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Bacteriophages
Introduction
Bacteriophages, or phages, are viruses that infect and replicate only within bacteria. They are the most abundant biological entities on Earth, with an estimated 1031 particles worldwide, vastly outnumbering their bacterial hosts.[1] They are found essentially everywhere bacteria live, from seawater and soil to the human gut, and they were recognized early on as a ubiquitous feature of the microbial world.[2][3]
Phages are strikingly diverse in size, morphology, and genome organization; their genomes range from a few thousand nucleotides of single-stranded DNA or RNA to several hundred kilobase pairs of double-stranded DNA.[4][5] Underlying that variety is a common design: a nucleic acid genome packaged within a phage-encoded protein capsid that protects the genome and delivers it into the next host cell.[4] Electron microscopy has cataloged hundreds of distinct particle types, and most phages characterized to date are tailed, double-stranded DNA viruses whose icosahedral “head” and helical “tail” give them their familiar appearance.[4][6]
The long-familiar morphology-based families-Myoviridae, Siphoviridae, and Podoviridae, grouped in the order Caudovirales—have been abolished by the International Committee on Taxonomy of Viruses, which now places tailed phages in the class Caudoviricetes and classifies phages by whole-genome relatedness rather than by particle shape.[7][6] Most phages infect only a single bacterial species, and frequently only particular strains within it.[8][4] Once a phage has attached to and entered a susceptible host, it follows one of two replication strategies, lytic or lysogenic.[2][9]
In a lytic cycle, the phage introduces its genome into the host cytoplasm and commandeers the cell's ribosomes and metabolic machinery to manufacture viral proteins.[8][4] The cycle ends when the host is lysed, typically through phage-encoded holins and endolysins that breach the membrane and cell wall, releasing a new generation of virions to infect neighboring bacteria.[8][10]
In a lysogenic cycle, the phage attaches and injects its genome as before, but rather than driving immediate lysis, the genome is integrated into the bacterial chromosome or maintained as an episomal element, where it is replicated and passed to daughter cells along with the host genome.[9][2] An integrated phage genome is termed a prophage, and a bacterium carrying one is a lysogen.[9]
Function
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Function
Although phages cannot infect or replicate in human cells, they are an integral part of the human microbiome and one of the main conduits of genetic exchange among bacteria, including between pathogenic and non-pathogenic strains. The transfer of bacterial genes from one cell to another inside a phage particle is called transduction, which proceeds by two classical routes, generalized and specialized.[11][12][11]
Generalized Transduction
In generalized transduction, fragments of host chromosomal DNA are packaged into phage capsids in place of the phage genome as the cell disintegrates at the end of a lytic infection.[11] If a particle carrying such bacterial DNA injects it into a healthy recipient, that DNA may recombine into the recipient's chromosome, altering its genome and that of its descendants.[11]
Specialized Transduction
Specialized transduction is a feature of temperate phages. When a prophage excises imprecisely from its integration site to launch a lytic cycle, it can take neighboring bacterial genes with it; because the lysogens in a population share the same integration site, their progeny phages carry the same host gene to every new host they infect.[11] A related but distinct process, lateral transduction, was described in Staphylococcus aureus in 2018: prophages that begin replicating in place before excising package long stretches of chromosomal DNA and transfer host genes, virulence determinants among them, at frequencies far exceeding classical transduction.[13]
Beyond moving genes around, phages shape microbial communities by preying on some bacterial species while leaving others untouched.[13] Metagenomic surveys show the healthy gut carries a large, highly individualized, and still largely uncultured community of phages, the gut “phageome,” within which crAssphage, identified in 2014, ranks among the most abundant human-associated viruses.[14][3] Researchers have tried to harness the bacteria-killing capacity of phages to treat infection for more than a century.[2] In the wild, however, phages appear to exert only transient control over their bacterial prey, in part because resistant mutants emerge quickly.[15]
Translating phage therapy into routine clinical practice has proved difficult for several reasons.[8][16][8] Wild bacterial populations are genetically diverse, and many strains resist one or more phages.[8] Resistance can arise from mutation or loss of the phage receptor or from dedicated bacterial immune systems.[8] The best known of these is CRISPR-Cas, which acts as an adaptive defense against phages and other mobile genetic elements and was later repurposed as a genome-editing tool.[17][18] CRISPR is only one option; bacteria deploy a striking range of anti-phage systems, including restriction-modification, abortive infection, and many others that have only been characterized in the past few years.[19] Phages also raise pharmacologic hurdles: they are more immunogenic than small-molecule antibiotics, can elicit neutralizing antibodies, and are cleared from the bloodstream by the reticuloendothelial system, all of which complicate dosing.[20]
For many years, these obstacles led investigators to assume that phage preparations were suited mainly to topical use, but the past decade has changed that picture. Intravenous phage therapy has been delivered successfully in compassionate-use cases, including a personalized cocktail that helped rescue a patient with a disseminated Acinetobacter baumannii infection and genetically engineered phages used against disseminated Mycobacterium abscessus in a young patient with cystic fibrosis.[21][22]
Controlled clinical evidence nonetheless remains limited, and results have been mixed. A phase 1/2 trial of a phage cocktail for burn-wound Pseudomonas aeruginosa infection (PhagoBurn) was underpowered and undermined by a drop in phage titer during manufacturing, and a placebo-controlled trial of intravesical phages for urinary tract infection did not demonstrate superiority over standard care.[23][24] Real-world experience from personalized, compassionate-use programs has been more encouraging; a 2024 multicenter series reported clinical improvement in most of 100 consecutive patients treated with tailored phage preparations, generally in combination with antibiotics.[25] Even so, no phage product is yet licensed as a drug in the United States or the European Union, and treatment is typically provided under expanded-access or with specialist oversight.[26]
Interest in pairing phages with antibiotics has grown as well, since sublethal concentrations of some antibiotics stimulate phage production and the two agents can act synergistically, an effect termed phage-antibiotic synergy.[27]
Clinical Significance
Phages carry clinical weight for several reasons. First, many of the most dangerous bacterial toxins are encoded not by the bacterial chromosome itself but by resident prophages, so that the host organism becomes toxigenic only after it has been lysogenized by the toxin-carrying phage.[12][11] Well-documented examples include the cholera toxin of Vibrio cholerae, carried on the filamentous phage CTX and still being characterized in pandemic strains; the diphtheria toxin of Corynebacterium diphtheriae, encoded by the tox gene of corynephage beta; the botulinum neurotoxins of Clostridium botulinum types C and D; the binary toxin (CDT)-associated genes identified on a Clostridioides difficile bacteriophage genome; and the Shiga toxins of Shigella and Shiga toxin-producing Escherichia coli, which are carried on inducible lambdoid prophages.[28][29][30][31][32][33] The Panton-Valentine leukocidin and several enterotoxins of Staphylococcus aureus are likewise prophage-encoded.[12] Without these phage-borne genes, the bacteria are considerably less virulent, or nonvirulent.[12] Why phages should carry toxin genes is not fully understood; the profuse watery diarrhea produced by cholera toxin plausibly helps disseminate both the bacterium and its phage, whereas the flaccid paralysis of botulism would seem, if anything, to hinder transmission.[12]
Second, phages are major vehicles for horizontal gene transfer, spreading not only toxin genes but also antimicrobial-resistance determinants between strains and species.[11][17] They can also be deliberately engineered to introduce genes into specific bacteria for clinical benefit, an approach still largely under development but already demonstrated with the genetically modified phages used in the Mycobacterium abscessus case.[22]
Third, detecting a phage can serve as a biomarker for the presence of its bacterial host in a complex sample. This principle underlies the long-standing use of coliphages as indicators of fecal contamination in water, where the presence of the phage implies that its enteric host is likely present as well.[34]
Phages have also been used to fingerprint bacterial strains. Because strains of a species differ in their susceptibility to a standardized panel of phages, phage typing, applied internationally to Staphylococcus aureus, for instance, allowed strains to be distinguished for epidemiologic tracking.[35] Although it has largely been supplanted by molecular methods such as multilocus sequence typing and pulsed-field gel electrophoresis, phage typing was for decades the reference standard for outbreak investigation.[35]
Enhancing Healthcare Team Outcomes
Bacteriophages rarely appear on a treatment plan, yet a working understanding of phage biology increasingly informs the interprofessional team- physicians, advanced practitioners, clinical microbiologists, infection preventionists, pharmacists, and nurses- responsible for managing bacterial infection and its complications. Because prophages carry toxin and antimicrobial-resistance genes between bacterial strains, the team is well served by selecting disinfection and isolation measures that inactivate viruses as well as bacteria, and by communicating those measures clearly whenever a phage-borne toxin disease, such as cholera or shigellosis, is suspected. Antimicrobial stewardship offers a concrete example: because certain antibiotics, fluoroquinolones in particular, can induce the Shiga toxin-encoding prophage of Escherichia coli and increase toxin production, pharmacists and clinicians do best to weigh antibiotic choice together in suspected Shiga toxin-producing E. coli infection and hemolytic uremic syndrome rather than in isolation. As phage therapy re-enters practice for multidrug-resistant infection, it depends even more on this coordination; expert consensus panels and the largest treated series to date describe delivery by multidisciplinary teams that link the treating clinician with clinical microbiology, dedicated phage laboratories, pharmacy, and regulatory staff, using structured communication to match each patient's isolate to a suitable phage, monitor the response, and manage supply under expanded-access.
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