Bacterial biofilms are critical contributors to chronic infections and antimicrobial resistance. Among the diverse extracellular matrix components, extracellular DNA (eDNA) and amyloid proteins have recently emerged as pivotal structural and functional molecules. Both individually contribute to biofilm stability and antibiotic tolerance, yet their cooperative roles remain underappreciated. This review aims to summarize current knowledge on the origins and functions of eDNA and amyloid proteins in biofilms, to highlight their molecular interactions, and to discuss how their synergistic effects promote biofilm-mediated resistance to antimicrobial agents. A comprehensive literature search was conducted using PubMed, Scopus, and Web of Science databases up to September 2025. Keywords included “biofilm”, “extracellular DNA”, “amyloid proteins”, “matrix”, and “antimicrobial resistance”. Relevant original research and review articles were systematically screened and critically analyzed to integrate emerging evidence on eDNA–amyloid interactions in bacterial biofilms. Current studies demonstrate that eDNA originates primarily from autolysis, active secretion, and host-derived DNA, while amyloid proteins are produced by multiple bacterial species, including Escherichia coli (curli), Pseudomonas aeruginosa (Fap), Bacillus subtilis (TasA), and Staphylococcus aureus (phenol-soluble modulins). Both molecules independently strengthen biofilm integrity and provide protective functions against antimicrobial agents. Importantly, recent evidence shows that eDNA can act as a nucleation template for amyloid fibrillation, while amyloid fibers stabilize and protect eDNA from degradation, creating a dense extracellular network. This synergistic eDNA–amyloid assembly enhances biofilm robustness, impedes antibiotic penetration, sequesters antimicrobial peptides, protects persister cells, and facilitates horizontal gene transfer of resistance determinants. The interplay between eDNA and amyloid proteins represents a central but underexplored mechanism driving biofilm-mediated antimicrobial resistance. Understanding this cooperative network not only deepens our mechanistic insights into bacterial pathogenesis but also highlights novel therapeutic targets. Strategies that disrupt eDNA–amyloid interactions may offer promising avenues for combating persistent biofilm-associated infections.

This study evaluated whether the combined Lactiplantibacillus plantarum CRL1506 and MPL16 nasal priming more effectively modulated the Toll-like receptor (TLR)-3- and respiratory syncytial virus (RSV)-mediated respiratory immune responses in mice than single strains. The interaction of single and combined strains with porcine alveolar macrophages (AMs) and porcine respiratory epithelial cells (PBE cells) in the context of TLR3 activation and porcine reproductive and respiratory syndrome virus (PRRSV) was also evaluated. The in vivo studies in mice revealed that the CRL1506 + MPL16 combination was more effective than the individual strains at reducing RSV replication, protecting the lung from TLR3-mediated inflammatory injury and modulating innate antiviral responses, particularly in AMs. In vitro, lactobacilli treatment also increased the resistance of porcine AMs to PRRSV infection. Notably, the CRL1506 + MPL16 combination was not more effective than the single strains in modulating AMs antiviral immunity. Complementary assays in PBE cells revealed that L. plantarum CRL1506 induced higher production of IFN-λ than the MPL16 strain in response to TLR3 activation. Thus, the superior in vivo protection against RSV seen with the L. plantarum CRL1506–MPL16 combination likely reflects complementary actions of lactobacilli: MPL16 would efficiently modulate AMs, whereas CRL1506 would be more effective to target respiratory epithelial cells driving greater IFN-λ production that further boosts AM antiviral activity. The results from the animal models of this work furnish the scientific basis for proposing future human trials to assess the efficacy of the CRL1506 + MPL16 combination in improving respiratory antiviral immunity.

Background/Objectives. Lactiplantibacillus plantarum CRL681 has previously demonstrated a strong antagonistic effect against Escherichia coli O157:H7 in food matrices; however, the molecular mechanisms underlying this activity remain poorly understood. Since initial interactions between beneficial bacteria and pathogens occur mainly at the cell surface and in the extracellular environment, the characterization of the bacterial secretome is essential for elucidating these mechanisms. In this study, the secretome of L. plantarum CRL681 was comprehensively characterized using an integrated in silico and in vitro approach. Methods. The exoproteome and surfaceome were analyzed by LC-MS/MS under pure culture conditions and during co-culture with E. coli O157:H7. Identified proteins were functionally annotated, classified according to subcellular localization and secretion pathways, and evaluated through protein–protein interaction network analysis. Results. A total of 275 proteins were proposed as components of the CRL681 secretome, including proteins involved in cell surface remodeling, metabolism and nutrient transport, stress response, adhesion, and genetic information processing. Co-culture with EHEC induced significant changes in the expression of proteins associated with energy metabolism, transport systems, and redox homeostasis, indicating a metabolic and physiological adaptation of L. plantarum CRL681 under competitive conditions. Notably, several peptidoglycan hydrolases, ribosomal proteins with reported antimicrobial activity, and moonlighting proteins related to adhesion were identified. Conclusions. Overall, these findings suggest that the antagonistic activity of L. plantarum CRL681 against E. coli O157:H7 would be mediated by synergistic mechanisms involving metabolic adaptation, stress resistance, surface adhesion, and the production of non-bacteriocin antimicrobial proteins, supporting its potential application as a bioprotective and functional probiotic strain.

C-reactive protein (CRP) and serum amyloid A (SAA) are classical acute-phase proteins that exemplify humoral innate immunity, the soluble arm of the host’s first-line defense. Beyond their traditional use as biomarkers of inflammation, both proteins function as active effectors against pathogens by binding microbial components, activating complements, and modulating inflammation. However, bacteria, viruses, and fungi have co-evolved diverse mechanisms to cope with or evade these host defenses. This review aims to summarize the current understanding of CRP and SAA as soluble innate immune effectors and to highlight pathogen strategies to counteract their antimicrobial pressure. We systematically surveyed and summarized evidence from experimental and clinical studies describing “function of CRP and SAA during infection”, “CRP and SAA in innate immune defense”, and “evasion mechanisms across bacterial, viral, and fungal pathogens”. CRP and SAA are rapidly upregulated in response to infection and contribute to pathogen recognition, opsonization, and inflammation. Pathogens, however, employ multiple coping strategies, including surface modification to block CRP binding, proteolytic degradation of acute-phase proteins, shielding within biofilms, and subversion of host signaling. These countermeasures enable microbes to reduce immune clearance and promote persistence. CRP and SAA represent central elements of humoral innate immunity, shaping the outcome of host–pathogen interactions. Pathogen adaptations to these proteins illustrate an ongoing evolutionary arms race between host defense and microbial survival. A deeper understanding of these processes may open avenues for novel therapeutic approaches, such as targeting microbial evasion factors or enhancing host acute-phase responses.

Streptococcus pneumoniae (S. pneumoniae) is a leading cause of pneumonia, and its interaction with host acute-phase proteins remains underexplored. Serum amyloid A1 (SAA1), an acute-phase protein, plays a crucial role in immune modulation. This study investigates the role of SAA1 in the early stages of respiratory infection by S. pneumoniae and its potential contribution to bacterial adaptation under acidic stress. We used a murine nasal infection model to simulate the early phase of S. pneumoniae invasion into the lower respiratory tract. Levels of SAA1 and C-reactive protein (CRP) in bronchoalveolar lavage fluid (BALF) and serum were quantified using ELISA. In vitro assays examined the effect of serum and recombinant SAA1 on bacterial survival under acidic conditions. Fluorescence-labeled recombinant SAA1 and microscopy were utilized to assess SAA1 internalized by S. pneumoniae. Following nasal infection, SAA1 levels in BALF were significantly reduced, whereas CRP levels remained unchanged. In vitro, serum enhanced S. pneumoniae’s resistance to acidic byproducts including formic, lactic, and acetic acids. Specifically, formic acid promoted bacterial uptake of SAA1, and this internalization improved bacterial tolerance to acidic conditions. Fluorescence microscopy confirmed that SAA1 is internalized by S. pneumoniae. S. pneumoniae can internalize SAA1 to bolster resistance to acid stress, particularly formic acid. This study reveals a novel host–pathogen interaction mechanism wherein S. pneumoniae exploits host acute-phase proteins for environmental adaptation, offering new insights into bacterial survival strategies during infection.

Objectives: Streptococcus pyogenes is involved in a wide range of diseases, including pharyngitis and life-threatening invasive infections. Increasing prevalence of antimicrobial resistance (AMR) has been reported worldwide in various bacteria, limiting the use of antibiotics in infection cases. The present study investigated the AMR of most prevalent S. pyogenes emm types, including emm89 strains in Japan. Methods: A total of 368 previously identified S. pyogenes isolates (311 emm89 strains and 57 of other emm types), which were previously isolated from patients with invasive and non-invasive infections throughout Japan, were used in the analyses. The minimum inhibitory concentrations of seven antibiotics, including penicillin-G, azithromycin (AZM) and clindamycin, were determined, and whole-genome sequences of AMR-associated genes were screened. Results: We identified 47 resistant strains, of which 91.49% (43/47) were resistant to AZM and/or clindamycin. A strong correlation was observed between non-invasive phenotypes and AMR. Whole-genome analysis indicated the wide distribution of three AMR-related genes, ermT, folP and lmrP, among the emm89 strains. Additionally, tetO was detected in tetracycline-resistance and soxS and mel was detected in chloramphenicol-resistance only in emm4 strains. Conclusions: The high prevalence of S. pyogenes resistance to AZM and/or clindamycin poses a threat to public health in Japan; thus, the development of next-generation antimicrobial therapies is imperative.