Bacteriophages are viruses that infect bacteria. A phage attaches to a specific receptor on the bacterial surface, injects its genome, and uses the host's own machinery to replicate before lysing the cell. Three features follow from that description and explain most of the practical interest and most of the practical difficulty: they kill bacteria rather than inhibiting them, they are specific to a narrow set of strains, and they can replicate in the presence of susceptible bacteria. Whether that is an advantage or a problem depends entirely on what you are trying to do.
Product referenced in this article
Bacteriophage Overview — application notes, specification table and document downloads for this range.
The infection cycle
A phage begins by recognising a receptor on the bacterial surface. That step is what determines host range, and it is why the same phage can be effective against one strain of a species and useless against another: the receptor is a specific structure, and strains differ in whether they carry it and in what form. The tail structure that makes the initial attachment is therefore the part of the genome that gets engineered when a broader range is wanted.
Once attached, the genome is injected and the host's ribosomes and enzymes are redirected to replicate it. New phage particles assemble, and the cell is lysed, releasing them. What happens next depends on the phage. In the lytic cycle that is the whole story, and the phage kills its host. In the lysogenic cycle the phage genome integrates into the bacterial chromosome and is copied along with it, so the cell survives and may not lyse at all. Temperate phages switch between the two, and the switch is not fully predictable.
Why specificity is both the strength and the problem
A phage kills rather than inhibits. That matters in medicine, where the aim is to remove a pathogen without damaging the surrounding flora, and where an antibiotic that suppresses growth leaves the organism alive to develop resistance. It also matters in any setting where the target population needs to be removed rather than held in check.
The cost is coverage. A receptor is a specific structure, and the population in a sample or a patient is a mixture of strains, not a single organism. A single phage will often cover only part of the population, which is why phage work is often done with cocktails, and why a library of phages with different receptor specificities is a practical asset. The engineering problem is to broaden range without making the phage recognise bacteria the application depends on, and that trade has not been fully resolved for general use.
What a phage library contains
A phage display library is a very large, very diverse population presented on the surface of phage particles, generated by fusing each variant gene to a coat-protein gene so that the encoded protein is displayed. The diversity typically runs to hundreds of millions to billions of independent variants, and because each variant is physically linked to the gene that encodes it, a phage that binds can be recovered and the gene sequenced from it.
That linkage is what makes the technique powerful and what distinguishes it from selection in animals or by hybridoma methods: the binding molecule and its gene are recovered together, with no cloning step in between. It is the standard route to antibody and binding-protein discovery, and it is the service we supply: presenting a target, screening a library, recovering and characterising the binders, and delivering the sequences in the format agreed for the work.
What phages are used for, and what we supply
Three application areas account for most of the work: anti-bacterial applications, where phages or engineered phages are developed against specific pathogens; resistance reversal, where a phage construct is used to select for bacteria that have regained antibiotic sensitivity; and delivery systems, where phages present antigens or other payloads. Each has a different engineering requirement, and the third is the most distant from a treatment.
What we supply is phage display and the related recombinant antibody work — library screening, binder recovery and characterisation. We are explicit that this is research-stage work and not a clinical product. A sequence recovered from a display campaign is a starting point for a project, not a treatment, and anyone presenting a display result as a therapy has misrepresented what the technique produces.
Frequently asked questions
- What is a bacteriophage?
- A virus that infects bacteria. It attaches to a specific receptor on the bacterial surface, injects its genome, uses the host's own machinery to replicate, and lyses the cell to release new particles. Some phages instead integrate their genome into the bacterial chromosome and are copied with it, which is called the lysogenic cycle, and temperate phages can switch between the two.
- Why do phages only work against some strains of a species?
- Because attachment depends on a specific receptor structure on the bacterial surface, and strains differ in whether they carry it and in what form. A phage selected against one isolate often will not attach to another. This is why phages are frequently used as cocktails, and why broadening host range by engineering the tail structure is a central research aim.
- How large is a phage display library and what does it give you?
- Typically hundreds of millions to billions of independent variants, each displayed on the phage surface and physically linked to the gene that encodes it. That linkage is the point: a phage that binds your target can be recovered and its gene sequenced directly, with no cloning step in between. It is the standard route to antibody and binding-protein discovery.
- Do you supply phage therapy or clinical phage products?
- No. We supply phage display and related recombinant antibody discovery as research-stage services: library screening, binder recovery and characterisation, with the sequences delivered in the agreed format. We do not supply clinical phage products, and a sequence recovered from a display campaign is a starting point for a project rather than a treatment.