Quercitrin Is a Novel Inhibitor of Salmonella enterica Serovar Typhimurium Type III Secretion System.

The purpose was to screen type III secretory system (T3SS) inhibitors of Salmonella enterica serovar Typhimurium (S. Typhimurium) from natural compounds. The pharmacological activities and action mechanisms of candidate compounds in vivo and in vitro were systematically studied and analyzed. Using a SipA-ß-lactamase fusion reporting system, we found that quercitrin significantly blocked the translocation of SipA into eukaryotic host cells without affecting the growth of bacteria. Adhesion and invasion assay showed that quercitrin inhibited S. Typhimurium invasion into host cells and reduced S. Typhimurium mediated host cell damage. ß-galactosidase activity detection and Western blot analysis showed that quercitrin significantly inhibited the expression of SPI-1 genes (hilA and sopA) and effectors (SipA and SipC). The results of animal experiments showed that quercitrin significantly reduced colony colonization and alleviated the cecum pathological injury of the infected mice. Small molecule inhibitor quercitrin directly inhibited the function of T3SS and provided a potential antibiotic alternative against S. Typhimurium infection. Importance: T3SS plays a crucial role in the bacterial invasion and pathogenesis of S. Typhimurium. Compared with conventional antibiotics, small molecules could inhibit the virulence factors represented by S. Typhimurium T3SS. They have less pressure on bacterial vitality and a lower probability of producing drug resistance. Our results provide strong evidence for the development of novel inhibitors against S. Typhimurium infection.

Acacetin Alleviates Listeria monocytogenes Virulence Both In Vitro and In Vivo via the Inhibition of Listeriolysin O.

Listeria monocytogenes is a ubiquitous Gram-positive foodborne pathogen that is responsible for listeriosis in both humans and several animal species. The bacterium secretes a pore-forming cholesterol-dependent cytolysin, listeriolysin O (LLO), a major virulence factor involved in the activation of cellular processes. The ability of LLO to lyse erythrocytes is a measure of LLO activity. We used hemolytic activity assay to screen the LLO inhibitors. Acacetin was found to be an LLO inhibitor, which is a di-hydroxy and mono-methoxy flavone present in various plants, including Black locust, Damiana, and Silver birch. As the features of acacetin are of low toxicity and have less acquired resistance, it comes to a hotspot in drug development. In our study, we report that acacetin antagonized the hemolytic activity of L. monocytogenes culture supernatants and purified LLO by directly interfering with the formation of oligomers without inhibiting the bacterial growth and the expression of LLO. Acacetin also relieved the injury of alveolar epithelial cells by inhibiting LLO activity. Further, acacetin significantly promoted the clearance of L. monocytogenes and alleviated the histopathological damage, thereby raising survival rate, which conferred mice with effective protection against L. monocytogenes infection. Using molecular docking and dynamics simulation, we further proved the mechanism of acacetin antagonizing LLO pore-forming activity by direct binding to the second membrane-inserting helix bundle (HB2) of LLO domain 3. These data suggested that acacetin recedes the virulence of L. monocytogenes both in vivo and in vitro, and this study provided a promising candidate and potential alternative for the prevention and treatment of L. monocytogenes infections.

Myricetin inhibits the type III secretion system of Salmonella enterica serovar typhimurium by downregulating the Salmonella pathogenic island I gene regulatory pathway.

Based on the in-depth study of type III secretion systems (T3SS) in pathogenic bacteria, approaches targeting T3SS have become new alternative strategies to combat drug-resistant bacterial infections. As an important food-borne pathogen, Salmonella enterica serovar Typhimurium (S. Typhimurium) injects effector proteins into host cells through the T3SS to disrupt cell signaling and host responses. In this study, myricetin was screened for its ability to block the translocation function of effector proteins (SipA/SipB) using cell biology and molecular biology methods. It exerted strong effects on inhibiting the expression of Salmonella pathogenicity island 1 (SPI-1)-associated effector proteins without affecting S. Typhimurium growth and thus prevented S. Typhimurium from invading HeLa cells and ultimately inhibited S. Typhimurium-mediated cell damage. In an animal experiment, myricetin comprehensively protected mice from death and pathological damage. A further analysis of the mechanism of action showed that myricetin interfered with the regulatory network of SPI-1-related genes, resulting in a significant decrease in the levels of key effector proteins, and thus inhibited T3SS-mediated virulence. In summary, this study provides a solution for clinical resistance to S. Typhimurium infection and potential candidate compounds. Myricetin, a potential T3SS inhibitor, possesses effective biological activity and exerts protective effects in vitro and in vivo. Myricetin will likely be developed as a novel type of antibiotic targeting S. Typhimurium infections in the future.

Isoalantolactone restores the sensitivity of gram-negative Enterobacteriaceae carrying MCR-1 to carbapenems.

Polymyxin B has been re-applied to the clinic as the final choice for the treatment of multidrug-resistant gram-negative pathogenic infections, but the use of polymyxin B has been re-assessed because of the emergence and spread of the plasmid-mediated mcr-1 gene. The purpose of this study was to search for an MCR inhibitor synergistically acting with polymyxin to treat the infection caused by this pathogen. In this study, we used the broth microdilution checkerboard method to evaluate the synergistic effect of isoalantolactone (IAL) and polymyxin B on mcr-1-positive Enterobacteriaceae. Growth curve analysis, time-killing assays and a combined disc test were used to further verify the efficacy of the combined drug. Colonization of the thigh muscle in mice, survival experiments and lung tissue section observations was used to determine the effect of synergy in vivo after Klebsiella pneumoniae and Escherichia coli infection. We screened a natural compound, IAL, which can enhance the sensitivity of polymyxin B to mcr-1-positive Enterobacteriaceae. The results showed that the combined use of polymyxin B and IAL has a synergistic effect on mcr-1-positive Enterobacteriaceae, such as K pneumoniae and E coli, not only in vitro but also in vivo. Our results indicate that IAL is a natural compound with broad application prospects that can prolong the service life of polymyxin B and make outstanding contributions to the treatment of gram-negative Enterobacteriaceae infections resistant to polymyxin B.

Quercetin, a pneumolysin inhibitor, protects mice against Streptococcus pneumoniae infection.

Pneumolysin (PLY), a pore-forming cytotoxin and a major virulence determinant, is a member of the cholesterol-dependent cytolysin (CDC) family and essential for promoting Streptococcus pneumoniae (S.pneumoniae) infection. Due to the action characteristics of hemolysin itself, the pneumolysin released after killing bacteria with conventional antibiotics still has the ability to damage host cells; therefore, drug treatments directly inhibiting hemolysin activity are the most effective. Hemolysis assays were used to confirm that quercetin can inhibit the activity of PLY, protecting cells in vitro, and an oligomerization assay was used to determine the mechanism of quercetin to suppress PLY activity. Live/Dead testing, lactate dehydrogenase (LDH) release analysis and a murine model of endonasal pulmonary infection were used to explore the capability of quercetin to protect cells and mice from S. pneumoniae-mediated damage in vivo and in vitro. The results indicated that quercetin significantly reduced PLY-induced hemolytic activity and cytotoxicity via repressing the formation of oligomers. In addition, treatment with quercetin can reduce PLY-mediated cell injury, improve the survival rate of mice infected with a lethal dose of S. pneumoniae, alleviate the pathological damage of lung tissue and inhibit the release of cytokines (IL-1ß and TNF-α) in bronchoalveolar lavage fluid. Considering the importance of these events in antimicrobial resistant S. pneumoniae pathogenesis, our results indicated that quercetin may be a novel potential drug candidate for the treatment of clinical pneumococcal infections.

Identification of the natural product paeonol derived from peony bark as an inhibitor of the Salmonella enterica serovar Typhimurium type III secretion system.

Salmonella enterica serovar Typhimurium (S. Typhimurium) is an important zoonotic pathogen in public health and food safety. The type III secretion system (T3SS) encoded by Salmonella pathogenicity island (SPI) is a sophisticated molecular machine that facilitates active invasion, intracellular replication, and host inflammation. Due to increasing antibiotic resistance, new therapeutic strategies that target the Salmonella T3SS have received considerable attention. In this study, paeonol was identified as an inhibitor of the S. Typhimurium T3SS. Paeonol significantly blocked the translocation of SipA into host cells and suppressed the expression of effector proteins without affecting bacterial growth in the effective concentration range. Additionally, S. Typhimurium-mediated cell injury and invasion levels were significantly reduced after treatment with paeonol, without cytotoxicity. Most importantly, the comprehensive protective effect of paeonol was confirmed in an S. Typhimurium mouse infection model. Preliminary mechanistic studies suggest that paeonol inhibits the expression of effector proteins by reducing the transcription level of the SPI-1 regulatory pathway gene hilA. This work provides proof that paeonol could be used as a potential drug to treat infections caused by Salmonella.

Inhibition of the type III secretion system by syringaldehyde protects mice from Salmonella enterica serovar Typhimurium.

The invasiveness of Salmonella enterica serovar Typhimurium (S. Typhimurium) is closely associated with the Salmonella pathogenicity island (SPI)-encoded type Ⅲ secretion system (T3SS), which can directly inject a series of effector proteins into eukaryotic cells to enable bacterial infection. In this study, syringaldehyde was identified as an effective inhibitor of the S. Typhimurium T3SS using an effector protein-lactamase fusion reporter system. Syringaldehyde treatment could inhibit the expression of important effector proteins (SipA, SipB and SipC) at a concentration of 0.18 mM without affecting bacterial growth. Additionally, significant inhibition of bacterial invasion and cellular injury was observed following the syringaldehyde treatment in the co-infection system of HeLa cells and S. Typhimurium. Furthermore, treatment with syringaldehyde provided systemic protection to mice infected with S. Typhimurium, reducing mortality (40.00%) and bacterial loads and relieving caecal damage and systemic inflammation. The results presented in this study indicate that syringaldehyde significantly affects T3SS activity and is a potential leading compound for treating S. Typhimurium infections.

Relationship between Eimeria tenella development and host cell apoptosis in chickens.

Coccidiosis causes considerable economic losses in the poultry industry. At present, the pathology of coccidiosis is preventable with anticoccidials and vaccination, although at considerable cost to the international poultry industry. The purpose of the present study was to elucidate the relationship between Eimeria tenella development and host cell apoptosis in chickens, which provides a theoretical basis for further study of the injury mechanism of E. tenella and the prevention and treatment of coccidiosis. Cecal epithelial cells from chick embryo were used as host cells in vitro. In addition, flow cytometry, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end labeling, and histopathological assays were used to detect the dynamic changes in E. tenella infection rates, DNA injury rates, and apoptosis rates in groups treated with and without the caspase-9 inhibitor Z-LEHD-FMK. Following E. tenella infection, we demonstrated that untreated cells had less apoptosis at 4 h and, inversely, more apoptosis at 24 to 120 h compared with control cells. Furthermore, after the application of Z-LEHD-FMK, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assays, and translation of phosphatidyl serines to the host cell plasma membrane surface, the treated group chick embryo cecal epithelial cells exhibited decreased apoptosis and DNA injuries (P<0.01) at 24 to 120 h. However, light microscopy showed that E. tenella infection rates of treated cells were higher (P<0.01) than untreated cells during the whole experimental period. Together, these observations suggest that E. tenella can protect host cells from apoptosis at early stages of development but can promote apoptosis during the middle to late stages. In addition, the inhibition of host cell apoptosis can be beneficial to the intracellular growth and development of E. tenella.

Characterization, genome analysis, and therapeutic evaluation of a novel Salmonella phage vB_SalS_JNS02: a candidate bacteriophage for phage therapy.

Phage therapy is gaining momentum as an alternative to antibiotics in the treatment of salmonellosis caused by Salmonella. In this study, a novel Salmonella phage, vB_SalS_JNS02, was isolated successfully from poultry farms in Shandong, China. The biological characteristics of vB_SalS_JNS02 were analysed, which revealed a short latent period of approximately 10 min and a burst size of 110 PFU/cell. Moreover, vB_SalS_JNS02 exhibited remarkable stability across a wide pH range (pH 3-12) and temperatures ranging from 30 to 80°C. Genome sequencing analysis provided valuable insights into the genetic composition of vB_SalS_JNS02, which consists of a double-stranded DNA genome that spans 42,450 base pairs and has a G + C content of 49.4%. Of significant importance, the genomic sequence of vB_SalS_JNS02 did not contain any genes related to lysogenicity, virulence, or antibiotic resistance. The phage's efficacy was evaluated in a larval challenge study. Treatment with the phage resulted in increased survival of Galleria mellonella larvae (100, 70, and 85%) (MOI 0.1) in the prophylactic treatment, co-infection treatment, and remedial treatment experiments, respectively. Another in vivo experiment investigated the potential application of the phage in broiler chickens and revealed that a single oral dose of vB_SalS_JNS02 (108 PFU/mL, 100 µL/chick) administered 3 h after S. enteritidis oral administration provided effective protection. The introduction of bacteriophage not only enhances the production of secretory immunoglobulin A (sIgA), but also induces alterations in the composition of the gut microbial community. Phage therapy increases the relative abundance of beneficial bacteria, which helps to maintain intestinal barrier homeostasis. However, it is unable to fully restore the disrupted intestinal microbiome caused by S. enteritidis infection. Importantly, no significant adverse effects were observed in the animal subjects following oral administration of the phage, and our findings highlight vB_SalS_JNS02 is a hopeful candidate as a promising tool to target Salmonella infections in poultry.

Isoflavone glucoside genistin, an inhibitor targeting Sortase A and Listeriolysin O, attenuates the virulence of Listeria monocytogenes in vivo and in vitro.

As a common intracellular facultative anaerobic Gram-positive bacterium, Listeria monocytogenes (L. monocytogenes) exhibits strong resistance to extreme environments, such as low temperature and a wide range of pH values, causing contamination in food production and processing. Sortase A (SrtA) and listeriolysin O (LLO), two crucial virulence factors of L. monocytogenes, are widely recognized as potential targets for the development of anti-L. monocytogenes infection drugs. In this study, we found that genistin simultaneously inhibits the peptidase activity of SrtA and the hemolytic activity of LLO without affecting the growth of L. monocytogenes, alleviating concerns about developing resistance. Furthermore, we demonstrated that genistin reduces L. monocytogenes biofilm formation and invasion of human colorectal cancer (Caco-2) cells. Subsequent mechanistic studies revealed that genistin inhibited LLO-mediated Caco-2 cell damage by blocking LLO oligomerization. Fluorescence quenching assay revealed the potential binding mode of SrtA and LLO to genistin. Genistin might bind to the active pocket of SrtA through residues Leu33, Asn29, and Met40, interacting with D1 domain of LLO involved in oligomerization and pore formation through residues Asn259. Studies in infection models revealed that genistin reduces mortality and pathological damage in mice infected with L. monocytogenes. These results indicate that genistin is a promising anti-virulence agent that could be considered an alternative candidate for the treatment of L. monocytogenes infection.

Inhibitory effect of hederagenin on Streptococcus pneumoniae pneumolysin in vitro.

Streptococcus pneumoniae is an important pathogen that causes otitis media, pneumonia, meningitis and bacteremia. As an important virulence factors of S. pneumoniae, pneumolysin (PLY) can penetrate cell membranes and lead to cell lysis and inflammation, which is one of the main causes of infection and damage of S. pneumoniae. Therefore, using pneumolysin as a target to study its inhibitors can provide a new treatment strategy for pneumococcal disease. This study analyzed the inhibitory effect of the natural compound hederagenin on PLY in vitro. The results show that hederagenin has great potential as a new strategy for the treatment of pneumococcal diseases.

Harmine, an inhibitor of the type III secretion system of Salmonella enterica serovar Typhimurium.

New therapeutic strategies for clinical Salmonella enterica serovar Typhimurium (S. Typhimurium) infection are urgently needed due to the generation of antibiotic-resistant bacteria. Inhibition of bacterial virulence has been increasingly regarded as a potential and innovative strategy for the development of anti-infection drugs. Salmonella pathogenicity island (SPI)-encoded type III secretion system (T3SS) represents a key virulence factor in S. Typhimurium, and active invasion and replication in host cells is facilitated by the secretion of T3SS effector proteins. In this study, we found that harmine could inhibit T3SS secretion; thus, its potential anti-S. Typhimurium infection activity was elucidated. Harmine inhibits the secretion and expression of T3SS effector proteins and consequently attenuates the S. Typhimurium invasion function of HeLa cells. This inhibition may be implemented by reducing the transcription of pathogenesis-related SPI-1 transcriptional activator genes hilD, hilC, and rtsA. Harmine improves the survival rate and bacterial loads of mice infected with S. Typhimurium. In summary, harmine, an effective T3SS inhibitor, could be a leading compound for the development of treatments for Salmonella infection.

Anti-inflammatory, antioxidant and anti-virulence roles of atractylodin in attenuating Listeria monocytogenes infection.

Background: Listeria monocytogenes (L. monocytogenes), as a pandemic foodborne pathogen, severely threatens food security and public health care worldwide, which evolves multiple bacterial virulence factors (such as listeriolysin O, LLO) for manipulating the immune response of L. monocytogenes-host interactions. Methods: Hemolysis assay was employed to screen a potential LLO inhibitor and the underlying mechanisms were investigated using molecular dynamics (MD) simulation and oligomerization assay. The effects of candidates on immune response were examined by qRT-PCR and immunoblotting analysis. Histological analysis, ELISA assay and biochemistry detection were conducted to assess in vivo efficacy of candidates. Results: In the present study, natural terpenoid atractylodin was characterized as an alternative drug candidate for the treatment of L. monocytogenes by the regulation of LLO function and host Nrf2/NLRP3 signaling pathway. Notably, in vivo infection model by L. monocytogenes also highlighted that atractylodin treatment provided effective therapeutic benefits, as evidenced by decreased bacterial burden and diminished inflammation. Congruently, the survival rate of L. monocytogenes-infection mice increased significantly from 10.0% to 40.0% by atractylodin treatment. Conclusion: Collectively, our study showed for the first time that atractylodin has tremendous potential to attenuate L. monocytogenes pathogenicity by blocking LLO pore formation and mediating the suppression of inflammation and oxidative stress, providing a promising therapeutic strategy and broadening the applications of atractylodin against L. monocytogenes infection.

Inhibition of the Type III Secretion System of Salmonella enterica Serovar Typhimurium via Treatment with Fraxetin.

The increasingly serious problem of bacterial drug resistance has led to the development of antivirulence agents. The Salmonella enterica serovar Typhimurium Salmonella pathogenicity island (SPI)-encoded type III secretion system (T3SS) and its effector proteins are important virulence factors for S. Typhimurium invasion and replication in host cells and for antivirulence drug screening. Fraxetin is isolated from Fraxinus spp. Extensive studies have reported its multiple pharmacological activities. However, it remains to be elucidated whether fraxetin affects the function of the S. Typhimurium T3SS. In this study, the anti-infection mechanism of fraxetin on S. Typhimurium and its T3SS was investigated. Fraxetin inhibited the S. Typhimurium invasion of HeLa cells without affecting the growth of bacteria in vitro. Further findings on the mechanism showed that fraxetin had an inhibitory effect on the S. Typhimurium T3SS by inhibiting the transcription of the pathogenesis-related SPI-1 transcriptional activator genes hilD, hilC, and rtsA. Animal experiments showed that fraxetin treatment protected mice against S. Typhimurium infection. Collectively, we provide the first demonstration that fraxetin may serve as an effective T3SS inhibitor for the development of treatments for Salmonella infection. IMPORTANCE The increasingly serious problem of bacterial antibiotic resistance limits the clinical application of antibiotics, which increases the need for the development of antivirulence agents. The type III secretion system (T3SS) plays a critical role in host cell invasion and pathogenesis of Salmonella and becomes a popular target for antivirulence agents screening. Our study found, for the first time, that fraxetin inhibited S. Typhimurium invasion by inhibiting the transcription of genes in a feed-forward regulatory loop. Further in vivo testing showed that fraxetin decreased bacterial burdens in the spleen and liver of S. Typhimurium-infected mice and improved survival outcomes in an in vivo mouse model of S. Typhimurium infection. Collectively, these results demonstrate that fraxetin inhibits S. Typhimurium infection by targeting the T3SS and may serve as a potential agent for the treatment of S. Typhimurium infection.

Commercialized artemisinin derivatives combined with colistin protect against critical Gram-negative bacterial infection.

The emergence and spread of the mcr-1 gene and its mutants has immensely compromised the efficient usage of colistin for the treatment of drug-resistant Gram-negative bacterial infection in clinical settings. However, there are currently no clinically available colistin synergis. Here we identify artemisinin derivatives, such as dihydroartemisinin (DHA), that produces a synergistic antibacterial effect with colistin against the majority of Gram-negative bacteria (FIC < 0.5) without induced resistance, particularly those carrying the mcr-1 gene. Mechanism analysis reveals the direct engagement of DHA with the active center of MCR-1 to inhibit the activity of MCR-1. Meanwhile, the results from transcriptome and electron microscope analysis show that DHA could also simultaneously affect the flagellar assembly and the energy metabolism of bacteria. Moreover, in the mouse infection models of Gram-negative bacteria, combination therapy shows remarkable treatment benefits, as shown by an improved survival rate, reduced morbidity, alleviated pathological injury and decreased bacterial loading. Due to the generally safe profile of specialized malaria medication administration in humans, artemisinin derivatives are a promising class of multi-target inhibitors on bacterial resistance and virulence that can be used to extend the usage life of colistin and to tackle the inevitability of serious bacterial infection with colistin.

Phloretin potentiates polymyxin E activity against gram-negative bacteria.

AIMS: The spread of plasmid-mediated polymyxin resistance has jeopardized the use of polymyxin, the last defender that combats infections caused by multidrug-resistant (MDR) gram-negative pathogens. MAIN METHODS: In this study, phloretin, as a monomeric compound extracted from natural plants, showed a good synergistic effect with polymyxin E against gram-negative bacteria, as evaluated by minimal inhibit concentration (MIC) assay and a series of assays, including growth curve, time-killing, and Western blot assays. A model of mice infected by Salmonella sp. stain HYM2 was established to further identify the synergistic effect of phloretin with polymyxin E. KEY FINDINGS: The results suggested that phloretin had the potential ability to recover the antibacterial sensitivity of polymyxin E from 64 µg/mL to no more than 2 µg/mL in E. coli ZJ478 or in Salmonella sp. stain HYM2 with a 32-fold decrease. A series of strains, including mcr-1-positive and mcr-1-negative strains, were treated with a combination of phloretin and polymyxin E, and the fractional inhibitory concentration (FIC) values were all found to be below 0.5. However, the combination of phloretin and polymyxin E did not lead to bacterial resistance. In vivo, the survival rate of infected mice reached nearly 80% with the combination treatment, and the cecal colony value also decreased significantly. SIGNIFICANCE: All the above results indicated that phloretin is a potential polymyxin potentiator to combat gram-negative stains.

Tannic Acid Inhibits Salmonella enterica Serovar Typhimurium Infection by Targeting the Type III Secretion System.

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a zoonotic pathogen that can cause food poisoning and diarrhea in both humans and animals worldwide. The Salmonella pathogenicity island (SPI) genes encoded type III secretion system (T3SS) is important for S. Typhimurium invasion and replication in host cells. Due to the increasing problem of antibiotic resistance, antibiotic treatment for clinical Salmonella infection has gradually been limited. Anti-virulence inhibitors are a promising alternative to antibiotics because they do not easily induce bacterial antibiotic resistance. Here, we systematically evaluated the therapeutic effect of tannic acid (TA) on Salmonella-infected mice and elucidated its anti-infection mechanism. TA treatment improved the survival rate of S. Typhimurium-infected mice and alleviated cecum pathological lesions. In addition, TA inhibited S. Typhimurium invasion to HeLa cells without affecting their growth. Further studies showed that TA could inhibit the expression of sipA and sipB. This inhibition may be implemented by inhibiting the transcription of key regulatory and structural genes of the T3SS. This study provides an alternative anti-virulence strategy for Salmonella infection treatment.

Acacetin inhibits Streptococcus pneumoniae virulence by targeting pneumolysin.

OBJECTIVES: Streptococcus pneumoniae (S. pneumoniae) is an important commensal and pathogenic bacterium responsible for pneumonia, meningitis and other invasive diseases. Pneumolysin (PLY) is the major virulence factor that contributes significantly to the interaction between S. pneumoniae and the host. KEY FINDINGS: In this study, the results of antibacterial analysis, the haemolysis test and the Western blotting assay showed that acacetin inhibited PLY-mediated pore-forming activity caused by S. pneumoniae culture precipitates and purified PLY without anti-S. pneumoniae activity. In addition, acacetin treatment inhibited PLY oligomerization without affecting the expression of PLY in S. pneumoniae culture supernatants. Live/dead cells and cytotoxicity assays suggested that acacetin significantly enhanced the survival rate of injured cells by inhibiting the biological toxicity of PLY without cytotoxicity in the coculture system. The in vivo mouse model of S. pneumoniae infection further demonstrated that acacetin treatment could significantly reduce the levels of inflammatory factors (INF-γ and IL-ß) in bronchoalveolar lavage fluid (BALF) and alleviate the pathological damage of lung injury. CONCLUSIONS: Taken together, the results presented in this study indicated that acacetin inhibited the pore-forming activity of PLY and reduced the virulence of S. pneumoniae in vivo and in vitro, which may provide a leading compound for the treatment of S. pneumoniae infection.

Tectorigenin reduces type IV pilus-dependent cell adherence in Clostridium perfringens.

Clostridium perfringens is an anaerobic, Gram-positive bacterium that causes a range of diseases in humans and animals around the globe. The type IV pilus (TFP) system plays a key role in the colonization and invasion of host cells, biofilm formation and gliding motility, which is vital for C. perfringens infection. Therefore, targeting TFP function may be a promising strategy for the treatment of C. perfringens infection. Here, we investigated the potential inhibitory effects of tectorigenin (TE), an isoflavone extracted from the rhizome of the Chinese herb Belamcanda chinensis (L.) DC, on gliding motility, biofilm formation, adherence to cells and antibacterial activity of C. perfringens. Tectorigenin significantly inhibited gliding motility, biofilm formation and adherence to Caco-2 cells without observable antibacterial activity against C. perfringens. In addition, we also demonstrated that the inhibitory effect of TE on TFP function appears to be partially achieved by the suppression of TFP-associated genes. These findings demonstrate that TE may have the potential to be developed as a new anti-virulence drug for C. perfringens infection, particularly for the targeting of TFP.

The effect of the mitochondrial permeability transition pore on apoptosis in Eimeria tenella host cells.

Although the mitochondrial permeability transition pore (MPTP) is associated with cellular apoptosis and necrosis, its effect in host response to Eimeria infections is not well understood. In an effort to better understand the effect of MPTP on apoptosis in Eimeria tenella host cells, an MPTP inhibitor (cyclosporin A) was used to inhibit MPTP opening in vitro. Cecal epithelial cells from chick embryos, which were either treated or non-treated with cyclosporin A, were used as Eimeria tenella host cells. In addition, primary chick embryo cecum epithelial cell culture techniques and flow cytometry were used to detect the dynamic changes in MPTP opening, mitochondrial transmembrane potential, and cell apoptosis rate of Eimeria tenella host cells. Compared with the control group, cytometric techniques showed that untreated host cells exhibited a significantly higher (P < 0.01) degree of MPTP opening but lower (P < 0.01 or P < 0.05) mitochondrial transmembrane potential. Moreover, untreated group cells had less apoptosis (P < 0.01) at 4 h and more apoptosis (P < 0.05 or P < 0.01) at 24 to 120 h as compared with control group cells. After the application of cyclosporin A, the degree of MPTP opening in the treated group was significantly lower (P < 0.01) at 4 to 120 h compared to the untreated group, whereas the treated group had higher (P < 0.05 or P < 0.01) mitochondrial transmembrane potentials at 24 to 120 h. Flow cytometry assays also showed that there was less (P < 0.05 or P < 0.01) apoptosis after 24 h in the treated group than in the untreated group. Taken together, these observations indicate that MPTP is a key node that plays a predominant role in the mitochondrial apoptosis pathway in the host cell induced by Eimeria tenella.