part of these activities. Furthermore, N.AHLs-deficient mutant strain was unable to degrade flesh cubes pseudo-tissues of A. bisporus. On the other hand, the virulence effect of complemented mutant was like to the parental WT strain. HPLC analysis revealed that some of the single components produced by WT strain were absent in N.AHLs-deficient mutant and others were highly reduced. The out-findings of the current research gave a spot into the regulatory role of N.AHLs and QS phenomenon in the biological activity of Bga bacterium.

Resistance Sources to Brown Blotch Disease (Pseudomonas tolaasii) in a Diverse Collection of Pleurotus Mushroom Strains

Brown blotch disease (BBD) caused by Pseudomonas tolaasii is one of the most devastating diseases of Pleurotus spp. worldwide. Breeding for resistant strains is the most effective method for controlling BBD. To identify resistant germplasm for BBD management, 97 strains comprising 21 P. cf. floridanus, 20 P. ostreatus, and 56 P. pulmonarius were screened by two different methods; namely, inoculation of the pathogen on the mushroom pileus (IMP) and on the spawned substrate (IMSS) under controlled conditions. Out of the 97 strains screened, 22 P. pulmonarius, and four P. cf. floridanus were moderately resistant to BBD using the IMP method. Eleven P. pulmonarius, six P. cf. florida, and one P. ostreatus strains were highly resistant to BBD using the IMSS method. All of the 97 strains showed varying degrees of susceptibility using the IMP method, but eight strains were completely resistant using the IMSS method. Combining these two methods, five strains were highly resistant (four P. pulmonarius and one P. cf. floridanus) and 11 were moderately resistant (eight P. pulmonarius and three P. cf. floridanus). The resistance sources to P. tolaasii identified in P. pulmonarius and P. cf. floridanus could be used for further breeding of Pleurotus spp.

Biological Investigations and Spectroscopic Studies of New Moxifloxacin/Glycine-Metal Complexes

Two novel ligand-metal complexes were prepared through the reaction of Zn(II) and Sn(II) with moxifloxacin (MOX) in the presence of glycine (Gly) to investigate their biological activities. IR, UV/VIS and ¹ H-NMR analysis have been carried out for insuring the chelation process. Results suggested that MOX and Gly react with the metal ions through the carbonyl oxygen atom and the oxygen atom of the carboxylic group of MOX and Gly. The antimicrobial activity was carried out against some common bacterial and fungal pathogens and the radical scavenging activity (RSA%) was evaluated using DPPH and ABTS methods. Phytotoxic effect of the prepared complexes was evaluated in vitro against Raphanus raphanistrum and Lepidium sativum. Hemolytic activity was tested against cell membrane of erythrocytes. Results showed that the two prepared complexes exhibited high antimicrobial activity against all tested phytopathogens and no significant phytotoxic effect has been observed. Only MOX-Zn(II) complex showed moderate hemolysis at 100 % concentration.

Phospholipolysis Caused by Different Types of Bacterial Phospholipases During Cold Storage of Bovine Raw Milk Is Prevented by N2 Gas Flushing

Cold storage aims to preserve the quality and safety of raw milk from farms to dairies; unfortunately, low temperatures also promote the growth of psychrotrophic bacteria, some of which produce heat-stable enzymes that cause spoilage of milk or dairy products. Previously, N2 gas flushing of raw milk has demonstrated significant potential as a method to hinder bacterial growth at both laboratory and pilot plant scales. Using a mass spectrometry-based lipidomics approach, we examined the impact of cold storage [at 6°C for up to 7 days, the control condition (C)], on the relative amounts of major phospholipids (phosphatidylethanolamine/PE, phosphatidylcholine/PC, phosphatidylserine/PS, phosphatidylinositol/PI, and sphingomyelin/SM) in three bovine raw milk samples, and compared it to the condition that received additional N2 gas flushing (N). As expected, bacterial growth was hindered by the N2-based treatment (over 4 log-units lower at day 7) compared to the non-treated control condition. At the end of the cold storage period, the control condition (C7) revealed higher hydrolysis of PC, SM, PE, and PS (the major species reached 27.2, 26.7, 34.6, and 9.9 μM, respectively), compared to the N2-flushed samples (N7) (the major species reached 55.6, 35.9, 54.0, and 18.8 μM, respectively). C7 samples also exhibited a three-fold higher phosphatidic acid (PA) content (6.8 μM) and a five-fold higher content (17.3 μM) of lysophospholipids (LPE, LPC, LPS, and LPI) whereas both lysophospholipids and PA remained at their initial levels for 7 days in N7 samples. Taking into consideration the significant phospholipid losses in the controls, the lipid profiling results together with the microbiological data suggest a major role of phospholipase (PLase) C (PLC) in phospholipolysis during cold storage. However, the experimental data also indicate that bacterial sphingomyelinase C, together with PLases PLD and PLA contributed to the degradation of phospholipids present in raw milk as well, and potential contributions from PLB activity cannot be excluded. Altogether, this lipidomics study highlights the beneficial effects of N2 flushing treatment on the quality and safety of raw milk through its ability to effectively hinder phospholipolysis during cold storage.

Diseases and pests noxious to Pleurotus spp. mushroom crops

The Pleurotus genus is one of most extensively studied white-rot fungi due to its exceptional ligninolytic properties. It is an edible mushroom that possesses biological effects, as it contains important bioactive molecules. It is a rich source of nutrients, particularly proteins, minerals as well as vitamins B, C and D. In basidiomycete fungi, intensive cultivations of edible mushrooms can often be affected by some bacterial, mold and virus diseases that rather frequently cause dramatic production loss. These infections are facilitated by the particular conditions under which mushroom cultivation is commonly carried out such as warm temperatures, humidity, carbon dioxide (CO₂) levels and presence of pests. There is not much bibliographic information related to pests of mushrooms and their substrates. The updated review presents a practical checklist of diseases and pests of the Pleurotus genus, providing useful information that may help different users.

In vitro study of biological activity of four strains of Burkholderia gladioli pv. agaricicola and identification of their bioactive metabolites using GC-MS

This research was carried out to study in vitro antibacterial activity of 4 strains of Burkholderia gladioli pv. agaricicola (Bga) against G+ve Bacillus megaterium and G−ve Escherichia coli, haemolytic activity against the cell membrane of erythrocytes, the production of extracellular hydrolytic enzymes and finally, the pathogenicity against Agaricus bisporus flesh blocks. Chemical structure of bioactive substances of the most bioactive strain (ICMP 11096) was established using gas chromatography–mass spectrometry (GC–MS). All the studied Bga strains inhibited the growth of the two tested bacteria although some growing substrates negatively influenced the antimicrobial substance production. The same Bga strains showed highly haemolytic activity and were able to produce 3 hydrolytic enzymes, i.e. chitinase, glucanase and protease. In pathogenicity assays, the considered Bga strains resulted virulent for A. bisporus. The GC–MS for compounds from Bga ICMP 11096 were compatible with the structure of two bioactive fatty acids identified as methyl stearate and ethanol 2-butoxy phosphate with mass spectrum m/e 298 and 398, respectively.

Pseudomonas protegens Pf-5 causes discoloration and pitting of mushroom caps due to the production of antifungal metabolites

Bacteria in the diverse Pseudomonas fluorescens group include rhizosphere inhabitants known for their antifungal metabolite production and biological control of plant disease, such as Pseudomonas protegens Pf-5, and mushroom pathogens, such as Pseudomonas tolaasii. Here, we report that strain Pf-5 causes brown, sunken lesions on peeled caps of the button mushroom (Agaricus bisporus) that resemble brown blotch symptoms caused by P. tolaasii. Strain Pf-5 produces six known antifungal metabolites under the control of the GacS/GacA signal transduction system. A gacA mutant produces none of these metabolites and did not cause lesions on mushroom caps. Mutants deficient in the biosynthesis of the antifungal metabolites 2,4-diacetylphloroglucinol and pyoluteorin caused less-severe symptoms than wild-type Pf-5 on peeled mushroom caps, whereas mutants deficient in the production of lipopeptide orfamide A caused similar symptoms to wild-type Pf-5. Purified pyoluteorin and 2,4-diacetylphloroglucinol mimicked the symptoms caused by Pf-5. Both compounds were isolated from mushroom tissue inoculated with Pf-5, providing direct evidence for their in situ production by the bacterium. Although the lipopeptide tolaasin is responsible for brown blotch of mushroom caused by P. tolaasii, P. protegens Pf-5 caused brown blotch-like symptoms on peeled mushroom caps through a lipopeptide-independent mechanism involving the production of 2,4-diacetylphloroglucinol and pyoluteorin.

Cyclic lipodepsipeptides produced by Pseudomonas spp. naturally present in raw milk induce inhibitory effects on microbiological inhibitor assays for antibiotic residue screening

Two Pseudomonas strains, identified as closely related to Pseudomonas tolaasii, were isolated from milk of a farm with frequent false-positive Delvotest results for screening putative antibiotic residues in raw milk executed as part of the regulatory quality programme. Growth at 5 to 7°C of these isolates in milk resulted in high lipolysis and the production of bacterial inhibitors. The two main bacterial inhibitors have a molecular weight of 1168.7 and 1140.7 Da respectively, are heat-tolerant and inhibit Geobacillus stearothermophilus var. calidolactis, the test strain of most of the commercially available microbiological inhibitor tests for screening of antibiotic residues in milk. Furthermore, these bacterial inhibitors show antimicrobial activity against Staphylococcus aureus, Bacillus cereus and B. subtilis and also interfere negatively with yoghurt production. Following their isolation and purification with RP-HPLC, the inhibitors were identified by NMR analysis as cyclic lipodepsipeptides of the viscosin group. Our findings bring to light a new challenge for quality control in the dairy industry. By prolonging the refrigerated storage of raw milk, the keeping quality of milk is influenced by growth and metabolic activities of psychrotrophic bacteria such as pseudomonads. Besides an increased risk of possible spoilage of long shelf-life milk, the production at low temperature of natural bacterial inhibitors may also result in false-positive results for antibiotic residue screening tests based on microbial inhibitor assays thus leading to undue production loss.

Pathogenic variation in isolates of Pseudomonas causing the brown blotch of cultivated mushroom, Agaricus bisporus

Twenty seven bacterial isolates were isolated from superficial brown discolorations on the caps of cultivated Agaricus bisporus. After White Line Assay (WLA) and the assist of Biolog computer-identification system, isolates were divided into groups: (I) comprised ninteen bacterial isolates that positively responded to a Pseudomonas “reactans” reference strain (NCPPB1311) in WLA and were identified as Pseudomonas tolaasii, (II) comprised two isolates which were WLA+ towards the reference strain (JCM21583) of P. tolaasii and were proposed to be P. “reactans”. The third group comprised six isolates, two of which weakly responded to the strain of P. tolaasii and were identified as P. gingeri whereas the other four were WLA-and identified as P. fluorescens (three isolates) and P. marginalis (one isolate). Isolates of P. tolaasii showed high aggressiveness compared with those of P. “reactans” in pathogenicity tests. Cubes of 1 cm³ of A. bisporus turned brown and decreased in size when were inoculated with 10 µl of P. tolaasii suspension containing 10⁸ CFU ml^-1, whereas a similar concentration of P. “reactans” caused only light browning. Fifty µl of the same concentration of P. tolaasii isolates gave typical brown blotch symptoms on fresh mushroom sporophores whereas the two P. “reactans” isolates caused superficial light discoloration only after inoculation with 100 µl of the same concentration. Mixture from both bacterial suspensions increased the brown areas formed on the pileus. This is the first pathogenicity report of P. tolasii and P. “reactans” isolated from cultivated A. bisporus in Egypt.

Genetic and functional characterization of cyclic lipopeptide white-line-inducing principle (WLIP) production by rice rhizosphere isolate Pseudomonas putida RW10S2

The secondary metabolite mediating the GacS-dependent growth-inhibitory effect exerted by the rice rhizosphere isolate Pseudomonas putida RW10S2 on phytopathogenic Xanthomonas species was identified as white-line-inducing principle (WLIP), a member of the viscosin group of cyclic lipononadepsipeptides. WLIP producers are commonly referred to by the taxonomically invalid name "Pseudomonas reactans," based on their capacity to reveal the presence of a nearby colony of Pseudomonas tolaasii by inducing the formation of a visible precipitate ("white line") in agar medium between both colonies. This phenomenon is attributed to the interaction of WLIP with a cyclic lipopeptide of a distinct structural group, the fungitoxic tolaasin, and has found application as a diagnostic tool to identify tolaasin-producing bacteria pathogenic to mushrooms. The genes encoding the WLIP nonribosomal peptide synthetases WlpA, WlpB, and WlpC were identified in two separate genomic clusters (wlpR-wlpA and wlpBC) with an operon organization similar to that of the viscosin, massetolide, and entolysin biosynthetic systems. Expression of wlpR is dependent on gacS, and the encoded regulator of the LuxR family (WlpR) activates transcription of the biosynthetic genes and the linked export genes, which is not controlled by the RW10S2 quorum-sensing system PmrR/PmrI. In addition to linking the known phenotypes of white line production and hemolytic activity of a WLIP producer with WLIP biosynthesis, additional properties of ecological relevance conferred by WLIP production were identified, namely, antagonism against Xanthomonas and involvement in swarming and biofilm formation.

Ostreolysin enhances fruiting initiation in the oyster mushroom (Pleurotus ostreatus)

Fruiting initiation in mushrooms can be triggered by a variety of environmental and biochemical stimuli, including substances of natural or synthetic origin. In this work ostreolysin, a cytolytic protein specifically expressed during the formation of primordia and fruit bodies of Pleurotus ostreatus, was applied to nutrient media inoculated with mycelium of P. ostreatus, and its effects on mycelial growth and fructification of the mushroom studied. The addition of ostreolysin slightly inhibited the growth of mycelium, but strongly induced the formation of primordia, which appeared 10 d earlier than in control plates supplemented with bovine serum albumin or with the dissolving buffer alone. Moreover, ostreolysin stimulated the subsequent development of primordia into fruit bodies. However, direct involvement of this protein in the sporulation of the mushroom is unlikely, as it was also detected in large amounts in the non-sporulating strain of P. ostreatus.

Phenotypic characterization of raw milk-associated psychrotrophic bacteria

Among the 68 isolates, selected from 13 raw-milk samples in Finland (that originate from farm, truck or silo tanks), 60 (88%) were psychrotrophs. All the isolates were characterized by the determination of their spoilage and phenotypic features: proteolytic and lipolytic activities, the production of lecithinases and hemolytic factors were considered. Phenotypic characterization of the isolates was mainly performed with API 20NE and BIOLOG GN2 identification systems; the results were system-dependent, although the presence of representatives of the Pseudomonas genus (for the majority of the isolates) was suggested by both systems. The results of the numerical profile analyses by API 20NE proposed that some strains might be members of Stenotrophomonas, Burkholderia and Acinetobacter genera; however, the identity of many isolates remained doubtful or controversial.

Biological characterization of white line-inducing principle (WLIP) produced by Pseudomonas reactans NCPPB1311

The biological activities of the lipodepsipeptides (LDP) white line-inducing principle (WLIP), produced by Pseudomonas reactans NCPPB1311, and tolaasin I, produced by R tolaasii NCPPB2192, were compared. Antimicrobial assays showed that both LDP inhibited the growth of fungi-including the cultivated mushrooms Agaricus bisporus, Lentinus edodes, and Pleurotus spp.--chromista, and gram-positive bacteria. Assays of the two LDP on blocks of Agaricus bisporus showed their capacity to alter the mushrooms' pseudo-tissues though WLIP was less active than that of tolaasin I. Contrary to previous studies, tolaasin I was found to inhibit the growth of gram-negative bacteria belonging to the genera Escherichia, Erwinia, Agrobacterium, Pseudomonas, and Xanthomonas. The only gram-negative bacterium affected by WLIP was Erwinia carotovora subsp. carotovora. Both WLIP and tolaasin I caused red blood cell lysis through a colloid-osmotic shock mediated by transmembrane pores; however, the haemolytic activity of WLIP was greater than that of tolaasin I. Transmembrane pores, at a concentration corresponding to 1.5 x C50, showed a radius between 1.5 and 1.7 +/- 0.1 nm for WLIP and 2.1 +/- 0.1 nm for tolaasin I. The antifungal activity of WLIP together with the finding that avirulent morphological variants of P. reactans lack WLIP production suggests that WLIP may play an important role in the interaction of the producing bacterium P. reactans and cultivated mushrooms.