Pseudomonas aeruginosa is an opportunistic pathogen responsible for chronic infections in both human and veterinary medicine, with biofilm formation and multidrug resistance posing major clinical challenges. The efficacy of two bacteriophages, JG003 and PTLAW1, alone and in combination, was evaluated. In vitro biofilms grown on abiotic 96-well plates showed significant reduction after treatment with individual bacteriophages or their combination, as confirmed by confocal microscopy. To better simulate physiological conditions, efficacy was assessed using an epidermal equivalent model and an ex vivo canine skin model. In the ex vivo system, bacteriophage treatment reduced bacterial load by 4 logs, as confirmed by scanning electron microscopy and immunofluorescence imaging. In the epidermal equivalent model, bacteriophage therapy decreased bacterial counts and CXCL8 levels without inducing cytotoxicity or disrupting the skin barrier. Integration of in vitro and ex vivo systems bridges the gap between traditional biofilm assays and in vivo studies. The use of Franz-type diffusion cells provides a physiologically relevant platform for evaluating topical bacteriophage delivery and skin permeation. These findings establish a reproducible preclinical framework for biofilm-targeted therapies, demonstrating that bacteriophage combinations effectively reduce Pseudomonas aeruginosa biofilms and inflammation on skin, supporting their potential for wound treatment in both human and veterinary medicine.

Streptococcus equi subsp. zooepidemicus (SEZ) causes diseases in various animals and humans. In pigs, it is an emerging pathogen that causes septicemia and other severe pathologies. SEZ expresses the M-like protein SzM, which recruits host proteins such as fibrinogen to the bacterial surface. The objective of this study was to investigate the survival mechanism of a zoonotic SEZ strain in porcine blood. The zoonotic SEZ C33 strain proliferated in porcine blood of 10-week-old piglets in vitro despite comparably high levels of IgM binding to the bacterial surface. Comparison with the isogenic szm deletion mutant showed that SzM_C33 expression is crucial for porcine blood survival. Flow cytometry analysis revealed that SEZ C33 was associated with porcine blood granulocytes, exhibiting a prominent generation of reactive oxygen species. Cytochalasin D treatment significantly inhibited this association. Furthermore, we found viable intracellular SEZ C33 in a penicillin/streptomycin protection assay in porcine blood and purified porcine granulocytes, in agreement with the microscopic detection of streptococci in neutrophils. Experimental infection with SEZ C33 confirmed the virulence of this strain in pigs, including one case of fibrino-suppurative meningoencephalitis and tarsitis. In conclusion, SEZ C33, originally isolated from a diseased human, demonstrated SzM-mediated intracellular survival in porcine blood granulocytes in vitro and is most likely involved in the pathogenesis of diseases such as meningitis and arthritis in vivo.

Bacterial persister cells are known to survive antibiotic treatment in the absence of typical resistance mechanisms. Persisters do not grow in the presence of antibiotics but can resume growth when antibiotic treatment is ceased, persisters are therefore often associated with chronic infections and treatment failure. In previous studies, it was shown that a lepA deletion strain of Escherichia coli was more tolerant to different classes of antibiotics due to reduced formation of reactive oxygen species (ROS). LepA is a highly conserved protein involved in translation, but the details of its role in the various mechanisms it participates in remain unclear. In this study, we performed killing assays to assess persister survival in a lepA deletion strain of Salmonella Typhimurium and FACS analysis to measure ROS formation in response to ciprofloxacin, β-lactam, and aminoglycoside antibiotics, representing three different classes of bactericidal drugs. Our results suggest that deletion of lepA reduces ROS formation and may increase persister survival after ciprofloxacin treatment, possibly due to reduced prophage induction. In contrast, reducing oxidative stress with the antioxidant glutathione was associated with increased persister formation and a larger subpopulation of wild type bacteria lacking prophage induction following ciprofloxacin treatment, whereas no additional beneficial effect was observed in the lepA mutant. These findings are consistent with a link between ROS-associated prophage induction and persister cell levels, providing a cautious perspective on how drug-induced ROS may influence bacterial survival in strains carrying prophages.

Persisters are phenotypically antibiotic-tolerant cells which can survive antibiotic exposure without acquiring antibiotic resistance. A proposed important factor in persistence is low intracellular ATP levels, which are thought to reduce the activity of antibiotic targets. However, previous studies demonstrated that persisters have comparable DNA damage as drug-sensitive bacteria after fluoroquinolone treatment. Furthermore, recent studies reported that endogenous prophages can reduce levels of antibiotic persistence in Salmonella after fluoroquinolone treatment. In this study, we examined prophage induction and persister cell survival of a prophage-free variant of Salmonella Typhimurium and strains harbouring a deletion of the tisAB genes, with tisB encoding the toxin from the tisB/istR-1 toxin-antitoxin system, known to reduce the intracellular ATP concentration. Treatment of the prophage-free variant with ciprofloxacin resulted in reduced killing and increased persistence as compared to the wild type. In addition, prophage induction and prophage mediated killing was significantly increased after deletion of tisAB following ciprofloxacin treatment. We also demonstrate that the recovery phase following the removal of ciprofloxacin, is crucial for the induction of endogenous prophages. Our results suggest that ATP-dependent prophage activation plays a significant role in DNA damage-mediated killing of bacteria. Low ATP levels can dampen the induction of prophages and increase the fraction of bacterial survivors after ciprofloxacin treatment.

Diazo peptides have been described earlier, however, due to their high reactivity have not been broadly used until today. Here, we report the preparation, properties, and applications of chemically stable internal diazo peptides. Peptidyl phosphoranylidene-esters and amides were found to react with triflyl azide primarily to novel 3,4-disubstituted triazolyl-peptides. Nonaflyl azide instead furnished diazo peptides, which are chemically stable from pH 1-14 as amides and from pH 1-8 as esters. Thus, diazo peptides prepared by solid phase peptide synthesis were stable to final deprotection with 95 % trifluoroacetic acid. Diazo peptides with the recognition sequence of caspase-3 were identified as specific, covalent, and irreversible inhibitors of this enzyme at low nanomolar concentrations. A fluorescent diazo peptide entered living cells enabling microscopic imaging and quantification of apoptotic cells via flow cytometry. Thus, internal diazo peptides constitute a novel class of activity-based probes and enzyme inhibitors useful in chemical biology and medicinal chemistry.

Streptococcus canis is a zoonotic agent that causes severe invasive diseases in domestic animals and humans, but little is known about its pathogenesis and virulence mechanisms so far. SCM, the M-like protein expressed by S. canis, is considered one of the major virulence determinants. Here, we report on the two distinct groups of SCM. SCM-1 proteins were already described to interact with its ligands IgG and plasminogen as well as with itself and confer antiphagocytic capability of SCM-1 expressing bacterial isolates. In contrast, the function of SCM-2 type remained unclear to date. Using whole-genome sequencing and subsequent bioinformatics, FACS analysis, fluorescence microscopy and surface plasmon resonance spectrometry, we demonstrate that, although different in amino acid sequence, a selection of diverse SCM-2-type S. canis isolates, phylogenetically representing the full breadth of SCM-2 sequences, were able to bind fibrinogen. Using targeted mutagenesis of an SCM-2 isolate, we further demonstrated that this strain was significantly less able to survive in canine blood. With respect to similar studies showing a correlation between fibrinogen binding and survival in whole blood, we hypothesize that SCM-2 has an important contribution to the pathogenesis of S. canis in the host.

Streptococcus suis is a porcine and zoonotic pathogen in the upper respiratory tract, expressing different capsular serotypes and virulence-associated factors. Given its genomic and phenotypic diversity, the virulence potential of S. suis cannot be attributed to a single factor. Since strong inflammatory response is a hallmark of S. suis infection, the objective of this study was to investigate the differences in transcriptional host responses to two serotype 2 and one serotype 9 strains. Both serotypes are frequently found in clinical isolates. We infected porcine precision-cut lung slices (PCLSs) with two serotype 2 strains of high (strain S10) and low (strain T15) virulence, and a serotype 9 strain 8067 of moderate virulence. We observed higher expression of inflammation-related genes during early infection with strains T15 and 8067, in contrast to infection with strain 10, whose expression peaked late. In addition, bacterial gene expression from infected PCLSs revealed differences, mainly of metabolism-related and certain virulence-associated bacterial genes amongst these strains. We conclude that the strain- and time-dependent induction of genes involved in innate immune response might reflect clinical outcomes of infection in vivo, implying rapid control of infection with less virulent strains compared to the highly virulent strain S10.

Formation of neutrophil extracellular traps was first described in 2004, showing that NETs are composed of decondensed chromatin fibers and nuclear and granule components. Free DNA is often used to quantify NETs, but to differentiate NETosis from necrotic DNA-release, immunofluorescence microscopy with NET-specific markers is required. Although evaluation by hand is time-consuming and difficult to standardize, it is still widespread. Unfortunately, no standardized method and only limited software tools are available for NET evaluation. This study provides an overview of recent techniques in use and aims to compare two published computer-based methods with hand counting. We found that the selected semi-automated quantification method and fully automated quantification via NETQUANT differed significantly from results obtained by hand and exhibited problems in detection of complex NET structures with partially illogical results. In contrast to that, trained persons were able to adapt to varying settings. Future approaches aimed at developing deep-learning algorithms for fast and reproducible quantification of NETs are needed.

Streptococcus suis , a common member of the porcine respiratory microbiota, can cause life-threatening diseases in pigs as well as humans. A previous study identified the gene trpX as conditionally essential for in vivo survival by intrathecal infection of pigs with a transposon library of S. suis strain 10. Here, we characterized trpX, encoding a putative tryptophan/tyrosine transport system substrate-binding protein, in more detail. We compared growth capacities of the isogenic trpX -deficient mutant derivative strain 10∆ trpX with its parent. Growth experiments in chemically defined media (CDM) revealed that growth of 10∆ trpX depended on tryptophan concentration, suggesting TrpX involvement in tryptophan uptake. We demonstrated that trpX is part of an operon structure and co-transcribed with two additional genes encoding a putative permease and ATPase, respectively. Bioinformatics analysis identified a putative tryptophan T-box riboswitch in the 5′ untranslated region of this operon. Finally, qRT-PCR and a reporter activation assay revealed trpX mRNA induction under tryptophan-limited conditions. In conclusion, our study showed that TrpX is part of a putative tryptophan ABC transporter system regulated by a T-box riboswitch probably functioning as a substrate-binding protein. Due to the tryptophan auxotrophy of S. suis , TrpX plays a crucial role for metabolic adaptation and growth during infection.

Interleukin-36α is a novel member of the IL-1 cytokine family that is highly expressed in epithelial tissues and several myeloid-derived cell types after induction. The transcription factor (TF) C/EBPβ binds specifically to an essential half-CRE•C/EBP motif in the Il36a promoter to induce Il36a expression upon LPS stimulation. C/EBPs regulate gene expression by binding to recognition sequences that can contain 5'-cytosine-phosphate-guanine-3' dinucleotides (CpG), whose methylation can influence TF binding and gene expression. Herein we show that the half-CRE•C/EBP element in the Il36a promoter is differentially methylated in the murine RAW264.7 macrophage cell line and in primary murine macrophages. We demonstrate that C/EBPβ binding to the half-CRE•C/EBP element in the Il36a promoter following LPS stimulation is insensitive to CpG methylation and that methylation of the CpG in the half-CRE•C/EBP element does not alter LPS-induced Il36a promoter activity which correlated with similar Il36a mRNA copy numbers and pro-IL-36α protein amount in both cell types. Taken together, our data indicate that C/EBPβ binding to the half-CRE•C/EBP element and subsequent gene activation occurs independently of the CpG methylation status of the half-CRE•C/EBP motif and underlines the potential of C/EBPs to recognize methylated as well as unmethylated motifs.

Apoptosis is an indispensable mechanism for eliminating infected cells and activation of executioner caspases is considered to be a point of no return. Streptococcus pneumoniae, the most common bacterial pathogen causing community-acquired pneumonia, induces apoptosis via its pore-forming toxin pneumolysin, leading to rapid influxes of mitochondrial calcium [Ca2+](m) as well as fragmentation, and loss of motility and membrane potential, which is accompanied by caspase-3/7 activation. Using machine-learning and quantitative live-cell microscopy, we identified a significant number of alveolar epithelial cells surviving such executioner caspase activation after pneumolysin attack. Precise single-cell analysis revealed the [Ca2+](m) amplitude and efflux rate as decisive parameters for survival and death, which was verified by pharmacological inhibition of [Ca2+](m) efflux shifting the surviving cells towards the dying fraction. Taken together, we identified the regulation of [Ca2+](m) as critical for controlling the cellular fate under pneumolysin attack, which might be useful for therapeutic intervention during pneumococcal infection.

Streptococcus suis is a common pathogen colonising the respiratory tract of pigs. It can cause meningitis, sepsis and pneumonia leading to economic losses in the pig industry worldwide. Cyclooxygenase-2 (COX-2) and its metabolites play an important regulatory role in different biological processes like inflammation modulation and immune activation. In this report we analysed the induction of COX-2 and the production of its metabolite prostaglandin E2 (PGE2) in a porcine precision-cut lung slice (PCLS) model. Using Western blot analysis, we found a time-dependent induction of COX-2 in the infected tissue resulting in increased PGE2 levels. Immunohistological analysis revealed a strong COX-2 expression in the proximity of the bronchioles between the ciliated epithelial cells and the adjacent alveolar tissue. The morphology, location and vimentin staining suggested that these cells are subepithelial bronchial fibroblasts. Furthermore, we showed that COX-2 expression as well as PGE2 production was detected following infection with two prevalent S. suis serotypes and that the pore-forming toxin suilysin played an important role in this process. Therefore, this study provides new insights in the response of porcine lung cells to S. suis infections and serves as a basis for further studies to define the role of COX-2 and its metabolites in the inflammatory response in porcine lung tissue during infections with S. suis.

Interleukin-36α(Il-36α) is a member of the novel Il-1-like proin-flammatory cytokine family that is highly expressed in epithelial tissues and several myeloid-derived cell types. We have recently shown that CCAAT enhancer binding proteinβ (C/EBPβ) binds specifically to an essential half cAMP response element (half-CRE)•C/EBP motif in the Il36A promoter to induce Il36A expression upon LPS stimulation. C/EBPs are transcription factors belonging to the basic leucine zipper (bZIP) family of transcriptional regulators. C/EBP proteins can form homo- and heterodimers and regulate gene expression by binding to C/EBP specific recognition sequences and composite sites that can contain 5′-cytosine-phosphate-guanine-3′dinucleotides (CpG). CpG methylation of such elements has been shown to influence transcription factor binding and gene expression. Here we show that the half-CREoC/EBP element in the Il36A promoter is differentially methylated in the murine RAW264.7 macrophage cell line and in primary murine macrophages. By using electrophoretic mobility gel shift and fluorescence polarization assays we demonstrate that C/EBPβ binding to the half-CRE•C/EBP element in the Il36A promoter following LPS stimulation is insensitive to CpG methylation. Transfection assays also show that methylation of the CpG in the half-CRE•C/EBP element does not alter LPS-induced Il36A promoter activity. A direct comparison of Il36A mRNA copy numbers as well as the pro-Il-36α protein level in RAW264.7 and primary macrophages revealed similar amounts in both cell types. Taken together, our data suggest that C/EBPβ binding to the half-CRE•C/EBP element and C/EBPβ mediated gene activation occurs independently of the CpG methylation status of the target DNA sequence and underline the potential of C/EBPβ to recognize methylated as well as unmethylated binding sites. ### Competing Interest Statement The authors have declared no competing interest.

Abstract Streptococcus suis, an emerging zoonotic pathogen, causes invasive diseases in pigs, including sepsis, meningitis, endocarditis, pneumonia, and arthritis. Importantly, similar pathologies are reported in human S. suis infections. In previous work, the locus SSU0375 of S. suis strain P1.7 had been identified as a conditionally essential gene by intrathecal experimental infection of pigs with a transposon library of S. suis. This study aimed to identify the function of the corresponding gene product. Bioinformatics analysis and homology modeling revealed sequence and structural homologies with the Streptococcus pneumoniae mid‐cell‐anchored protein Z (MapZ) that is involved in cell division in different bacterial species. Indeed, depletion of this locus in S. suis strain 10 revealed a growth defect as compared to the wild type. Electron microscopy analysis of the corresponding mutant demonstrated morphological growth defects as compared to the wild‐type strain, including an irregular cell shape and size as well as mispositioned division septa. Light microscopy and subsequent quantitative image analysis confirmed these morphological alterations. In the genetic rescue strain, the wild‐type phenotype was completely restored. In summary, we proposed that SSU0375 or the corresponding locus in strain 10 encode for a S. suis MapZ homolog that guides septum positioning as evidenced for other members of the Streptococci family.

Mycobacterium avium subspecies paratuberculosis (MAP) causes Johne’s disease (JD), an incurable chronic intestinal bowel disease in ruminants. JD occurs worldwide and causes enormous economic burden in dairy industry. Research on JD pathobiology is hampered by its complexity which cannot completely be mimicked by small animal models. As a model the mouse allows dissecting some pathogenicity features of MAP. However, for unknown reasons MAP exhibits reduced growth in granulomas of infected mice compared to other Mycobacterium avium subspecies. Here, we characterized immune reactions of MAP-infected C57BL/6 mice. After infection, mice appeared fully immunocompetent. A strong antigen-specific T cell response was elicited indicated by IFNγ production of splenic T cells re-stimulated with MAP antigens. Function of splenic dendritic cells and proliferation of adoptively transferred antigen-specific CD4+ T cells was unaltered. Isolated splenic myeloid cells from infected mice revealed that MAP resides in CD11b+ macrophages. Importantly, sorted CD11b+CD11c− cells expressed high level of type 2 nitric oxide synthase (NOS2) but only low levels of pro- and anti-inflammatory cytokines. Correspondingly, MAP-infected MAC2 expressing myeloid cells in spleen and liver granuloma displayed strong expression of NOS2. In livers of infected Nos2−/−mice higher bacterial loads, more granuloma and larger areas of tissue damage were observed 5 weeks post infection compared to wild type mice. In vitro, MAP was sensitive to NO released by a NO-donor. Thus, a strong T cell response and concomitant NOS2/NO activity appears to control MAP infection, but allows development of chronicity and pathogen persistence. A similar mechanism might explain persistence of MAP in ruminants.

Streptococcus canis is a zoonotic agent that causes serious invasive diseases in domestic animals and humans, but knowledge about its pathogenic potential and underlying virulence mechanisms is limited. Here, we report on the ability of certain S. canis isolates to form large bacterial aggregates when grown in liquid broth. Bacterial aggregation was attributed to the presence and the self-binding activity of SCM, the M protein of S. canis, as evaluated by bacterial sedimentation assays, immunofluorescence- and electron microscopic approaches. Using a variety of truncated recombinant SCM fragments, we demonstrated that homophilic SCM interactions occur via the N-terminal, but not the C-terminal part, of the mature M protein. Interestingly, when incubated in human plasma, SCM forms soluble protein complexes comprising its known ligands, immunoglobulin G (IgG) and plasminogen (Plg). Co-incubation studies with purified host proteins revealed that SCM-mediated complex formation is based on the interaction of SCM with itself and with IgG, but not with Plg or fibrinogen (Fbg), well-established constituents of M protein-mediated protein complexes in human-associated streptococci. Notably, these soluble, SCM-mediated plasma complexes harbored complement factor C1q, which can induce complement breakdown in the periphery and therefore represent another immune evasion mechanism of SCM.