Background: Addressing the Blind Spots of Single Acr Target Exploration and the Data Bottleneck in CRISPR Immune Inhibition for Multidrug-Resistant Enterobacterales R&D

Existing Anti-CRISPR (Acr) protein research has been constrained by a static and linear discovery pipeline biased towards single candidates derived from Pseudomonas phages, failing to systematically capture the distribution patterns of Acr gene loci within mobile genetic elements (MGEs) across the entire Enterobacterales order, thus exposing a critical blind spot. Specifically, the textbook notion that the Type I-E CRISPR-Cas system in Escherichia coli K12 is transcriptionally silent has prevailed, leaving unanswered whether this system is a genuine target within MGEs. Although the role of Acr modules in neutralizing host CRISPR defenses during the acquisition of multidrug-resistant plasmids in ESKAPE pathogens, including Klebsiella pneumoniae, is a clinically urgent question, conventional single phage-host pair plaque assays cannot capture the actual prevalence and mechanistic diversity of cross-species shared Acr clusters. This has created a critical data bottleneck in establishing next-generation anti-infective strategies targeting the blockade of Acr-mediated horizontal gene transfer (HGT).

Discovery: Implementation of a Pan-Enterobacterales Comparative Genomics Census and Empirical Validation of a Multisubunit Resolution Interference Tensor Synchronized within the Cascade Complex

The Skoltech-US-China collaborative research team (Artem Isaev, PI, PNAS Vol. 123 Issue 24, June 2026) conducted a pan-Enterobacterales comparative genomics census of prophages and conjugative plasmids, quantitatively demonstrating that the acrIE9–acrIE10 genetic linkage module is the most widely distributed Anti-CRISPR cluster within this order, and elucidating the co-occurrence pattern of acrIE13, which possesses an atypical helix-turn-helix-like (HTH) architecture. The crystal structure of AcrIE9 was determined at 1.73 Å resolution using selenomethionine anomalous diffraction, revealing an α/β fold composed of an N-terminal antiparallel β-sheet and a C-terminal long α-helix, which was identified as a completely novel protein fold in a DALI server-based structural similarity search, returning no existing registered structures. Solution-state monomer-dimer dynamic equilibrium was demonstrated by SEC-MALS, revealing a 29-hydrogen-bond-based homodimer assembly that buries 32.5–34% of the surface area per protomer. Critically, AlphaFold3 multimeric modeling predicted that the AcrIE9 dimer forms a multivalent interface with three individual Cas7e subunits within the assembled Cascade complex, consistent with the lack of direct interaction detected in analytical SEC with individual Cas7e alone—that is, AcrIE9 employs a multisubunit targeting mechanism that recognizes the composite interface of the fully assembled Cascade rather than a single subunit. This composite interface blockade fundamentally prevents the Cascade from binding to its target DNA, completely eliminating CRISPR interference prior to Cas3 nuclease recruitment. AcrIE10 employs a similar mechanism while exhibiting non-redundant synergy with AcrIE9, and the newly identified AcrIE13 raises the possibility of direct targeting of bacterial DNA, disruptively expanding the existing paradigm of Acr protein function (direct binding to immune proteins).

Establishment of a Model for Cascade Complex Assembly-Dependent Multivalent Inhibition Tuning and Precise Layered Modulation of Reversible Host CRISPR Immune Homeostasis

This study molecularly stratified Enterobacterales strains harboring Type I-E CRISPR-Cas systems based on Acr cluster composition (acrIE9 alone, acrIE9–IE10 dual, acrIE9–IE10–IE13 triple) to construct a correlation matrix of MGE acquisition susceptibility and residual CRISPR defense capacity. The multivalent binding of AcrIE9 to the Cascade 6×Cas7e repeat subunit backbone substantially up-clamps the free energy barrier of the rate-limiting R-loop formation, and this inhibition exhibits reversibility dependent on Acr expression levels—that is, CRISPR interference is completely blocked by high concentrations of Acr secreted during the early stages of phage infection, but the host immune system can autonomously restore itself after the Acr gene locus is deleted and the prophage is lost. This suggests that a binary switching dynamic of resistance plasmid acquisition (Acr ON → CRISPR neutralization) and immune defense restoration (Acr OFF → CRISPR reactivation) at the bacterial population level directly drives the temporal variation of infection dynamics within hospitals. The possibility that the HTH-like domain of AcrIE13 targets bacterial DNA itself represents a topological shift that expands the downstream effect space of Acr function from binding to immune proteins to direct regulation of DNA, necessitating cross-validation with transcriptional regulatory network disturbance tensors. The demonstration of interspecies Acr compatibility between E. coli and K. pneumoniae confirms that the host range of horizontally transferred Acr modules within Enterobacterales extends beyond a single species, realizing a scenario of CRISPR immune homeostasis breakdown in a polymicrobial environment within hospitals.

Prospects: Establishment of a Standard for Programmable Anti-CRISPR Surveillance Systems and Launch of a Digital Governance System for Next-Generation Anti-Infective Investigational New Drug (IND) Applications

This study provides the basis for a complete reset of the governance of antibacterial drug R&D, shifting from a static, post-hoc, symptomatic antibiotic pipeline to a programmable surveillance infrastructure based on an AI-powered multidimensional Acr distribution tensor. Given that Acr modules function as gatekeepers in neutralizing CRISPR defenses during the spread of carbapenem-resistant Klebsiella pneumoniae plasmids, which are listed as a top-priority critical pathogen in the WHO priority pathogen list (2024 revision), global multinational anti-infective pipelines should be expanded to a dual surveillance system that monitors both resistance genes and anti-immunity genes (including Acr). In the Phase I/II pipeline of CRISPR-Cas3-based antibacterial phage therapeutics from Locus Biosciences (now a bioMérieux subsidiary), such as crPhage and LBP-EC01, the intrinsic Acr profile of the target strain is emerging as a key confounding variable in treatment response rates, and a companion diagnostic (CDx) panel interface that pre-screens for this should be incorporated into the essential data package for IND applications. The 1.73 Å structural coordinates of AcrIE9 and the AlphaFold3 multimeric prediction model are directly applicable as seed assets for structure-based drug design (SBDD) of Acr-neutralizing antibodies or small-molecule inhibitors, and can be combined with a strategy of competitive inhibition using Cascade complex interface mimetic peptides to derive cGMP process correction factors that eliminate batch-to-batch Acr neutralization titer variations. Ultimately, the pan-Enterobacterales Acr census database will be linked to NCBI AMRFinderPlus and CARD (Comprehensive Antibiotic Resistance Database) to function as a digital healthcare governance asset that determines the presence or absence of Acr in clinical isolates in real time based on genomic sequencing, forming a master reference architecture that disruptively shortens the FDA/EMA cGMP approval timeline for CRISPR-based therapeutics.