What are Bacteriophages?

Bacteriophage 101

Bacteriophages — often simply called phages — are viruses that infect and kill bacteria. They are the most abundant biological entities on Earth, as nearly every single bacterial strain has at least one of these bacterial viruses capable of infecting it. In this dynamic, bacteria are working as the host and phage as the bacterial pathogen. As biological entities, phages have been evolving to better infect bacteria, and bacteria have evolved ways to counter that. This host-phage dynamic has been ongoing for millions of years and has led to an immense arms race between bacterial and phage properties, and continues to shape microbial ecosystems, driving bacterial evolution, and nowadays offering great value to science and biotechnology. However, phages are so versatile and effective, that they tend to easily overcome bacterial defences and truly are the ones keeping global bacterial populations in order. 

So, what is a bacteriophage? A bacteria killer.

One important feature of bacteriophages is bacteriophage specificity. They tend to be extremely picky of their host, targeting specific bacterial strains with high precision and are unable to infect other cells, including bacteria outside their host range or e.g., human cells. Because of this accuracy, despite being viruses, bacteriophages pose no direct threats to humans – a key feature why phages are an increasingly versatile asset in biotechnology.

Bacteriophage structure

The structure of bacteriophages varies, but many share key components. A typical tailed bacteriophage virus has:

• A protein capsid head that encloses the genome (DNA or RNA).
• A tail structure that attaches to the bacterial surface.
• Additional fibers or spikes used for host recognition.

These bacteriophage parts, for example, effect the phage’s host range.

Bacteriophage life cycles

Phages can generally follow two main life cycles, known as the bacteriophage lytic cycle and the lysogenic cycle:

• In the lytic bacteriophage cycle, infection leads to rapid replication and eventual lysis of the host cell. The lytic cycle typically follows key steps: adsorption, DNA entry, DNA replication, transcription and translation, phage assembly, and host cell lysis. Bacteriophages operating in this manner are also called lytic phages, and they are often harnessed in applications that require efficient bacterial killing, such as infection treatment in humans.

• In the lysogenic bacteriophage cycle, the phage genome integrates into the bacterial chromosome, becoming a prophage. Prophages can remain dormant until triggered by phage induction, at which point they enter the lytic cycle. Bacteriophages capable of operating in this manner are called temperate bacteriophages.

Both pathways are central to bacteriophage biology and explain how phages shape bacterial populations.

Types and classification

Phages can be classified in several ways:

• By life cycle: lytic vs lysogenic phages.
• By morphology: filamentous, tailed, or icosahedral structures.
• By host range: some infect broad groups of bacteria, others are highly specific.

Scientists sometimes use bacteriophage typing or phage typing methods to distinguish between bacterial strains, based on which types of bacteriophage can infect them.

Why bacteriophages matter

Bacteriophages are more than just curiosities of microbiology. Their life cycles, structures, and diversity tell us much about the microbial world, and their ability to precisely target bacteria makes them invaluable tools. Whether considered as lytic phages for antibacterial use, temperate phages that contribute to bacterial evolution, or prophages hidden within genomes, phages remain central to understanding and shaping microbial ecosystems.

Perhaps most intriquingly, their natural mode of action enables highly specific infection treatment. As the threat of AMR escalates, phages offer a great alternative to conventional antibiotics against resistant pathogens. 

We at PrecisionPhage aim to develop and offer state-of-the-art solutions in order to unlock the true potential of bacteriophages in infection treatment, biosafety, and beyond.

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