How Bacteria and Viruses Evolve to Defend Against Each Other

Bacterial cells often rely on extracellular structures, such as capsules and biofilms, as physical and biochemical barriers against external threats, including bacteriophage infections. These surface-associated structures provide robust defenses, obstructing phage adsorption and impeding their ability to establish infection. This multifaceted resistance is a critical survival strategy for bacteria in diverse environments and exemplifies the co-evolutionary arms race between phages and their bacterial hosts.

Capsules are dense, often polysaccharide-rich layers surrounding bacterial cells, functioning as the first line of defense against phages. They form a physical barrier and block access to specific receptors on the bacterial surface, preventing phage adsorption and subsequent infection. Capsular polysaccharides are highly variable among bacterial species, contributing to strain-specific resistance (Shabbir et al. 2016; Sørensen et al. 2021). For example, the capsule of Escherichia coli K1 effectively prevents the attachment of phages that target the lipopolysaccharides (LPS) underneath the capsule (Scholl et al. 2005). However, this defense is counterbalanced by the emergence of K1-specific phages (e.g., K1F and K1-5) that encode capsule-degrading endosialidases, illustrating a highly specialized co-evolutionary interaction between the two.

A comparable yet more complex capsule-mediated defense strategy is observed in Klebsiella pneumoniae, a pathogen characterized by exceptional capsular diversity encompassing more than 80 distinct serotypes. In K. pneumoniae, the thick polysaccharide capsule impedes phage adsorption by shielding conserved outer membrane structures and enhances resistance to immune clearance, antimicrobial peptides, and environmental stress. This extensive capsular heterogeneity significantly restricts the ability of phages to evolve into broadly effective receptor-binding proteins, thereby limiting the phage host range and infection efficiency (Y.-J. Pan et al. 2019a, b). Although certain K. pneumoniae-specific phages encode depolymerases capable of degrading defined capsule types, resistance frequently emerges through capsule modification, downregulation, or complete loss of capsule synthesis. While such adaptations reduce phage susceptibility, they often incur fitness and virulence trade-offs, underscoring the evolutionary balance between capsule-mediated defense and pathogenicity.

Another extracellular structural defense mechanism involves the biofilm formation. Biofilms are complex, multicellular communities of bacteria encased in an extracellular polymeric substance (EPS) matrix, which consists of polysaccharides, proteins, lipids, and extracellular DNA (eDNA). Biofilms are a hallmark of bacterial survival in hostile environments, providing resistance to antibiotics, host immune responses and phage predation. The EPS matrix acts as a physical and chemical shield, trapping phages and preventing their penetration into deeper layers of the biofilm(Karygianni et al. 2020; Wang et al. 2023a, b, c).

The structural complexity of biofilms presents multiple barriers to phages. In Pseudomonas aeruginosa biofilms, the EPS component alginate plays a critical role in restricting phage diffusion and adsorption (Hanlon et al. 2001). Similarly, Staphylococcus epidermidis biofilms produce poly-N-acetylglucosamine, which enhances resistance to phages by limiting access to bacterial receptors (Le et al. 2018).

Biofilm-mediated phage resistance is a multifaceted strategy that protects bacterial communities from phage predation. One primary mechanism is the physical shielding provided by the extracellular polymeric substance (EPS) matrix, which impedes phage diffusion, preventing them from accessing and binding to bacterial receptors (Ferriol-González and Domingo-Calap 2020). This barrier is particularly effective against lytic phages that require direct contact with their host cells for propogation. Studies on Pseudomonas fluorescens have demonstrated that biofilms can trap and inactivate phages within the EPS matrix, effectively neutralizing their infectious potential (Dunsing et al. 2019). Additionally, the inherent phenotypic and genetic heterogeneity within biofilm communities further enhances their resistance. Subpopulations within biofilms may downregulate phage receptors or acquire mutations that confer resistance, ensuring that at least a fraction of the bacterial population survives even under intense phage attack (Harper et al. 2014). Another significant defense mechanism involves extracellular DNA (eDNA) within the biofilm matrix, which can bind to and immobilize phages, effectively trapping them and preventing infection. This “phage-trapping” mechanism has been observed in Pseudomonas aeruginosa biofilms, where eDNA contributes to phage sequestation, significantly reducing their efficacy (Whitchurch et al. 2002). Together, these mechanisms make biofilms a formidable barrier to phage infection and a key survival strategy for bacterial communities.

The downregulation of phage receptors on cell surfaces by reducing the expression of these bacterial receptors effectively decreases the likelihood of phage adsorption and subsequent infection. This mechanism plays a crucial role in bacterial survival and is often regulated by quorum sensing and other metabolic adaptations.One of the primary ways in which bacteria regulate the expression of phage receptors is through quorum sensing, a system that adjusts gene expression based onbased on cell population density. For example, in Vibrio anguillarum, quorum sensing downregulates the expression of the OmpK receptor at high cell densities, thereby reducing the susceptibility of the bacterium to phage KVP40 (Tan et al. 2015). A similar mechanism has been observed in Escherichia coli, where quorum-sensing signals lead to a reduction in the number of λ phage receptors on the bacterial surface (Ding et al. 2021; Wang et al. 2022). This significantly decreases the rate of adsorption. By modulating receptor expression in response to population density, bacteria can limit phage infections when they are most vulnerable while optimizing receptor availability under conditions that favor growth and resource acquisition.

In addition to quorum sensing, bacteria employ non-mutational defense mechanisms to evade phage infection. Unlike genetic mutations that permanently alter receptor structure, non-mutational strategies allow bacteria to reversibly modify their surface receptors in response to environmental cues. For example, Vibrio alginolyticus downregulates specific receptors and transporters in its membrane to avoid infection by lytic bacteriophages (Skliros et al. 2023). This form of defense is part of a broader metabolic adaptation that includes changes in quorum-sensing regulatory proteins and other cellular processes. By selectively suppressing the expression of receptors required for phage adsorption, bacteria can maintain their overall fitness while mitigating the risk of phage attacks.However the downregulation of phage receptors has broader implications for bacterial physiology and virulence. In some cases, the same receptors that phages exploit for infection also play essential roles in nutrient uptake, biofilm formation, or interactions with host organisms. In Vibrio anguillarum, for example, phage-resistant variants that downregulate the OmpK receptor exhibit reduced virulence and altered biofilm formation. This suggests that receptor downregulation, although beneficial for phage resistance, involves trade-offs that may affect bacterial survival in other contexts. Similarly, in Vibrio alginolyticus, changes in receptor expression have been linked to modifications in quorum-sensing pathways, indicating that phage resistance strategies can influence broader regulatory networks within bacterial populations (Høyland-Kroghsbo et al. 2013).

The trade-offs associated with receptor downregulation highlight the complex interplay between bacterial defense mechanisms and their ecological fitness. While reducing receptor expression helps bacteria evade phage infection, it can also impose metabolic costs and limit their ability to exploit certain environmental resources. These trade-offs underscore the dynamic nature of bacterial evolution, where survival strategies must balance the immediate threat of phage predation with the long-term need for growth and adaptation.While surface-associated defenses serve as the primary barrier limiting phage attachment and entry, intracellular defense mechanisms are activated as a secondary protective layer upon successful phage genome delivery into the bacterial cytoplasm.

Restriction-modification (R-M) systems are the cornerstone of bacterial intracellular defense, offering a highly specific mechanism to recognize and degrade foreign DNA, including phage genomes. These systems consist of two main components: restriction endonucleases that cleave DNA at specific recognition sequences and methyltransferases that modify the bacterial genome by adding methyl groups to specific bases, thus distinguishing self-DNA from non-self-DNA (Fig. 4).

R-M systems are classified into four types based onbased on their structure, mode of action, and recognition sequence complexity. Type II R‒M systems, such as EcoRI in Escherichia coli, have been extensively studied for their precise cleavage activity and wide application in molecular biology (R. J. Roberts et al. 2003). Similiar to EcoKI system, type I systems combine restriction and modification activities within a complex enzyme and require ATP for their function, whereas type III systems, such as EcoP15I, require two recognition sites to trigger cleavage (Murray 2000). Type IV systems add another layer of protection by targeting modified DNA, such as methylated or hydroxymethylated DNA, which is characteristic of certain phages (Loenen et al. 2014). For example, the PvuRts1I enzyme specifically recognizes glucosylated hydroxymethyl cytosine, a modification found in phage T4 DNA.These systems provide a robust initial response to invading phages, effectively degrading the viral genome and halting infection. However, their specificity also imposes limitations, as mutations in phage DNA sequences or methylation patterns can render R-M systems ineffective, driving the coevolution of bacterial defenses and phage counter-defenses.

Abortive infection (Abi) systems operate as a last-resort defense mechanism, sacrificing infected bacterial cells to prevent the spread of phages within a population. These altruistic systems disrupt critical cellular processes following phage infection, often leading to host cell death but ensuring that progeny phages are not released(Aframian and Eldar 2023; Lopatina et al. 2020). This strategy protects the surrounding bacterial population by halting viral spread. Abi systems achieve this by distrupting essential cellular processes, including translation, transcription, and replication, or by inducing membrane leakage(Fineran et al. 2009). However, the precise mechanisms by which bacteria recognize phage infection to trigger the Abi response remain largely unknown.

Nucleotide-based immunity systems represent a recently discovered class of bacterial defenses that offer broad-spectrum protection against phages. These systems, including BREX and DISARM, are encoded within bacterial genomes and involve complex multiprotein machinery that targets phage DNA during infection.

The BREX system is a widespread bacterial immune defense that operates by modifying specific DNA motifs through methylation, effectively marking the bacterial genome as “self.” This prevents phages from replicating their DNA within host cells. Unlike traditional restriction-modification (R-M) systems, BREX does not degrade or cleave the phage DNA. Instead, it modifies bacterial DNA at non-palindromic TAGGAG motifs through methylation, mediated by BrxX, a DNA methyltransferase. This modification acts as a molecular signature that distinguishes bacterial DNA from foreign genetic material, preventing phage replication(Beck et al. 2022).

BREX typically consists of six core genes, including BrxX, the primary DNA methyltransferase responsible for adenine methylation within specific motifs, and BrxA, a DNA-binding protein that facilitates methylation specificity and DNA recognition. Additional regulatory and structural proteins, BrxB, BrxC, BrxD, and BrxE, contribute to system the function and stability of the system(Goldfarb et al. 2015; Picton et al. 2021). Some phages encode anti-defense proteins to overcome BREX-mediated defenses. For example, T7 phage protein Ocr, mimics DNA and binds to BrxX, inhibiting its methylation activity. This enables the phage to bypass the bacterial immune response and replicate successfully(Li et al. 2024). BREX systems are found in approximately 10% of all sequenced microbial genomes. These genes were classified into six subtypes based onbased on gene composition and organization, illustrating their evolutionary diversity. This system provides robust and non-degradative protection against phage infection, making it an essential component of bacterial defense. For example, BREX systems in Escherichia coli and Bacillus subtilus have been shown to block a wide range of phages through an undefined but highly effective mechanism (Goldfarb et al. 2015).

The DISARM system represents a sophisticated nucleotide-based defense mechanism that provides broad protection against phages through a combination of DNA methylation and restriction (Athukoralage and White 2022; Dot et al. 2023; Isaev et al. 2021). Unlike BREX, which blocks replication without cleaving DNA, DISARM actively restricts phage DNA for preventing infection. DISARM uses a multiprotein complex to distinguish bacterial DNA from foreign DNA. It modifies bacterial DNA at CCWGG motifs through methylation, preventing the function and stability of the systemby its restriction enzymes (Zhang et al. 2025). When un-methylated phage DNA enters the cell, the system triggers its restriction and degradation, thereby neutralizing the invading genetic material. The DISARM system is composed of five core genes: DrmA and DrmB, which form the core DNA methylation complex responsible for modifying bacterial DNA; a helicase domain that facilitates DNA unwinding during restriction; a phospholipase D (PLD) domain that provides additional enzymatic activity for DNA cleavage; and the DUF1998 domain, a protein of unknown function that is believed to contribute to system stability and phage recognition (Bravo et al. 2022).

DrmAB remains autoinhibited by a trigger loop (TL) within DrmA. Upon binding to unmethylated phage DNA, the TL is dislodged, triggering structural rearrangements that activate the restriction enzyme complex. This results in the targeted degradation of foreign DNA and preventphage replication. Compared with BREX systems, DISARM systems are more widely distributed across bacterial and archaeal species. Although many phages have evolved mechanisms to evade other restriction-modification systems, there is limited evidence of specific phage countermeasures against DISARM, making it a particularly robust immune mechanism (Bravo et al. 2022; Ofir et al. 2018).

Both BREX and DISARM utilize nucleotide-based modifications to defend bacteria against phages but employ distinct strategies to do so. BREX functions by marking bacterial DNA and preventing phage replication through methylation, whereas DISARM takes a more aggressive approach by degrading foreign DNA. BREX relies on BrxX methyltransferase and BrxA for DNA modification, whereas DISARM includes additional enzymatic components such as a helicase domain and a phospholipase D domain (X. Xu and Gu 2024). Phages have evolved countermeasures, such as Ocr proteins, to inhibit these defense systems; however DISARM appears to have fewer known phage evasion strategies. Understanding these systems not only enhances our knowledge of bacterial immunity but also has potential applications in biotechnology, synthetic biology, and phage therapy for the control bacterial infections.

The clustered regularly interspaced short palindromic repeats (CRISPR) CRISPR-associated proteins (CRISPR‒Cas) system represents an advanced adaptive defense mechanism in bacteria and archaea, offering precise protection against invading genetic elements such as bacteriophages and plasmids. Unlike innate mechanisms, which provide broad and nonspecific defenses, the CRISPR‒Cas system allows bacteria to develop sequence-specific immunity. By incorporating fragments of phage DNA into their genome, bacteria can “remember” past infections and mount targeted responses against recurring threats. This remarkable capability underscores the sophistication and evolutionary success of the CRISPR‒Cas system (Barrangou et al. 2007; Barrangou and Marraffini 2014).

CRISPR‒Cas systems are categorized into two main classes based on their structure and function. Class 1 systems, which include Types I, III, IV, and VII, utilize multiprotein complexes for interference (Makarova et al. 2025). For example, Type I systems use a cascade complex to recognize and cleave foreign DNA. In contrast, Class 2 systems employ a single effector protein such as Cas9 (Type II), Cas12 (Type V), or Cas13 (Type VI) to execute these tasks (Koonin et al. 2017). This simplicity has made Class 2 systems a focal point in genetic engineering and biotechnological applications. The diversity of CRISPR‒Cas systems enables bacteria to tailor their defenses against the vast array of phages they encounter, providing a significant survival advantage to the bacteria.

The CRISPR‒Cas system operates through three interconnected phases: adaptation, crRNA maturation and interference (Ahmed et al. 2024). During the adaptation phase, bacterial cells capture and integrate short DNA fragments (spacers) from invading phages into their CRISPR arrays. This process is mediated by Cas1 and Cas2 proteins and establishes a molecular memory of the infection. In subsequent encounters, the CRISPR array is transcribed into precursor CRISPR RNAs (precrRNAs), which are processed into mature crRNAs. These guide RNAs direct Cas proteins to invading the phage DNA, enabling precise cleavage and neutralization. This process, often termed the interference phase, effectively halts phage infection (Hille et al. 2018). Over time, the CRISPR array expands, maintaining a chronological record of past infections, which forms the basis of adaptive immune memory (Fig. 6).

The CRISPR‒Cas system significantly enhances bacterial immunity by providing specific and durable protection against phages. For example, Streptococcus pyogenes employs its Type II CRISPR‒Cas9 system to target and neutralize specific DNA sequences, demonstrating the precision of this mechanism. However, although the system offers many advantages, it is not without limitations. Phages can develop mutations in their target sequences, such as protospacer or protospacer-adjacent motifs (PAMs), allowing them to evade detection and cleavage by CRISPR‒Cas. Additionally, the metabolic cost of maintaining and operating the system may reduce bacterial fitness in environments with low levels of phage pressure. Furthermore, some phages produce anti-CRISPR (Acr) proteins that inhibit CRISPR‒Cas activity, thereby negating their defensive capabilities (Barrangou and Marraffini 2014; Makarova et al. 2020).

Despite these challenges, the CRISPR‒Cas system remains a cornerstone of bacterial defense, demonstrating unparalleled adaptability and specificity (Table 1). The evolutionary arms race between bacteria and phages drives constant innovation in these systems, offering valuable insights into microbial survival strategies. This ongoing conflict has also fuelled advancements in biotechnology, where CRISPR–Cas systems are now harnessed for genome editing, disease modeling, and therapeutic development, further highlighting their importance beyond natural bacterial immunity.

Bacteriophages have evolved multiple strategies to counter bacterial receptor-based defenses. When bacteria modify or mask their receptors to prevent phage attachment, phages respond by adapting to the new receptors via receptor mimicry, breaking down protective extracellular barriers, or evolving to recognize the modified receptors (Dunne et al. 2021; Nobrega et al. 2018).A common approach that phages use to adapt to new receptors by mutating their receptor-binding proteins (RBPs), also known as receptor mimicry, where phages evolve proteins that mimic bacterial receptors, allowing them to bind to and infect bacterial cells despite bacterial resistance mechanisms. Bacteriophages use receptor-binding proteins (RBPs) in their tail fibers or tail spikes to recognize and attach to bacterial surface receptors (de Jonge et al. 2019; Dunne et al. 2019). Tail spikes also contain enzymes that degrade protective polysaccharides, such as capsules or extracellular polysaccharides (EPS), exposing secondary receptors for irreversible adsorption (X. Wang et al. 2021a, b). Podoviridae phages often have depolymerases in their tail spike proteins (TSPs) that breakdown host polysaccharides, allowing DNA injection and infection(Knirel et al. 2020).RBPs function as keys that fit into bacterial receptor locks. A single RBP can recognize multiple receptors, and a single receptor can interact with various RBPs. If a receptor is lost or modified, phage binding is blocked, enabling bacterial resistance to phage infection. However, phages can mutate their RBPs to target new receptorsand adapt to bacterial defenses. The host range of phages can be modified by altering their tail proteins (Philippe et al. 2023; Shuwen and Kefeng 2022).For example, phage λ, which initially binds to the LamB receptor on E. coli, can evolve to recognize the OmpF receptor through a series of mutations when LamB expression is reduced (Samson et al. 2013). Similarly, phages can modify their tail fibers or adsorption proteins to recognize alternative receptors when primary receptors are inaccessible. The phage ΦX174 adapts to changes in its host’s lipopolysaccharide (LPS) structure by mutating key proteins involved in attachment (Romeyer Dherbey et al. 2023).

Phages also utilize diversity-generating retroelements (DGRs) to increase receptor adaptability. DGRs consist of reverse transcriptase and two repeat regions: a variable repeat (VR) and a template repeat (TR). Mutations introduced in the VR region create variations in phage tail fiber proteins, allowing phages to recognize different receptors. This mechanism can generate up to 10¹⁴ tail fiber variations, significantly increasing the chances of successful bacterial infections (Benler et al. 2018). Another strategy involves the hydrolysis of extracellular components that shield bacterial receptors. Phages produce enzymes, such as endosialidases, glycosidases, and EPS-degrading enzymes, to break down capsules and exopolysaccharides, exposing hidden receptors for binding(Abd-Allah et al. 2021). Bacterial capsules, composed of polysaccharides, are highly effective barriers against phage adsorption and infection. However, many phages counter this defense by producing capsule-degrading enzymes, such as depolymerases. These enzymes break down the capsular polysaccharides, exposing the underlying receptors and enabling phage attachment. For example, phage K1F uses a specific depolymerase to degrade the capsule of Escherichia coli K1, allowing it to bind to its receptor and initiate infection (Scholl et al. 2005). Another example is phage phiKZ, which produces multiple polysaccharide-degrading enzymes that efficiently penetrate bacterial capsules (Cornelissen et al. 2012).

Capsule-degrading enzymes not only facilitate infection but also contribute to the dispersal of bacterial populations, increasing their susceptibility to secondary infections. These findings demonstrate the dual role of these enzymes in overcoming bacterial defense mechanisms and reshaping microbial communities. For example, phage E79 produces an alginate lyase enzyme that breaks down alginate, a key EPS component in Pseudomonas aeruginosa biofilms, thereby improving its infectivity and facilitating biofilm disruption (Hanlon et al. 2001).In addition to enzymatic degradation, some phages exhibit enhanced persistence in biofilm environments. This persistence increases the likelihood of phage encounters with susceptible cells, thereby initiating infection. For example, phages targeting Staphylococcus aureus biofilms have been shown to persist in the biofilm environment for a long enough period to locate and infect viable cells (Harper et al. 2014). Furthermore, certain phages evolve to preferentially infect biofilm-associated bacteria by recognizing specific biofilm-related receptors or exploiting structural weaknesses in the biofilm.

Through mutational adaptation, enzymatic degradation of bacterial barriers, and receptor-binding protein diversification, phages continuously evolve to overcome bacterial defenses, ensuring their ability to infect host cells despite bacterial resistance strategies. In addition, the genetic engineering of RBPs provides a strategy to expand or change phage specificity, thereby enhancing their potential for antibacterial applications.

One of the key counter defenses employed by phages is the production of anti-CRISPR (Acr) proteins. These proteins inhibit bacterial CRISPR‒Cas systems and effectively neutralize their adaptive immunity. Acr proteins function by either blocking phage DNA recognition by the CRISPR complex or preventing DNA cleavage. For example, AcrIIA4 targets the widely studied Cas9 protein of Streptococcus pyogenes, directly binds to its nuclease domain, and inhibits its ability to cleave DNA (Pawluk 2016). Another example is AcrIF1, which inhibits the Type I‒F CRISPR‒Cas system in Pseudomonas aeruginosa by interfering with the cascade complex responsible for DNA binding (Bondy-Denomy et al. 2013). The diversity of Acr proteins across phages highlights their ability to adapt to different types of CRISPR‒Cas systems, demonstrating the evolutionary pressure exerted by bacterial adaptive immunity. In addition to Acr proteins, some phages encode mini-CRISPR arrays, which produce small CRISPR-derived RNAs that silence host CRISPR–Cas defenses by guiding interference complexes away from phage targets (Camara-Wilpert et al. 2023). Together, these strategies illustrate the diversity of phage anti-CRISPR mechanisms and highlight the evolutionary pressure exerted by bacterial adaptive immunity.

The restriction-modification mechanism is one of the first-line defense mechanisms developed by bacteria. Bacteriophages have developed multiple mechanisms to evade bacterial R–M defenses, including genome modifications, site masking, molecular mimicry, enzyme activation, and cofactor degradation.The key mechanism involves the use of restriction endonucleases (REases) to recognize and cleave specific phage DNA sequences; however phages have developed mechanisms to evade or disable these defenses. One key strategy involvess modifying their genomes with phages to prevent recognition by bacterial REases. Many phages introduce random point mutations in their restriction sites, thereby reducing the likelihood of cleavage. For example, in Bacillus species phages such as SPO1, replacing thymine with hydroxymethyluracil prevents many REases from recognizing phage DNA. Phages also incorporate modified bases such as N6-methyladenine, hydroxymethylcytosine, and glucosylated hydroxymethylcytosine (Drozdz et al. 2012; Krüger and Bickle 1983).

Another common strategy is to mask the restriction sites to protect the phage genome. The coliphage P1, for example, injects DarA and DarB proteins, which bind to its DNA and shield restriction sites from enzymatic cleavage. Some phages also use molecular mimicry to evade restriction systems(Iida et al. 1987). The coliphage T7 produces the Ocr protein, which mimics the DNA phosphate backbone and binds to both the methyltransferase (MTase) and REase domains of the EcoKI enzyme. Ocr has a greater affinity for EcoKI than does phage DNA, leading to enzyme inactivation and protecting the phage from restriction (Atanasiu et al. 2002; Safari et al. 2020).

Phages also manipulate bacterial modification enzymes to safeguard their genomes. Some phages acquire methyltransferases (MTases) through horizontal gene transfer, allowing them to methylate their DNA and evade restriction enzymes. For example, the Lactococcus lactis phage Φ50 acquired a type II MTase through recombination with a resident plasmid, providing broad resistance to bacterial R–M systems. Additionally, phage λ encodes the ral gene, which enhances the activity of bacterial MTases such as EcoK and EcoB, leading to increased methylation of the phage genome and resistance to cleavage. Similarly, the Lar protein, found in a hybrid phage λ (λ reverse), not only increases MTase activity but also inhibits restriction enzyme activity(Hill et al. 1991). Some phages directly degrade essential R–M cofactors to disable bacterial defenses. For example, coliphage T3 produces S-adenosyl-L-methionine hydrolase, which destroys an essential cofactor required for restriction of enzyme function(Studier and Movva 1976). Similarly, the Stp protein of phage T4 induces conformational changes in the REase EcoprrI, rendering it inactive(Penner et al. 1995).Certain phages encode their own MTases, which function independently of bacterial enzymes. Phage SPR, for example, encodes an MTase that methylates cytosines at three distinct recognition sites: GGCC, CCGG, and C(A/T)GG. Phage Mu employs a different strategy by encoding the methyl-carbamoylase Mom, which attaches an acetamide group to adenine at specific recognition sites, further protecting the phage genome from restriction (Pósfai et al. 1984).

Despite these sophisticated counter-defenses, bacteria have evolved second-line strategies to counteract anti-R–M proteins. When restriction mechanisms fail, bacteria can trigger growth arrest or programmed cell death to prevent phage replication. DNA adenine methylase (Dam), an anti-R–M protein, is a key activator of bacterial defense systems such as retron-Sen2, which induces cell growth arrest. Additionally, Dam can activate other bacterial defense mechanisms, including Dazbog and Nhi-like systems (Blower et al. 2012).

Abortive infection (Abi) is a bacterial defense mechanism that inhibits key phases of a phage’s life cycle, such as replication, transcription, and virion assembly. Abi frequently causes the death of phages and infected bacterial cells, in contrast to other resistance strategies.

All Abi systems require a toxic protein that becomes active upon phage infection, even though all Abi systems function differently. Endoribonucleases, ion channel disruptors, and transcriptional repressors are among the proteins that either inhibit bacterial growth or cause cell death, which stops phage reproduction. However, phages have developed strategies to overcome these barriers. For example, phage bIL66 suppresses the ORF3 protein, an essential enzyme for phage growth, by producing the ORF1 protein, which sets off the AbiD1 system. To bypass this, the phage mutates ORF1, preventing AbiD1 activation (Bondy-Denomy et al. 2013). Similarly, Lactococcus phages evade the AbiK system through mutations in sak genes, which encode DNA repair proteins. To resist the AbiV system, phages acquire mutations in sav, a gene that normally interacts with AbiV to shut down protein synthesis(Haaber et al. 2010; Scaltriti et al. 2011).Additionally, research on three T4-like phages revealed that they evolved spontaneous escape mutants to overcome ToxINPa-mediated abortive infection. These mutants avoid cell death through three different strategies: mutation of asiA, a transcriptional coactivator; alteration of orf84, a conserved gene; or deletion of a large Sect. (6.5–10 kb) of their genome(Chen et al. 2017).In a more recent discovery, the phage FTE was found to counteract the ToxIN Abi system in Pectobacterium atrosepticum by producing an RNA molecule that mimics an antitoxin, effectively neutralizing the toxic protein ToxN. This demonstrates that the evolutionary battle between bacteria and phages is ongoing, with each side constantly developing new strategies to outmaneuver the other (Blower et al. 2012).