Growth performance of Listeria monocytogenes and background microbiota from mushroom processing environments

Interaction between Listeria monocytogenes and resident background microbiota may occur in food processing environments and may influence the survival of this pathogen in a factory environment. Therefore the aim of this study was to characterize the growth performance of microbiota isolated from the processing environments of frozen sliced mushrooms, and to investigate the competitive performance of L. monocytogenes when co-cultured with accompanying environmental microbiota. Acinetobacter, Enterobacteriaceae, Lactococcus and Pseudomonas were the most prominent background microbiota isolated from the processing environment of frozen sliced mushrooms. All individual microbiota strains were able to grow and form biofilm in filter-sterilized mushroom medium, with the mannitol-consumers Raoultella and Ewingella as top performers, reaching up to 9.6 and 9.8 log CFU/mL after 48 h incubation at room temperature. When L. monocytogenes mushroom isolates were co-cultured with the microbiota strains, L. monocytogenes counts ranged from 7.6 to 8.9 log CFU/mL after 24 h of incubation, while counts of the microbiota strains ranged from 5.5 to 9.0 log CFU/mL. Prolonged incubation up to 48 h resulted in further increase of L. monocytogenes counts when co-cultured with non-acidifying species Pseudomonas and Acinetobacter reaching 9.1 to 9.2 log CFU/mL, while a decrease of L. monocytogenes counts reaching 5.8 to 7.7 log CFU/mL was observed in co-culture with Enterobacteriaceae and acidifying Lactococcus representatives. In addition, L. monocytogenes grew also in spent mushroom media of the microbiota strains, except in acidified spent media of Lactococcus strains. These results highlight the competitive ability of L. monocytogenes during co-incubation with microbiota in fresh and in spent mushroom medium, indicative of its invasion and persistence capacity in food processing factory environments.

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.

Comparative Genomic Analysis Provides Insights into the Phylogeny, Resistome, Virulome, and Host Adaptation in the Genus Ewingella

Ewingella americana is a cosmopolitan bacterial pathogen that has been isolated from many hosts. Here, we sequenced a high-quality genome of E. americana B6-1 isolated from Flammulina filiformis, an important cultivated mushroom, performed a comparative genomic analysis with four other E. americana strains from various origins, and tested the susceptibility of B6-1 to antibiotics. The genome size, predicted genes, and GC (guanine-cytosine) content of B6-1 was 4.67 Mb, 4301, and 53.80%, respectively. The origin of the strains did not significantly affect the phylogeny, but mobile genetic elements shaped the evolution of the genus Ewingella. The strains encoded a set of common genes for type secretion, virulence effectors, CAZymes, and toxins required for pathogenicity in all hosts. They also had antibiotic resistance, pigments to suppress or evade host defense responses, as well as genes for adaptation to different environmental conditions, including temperature, oxidation, and nutrients. These findings provide a better understanding of the virulence, antibiotic resistance, and host adaptation strategies of Ewingella, and they also contribute to the development of effective control strategies.

Bacteria colonising Penstemon digitalis show volatile and tissue-specific responses to a natural concentration range of the floral volatile linalool

Bacteria on floral tissue can have negative effects by consuming resources and affecting nectar quality, which subsequently could reduce pollinator visitation and plant fitness. Plants however can employ chemical defences to reduce bacteria density. In North American, bee-pollinated Penstemon digitalis, the nectar volatile S-(+)-linalool can influence plant fitness, and terpenes such as linalool are known for their antimicrobial properties suggesting that it may also play a role in plant–microbe interactions. Therefore, we hypothesized linalool could affect bacterial growth on P. digitalis plants/flowers. Because P. digitalis emits linalool from nectar and nectary tissue but not petals, we hypothesised that the effects of linalool could depend on tissue of origin due to varying exposure. We isolated bacteria from nectary tissue, petals and leaves, and compared their growth relative to control using two volatile concentrations representing the natural emission range of linalool. To assess whether effects were specific to linalool, we compared results with the co-occurring nectar volatile, methyl nicotinate. We show that response to floral volatiles can be substance and tissue-origin specific. Because linalool could slow growth rate of bacteria across the P. digitalis phyllosphere, floral emission of linalool could play a role in mediating plant–bacteria interactions in this system.

Effect of fruiting body bacteria on the growth of Tricholoma matsutake and its related molds

Tricholoma matsutake (pine mushroom, PM) is a prized mushroom in Asia due to its unique flavor and pine aroma. The fruiting body of PM forms only in its natural habitat (pine forest), and little is known regarding the natural conditions required for successful generation of the fruiting bodies in this species. Recent studies suggest that microbial interactions may be associated with the growth of PM; however, there have been few studies of the bacterial effects on PM growth. In this study, we surveyed which bacteria can directly and indirectly promote the growth of PM by using co-cultures with PM and molds associated with the fruiting body. Among 16 bacterial species isolated from the fruiting body, some species significantly influenced the mycelial growth of PM and molds. Most bacteria negatively affected PM growth and exhibited various enzyme activities, which suggests that they use the fruiting body as nutrient source. However, growth-promoting bacteria belonging to the Dietzia, Ewingella, Pseudomonas, Paenibacillus, and Rodococcus were also found. In addition, many bacteria suppressed molds, which suggests an indirect positive effect on PM as a biocontrol agent. Our results provide important insights toward a better understanding of the microbial interactions in the fruiting body of PM, and indicate that growth-promoting bacteria may be an important component in successful cultivation of PM.

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.

Characterization of the rcsA Gene from Pantoea sp. Strain PPE7 and Its Influence on Extracellular Polysaccharide Production and Virulence on Pleurotus eryngii

RcsA is a positive activator of extracellular polysaccharide (EPS) synthesis in the Enterobacteriaceae. The rcsA gene of the soft rot pathogen Pantoea sp. strain PPE7 in Pleurotus eryngii was cloned by PCR amplification, and its role in EPS synthesis and virulence was investigated. The RcsA protein contains 3 highly conserved domains, and the C-terminal end of the open reading frame shared significant amino acid homology to the helix-turn-helix DNA binding motif of bacterial activator proteins. The inactivation of rcsA by insertional mutagenesis created mutants that had decreased production of EPS compared to the wild-type strain and abolished the virulence of Pantoea sp. strain PPE7 in P. eryngii. The Pantoea sp. strain PPE7 rcsA gene was shown to strongly affect the formation of the disease symptoms of a mushroom pathogen and to act as the virulence factor to cause soft rot disease in P. eryngii.

Mushroom tumor: a new disease on Flammulina velutipes caused by Ochrobactrum pseudogrignonense

Mushroom tumor on Flammulina velutipes has become the main disease during the off-season cultivation of F. velutipes while the causal organism has remained unknown. The present study was aimed at identifying the pathogen confirming its pathogenisity following Koch's Postulates, characterizing it using morphological, physiological, biochemical and molecular features, and studying its current distribution. We determined that mushroom tumor is a new bacterial infection disease caused by Ochrobactrum pseudogrignonense. It produces tumor-like structures on the surface of the substrate, and inhibits the formation of primordia and fruiting of F. velutipes. The molecular studies showed that this new pathogen is closely related to Ochrobactrum based on 16S rRNA sequences. This is the first time that Ochrobactrum has been shown to be a pathogen of a mushroom.

Characterization of the wzc gene from Pantoea sp. strain PPE7 and its influence on extracellular polysaccharide production and virulence on Pleurotus eryngii

To characterize of the pathogenicity gene from the soft rot pathogen Pantoea sp. PPE7 in Pleurotus eryngii, we constructed over 10,000 kanamycin-resistant transposon mutants of Pantoea sp. strain PPE7 by transposon mutagenesis. One mutant, Pantoea sp. NPPE9535, did not cause a soft rot disease on Pleurotus eryngii was confirmed by the pathogenicity test. The transposon was inserted into the wzc gene and the disruption of the wzc gene resulted in the reduction of polysaccharide production and abolished the virulence of Pantoea sp. strain PPE7 in P. eryngii. Analysis of the hydropathic profile of this protein indicated that it is composed of two main domains: an N-terminal domain including two transmembrane α-helices and a C-terminal cytoplasmic domain consisting of a tyrosine-rich region. Comparative analysis indicated that the amino acid sequence of Wzc is similar to that of a number of proteins involved in the synthesis or export of polysaccharides in other bacterial species. Purified GST-Wzc was found to affect the phosphorylation of tyrosine residue in vivo. These results showed that the wzc gene might play an important role in the virulence of Pantoea sp. strain PPE7 in P. eryngii.

Fungicide Sensitivity and Characterization of Cobweb Disease on a Pleurotus eryngii Mushroom Crop Caused by Cladobotryum mycophilum

In 2009-2010, unusual symptoms were observed on Pleurotus eryngii grown in mushroom farms in Gyeongnam Province, Republic of Korea. One of the main symptoms was a cobweb-like growth of fungal mycelia over the surface of the mushroom. The colonies on the surface rapidly overwhelmed the mushrooms and developed several spores within 3-4 days. The colonized surface turned pale brown or yellow. The fruit body eventually turned dark brown and became rancid. Koch's postulates were completed by spraying and spotting using isolated strains. The phylogenetic tree obtained from the internal transcribed spacer sequence analysis showed that the isolated fungal pathogen corresponded to Cladobotryum mycophilum (99.5%). In the fungicide sensitivity tests, the ED50 values for the isolate with respect to benomyl and carbendazim were from 0.29 to 0.31 ppm. Benzimidazole fungicides were most effective against C. mycophilum, a causal agent of cobweb disease in P. eryngii.

Microbially induced diseases of Agaricus bisporus: biochemical mechanisms and impact on commercial mushroom production

The button mushroom, Agaricus bisporus (Lange) Imbach, the most common cultivated mushroom, is susceptible to a wide range of virus, bacterial, and fungal diseases. However, only some diseases were studied for the mechanisms involved in the host-microorganism interaction. This review deals with biochemical mechanisms related to cavity disease (Burkholderia gladioli) and to the interaction between A. bisporus and the causal agents responsible for the most severe diseases, namely the bacteria Pseudomonas tolaasii and Pseudomonas reactans and the fungi Trichoderma aggressivum and Lecanicillium fungicola.

Occurrence of Internal Stipe Necrosis of Cultivated Mushrooms (Agaricus bisporus) Caused by Ewingella americana in Korea

The internal stipe necrosis of cultivated mushrooms (Agaricus bisporus) is caused by the bacterium Ewingella americana, a species of the Enterobacteriaceae. Recently, Ewingella americana was isolated from cultivated white button mushrooms in Korea evidencing symptoms of internal stipe browning. Its symptoms are visible only at harvest, and appear as a variable browning reaction in the center of the stipes. From these lesions, we isolated one bacterial strain (designated CH4). Inoculation of the bacterial isolate into mushroom sporocarps yielded the characteristic browning symptoms that were distinguishable from those of the bacterial soft rot that is well known to mushroom growers. The results of Gram stain, flagellal staining, and biochemical tests identified these isolates as E. americana. This was verified by pathogenicity, physiological and biochemical characteristics, and the results of an analysis of the 16S rRNA gene sequences and the fatty acids profile. This is the first report of the isolation of E. americana from cultivated white button mushrooms in Korea.

Bacterial/Fungal interactions: from pathogens to mutualistic endosymbionts

A fundamental issue in biology is the question of how bacteria initiate and maintain pathogenic relationships with eukaryotic hosts. Despite billions of years of coexistence, far less is known about bacterial/fungal interactions than the equivalent associations formed by either of these types of microorganisms with higher eukaryotes. This review highlights recent research advances in the field of bacterial/fungal interactions, and provides examples of the various forms such interactions may assume, ranging from simple antagonism and parasitism to more intimate associations of pathogenesis and endosymbiosis. Information derived from the associations of bacteria and fungi in the context of natural and agronomic ecosystems is emphasized, including interactions observed from biological control systems, endosymbiotic relationships, diseases of cultivated mushrooms, and model systems that expand our understanding of human disease. The benefits of studying these systems at the molecular level are also emphasized.

The general secretory pathway of Burkholderia gladioli pv. agaricicola BG164R is necessary for cavity disease in white button mushrooms

Cavity disease in white button mushrooms is caused by Burkholderia gladioli pv. agaricicola. We describe the isolation and characterization of six mutants of the strain BG164R that no longer cause this disease on mushrooms. The mutations were mapped to genes of the general secretory pathway (GSP). This is the first report of the association of the type II secretion pathway with a disease in mushrooms. Phenotypes of the six avirulent mutants were the following: an inability to degrade mushroom tissue, a highly reduced capacity to secrete chitinase and protease, and a reduced number of flagella. Using these mutants, we also made the novel observation that the factors causing mushroom tissue degradation, thereby leading to the expression of cavity disease, can be separated from mycelium inhibition because avirulent mutants continued to inhibit the growth of actively growing mushroom mycelia. The GSP locus of B. gladioli was subsequently cloned and mapped and compared to the same locus in closely related species, establishing that the genetic organization of the gsp operon of B. gladioli pv. agaricicola is consistent with that of other species of the genus. We also identify the most common indigenous bacterial population present in the mushroom fruit bodies from a New Zealand farm, one of which, Ewingella americana, was found to be an apparent antagonist of B. gladioli pv. agaricicola. While other investigators have reported enhanced disease symptoms due to interactions between endogenous and disease-causing bacteria in other mushroom diseases, to the best of our knowledge this is the first report of an antagonistic effect.

Cloning of a chitinase gene from Ewingella americana, a pathogen of the cultivated mushroom, Agaricus bisporus

We have isolated a gene encoding a chitinase (EC 3.2.1.14) from Ewingella americana, a recently described pathogen of the mushroom Agaricus bisporus. This gene, designated chiA (EMBL/Genbank/DDBJ accession number X90562), was cloned by expression screening of a plasmid-based E. americana HindIII genomic library in Escherichia coli using remazol brilliant violet-stained carboxymethylated chitin incorporated into selective medium. The chiA gene has a 918-bp ORF, terminated by a TAA codon, with a calculated polypeptide size of 33.2 kDa, likely corresponding to a previously purified and characterised 33-kDa endochitinase from E. americana. The deduced amino acid sequence shares 33% identity with chitinase II from Aeromonas sp. No. 10S-24 and 7.8% identity with a chitinase from Saccharopolyspora erythraeus. Homology to other chitinase sequences was otherwise low. The peptide sequence deduced from chiA lacks a typical N-terminal signal sequence and also lacks the chitin binding and type III fibronectin homology units common to many bacterial chitinases. The possibility that this chitinase is not primarily adapted for the environmental mineralisation of pre-formed chitin, but rather for the breakdown of nascent chitin, is discussed in the context of mushroom disease.

Aspects of the pathology and etiology of 'drippy gill' disease of the cultivated mushroom Agaricus bisporus

Agaricus bisporus sporocarps exhibiting characteristic 'drippy gill' symptoms from a natural outbreak were examined. Discrete bacterial droplets on the hymenial lamellae often coalesced to form ribbons of bacterial ooze. Longitudinal splits on the stipe were lined with a similar bacterial ooze. Bacteria isolated from both the hymenium and stipe were identified as Pseudomonas agarici, and were confirmed to be the causal organism by satisfying Koch's postulates. By light and transmission electron microscopy, the causal bacteria were found to colonize the extrahyphal spaces and degrade the extracellular matrix within affected sporocarps. Degradation of the extracellular matrix was shown to reduce the integrity of the sporocarp, and result in stipe splitting and hymenium disruption. In artificial inoculations of the pileus, bacteria were shown to exist predominantly in sporocarp tissue below the point of inoculation and above affected areas of the hymenium, indicating an approximately vertical passage through the sporocarp via the extracellular spaces. The dissolution of the extracellular matrix, and the observed failure of the bacterium to produce a toxin active against A. bisporus, allow the bacteria to pass through protective membranes unnoticed, and infect the stipe and hymenium prior to veil break. These observations dispel previous assumptions of intrahyphal existence and transmission. In the few instances in which the bacteria were observed to be intrahyphal, the host fungal cell wall was often broken, suggesting intrahyphal existence was opportunistic rather than obligatory. The taxonomic position of a bacterium isolated previously from sporocarps exhibiting symptoms similar to those of drippy gill was determined by examining the biochemical and nutritional profiles of the bacterium, and comparing them with other Pseudomonas agarici isolates.