What if I told you that there was a virus that kills trillions every year and I suspect you have never heard of it, would that peak your interest?

In this post, I wanted to explore one of the weirdest viruses known as a Bacteriophage. As always this is very much an introductory post and is designed to provide some general information, it is a combination of some info, likely rambling and my thoughts. For a more detailed explanation, you will undoubtedly find a significant amount of information on the internet.

I promise I didn't mean to clickbait you, bacteriophages are real, they are viruses and they kill trillions a day, trillions of bacteria that is. Bacteriophages are viruses that infect Bacteria and Archaea.

What are Bacteriophages?

A bacteriophage (colloquially known as a ‘phage') is a virus that infects and replicates within bacteria and archaea, interestingly the word bacteriophage means ‘bacteria eater' as bacteriophages end up destroying the host cell.

Interestingly, Bacteriophages are one of the most common (and also diverse entities), they are found wherever bacteria exist, and it is estimated that there are more than 10^31 bacteriophages on the planet (which equates to more than every other organism on Earth).

Image Source: Dr. Victor Padilla-Sanchez, PhD – Atomic structural model of bacteriophage T4 in UCSF Chimera software using pdbs of the individual proteins.

Without going into too much technical information on a phage, they basically consist of a top end (head) which consists of the DNA. There is then the body which consists of the collar and sheath, towards the bottom of the bacteriophage is then the tube, baseplate, spikes and fibres (essentially the legs).

There are a significant number of bacteriophages, these are classified based on criteria from the International Committee on Taxonomy of Viruses (ICTV) according to morphology and nucleic acid.

How do Bacteriophages Infect?

There are multiple stages associated with infection by bacteriophages, similar to viruses this is usually broken down into steps such as ‘attachment and penetration', ‘synthesis of proteins and nucleic acid', ‘virion assembly' and finally ‘release of virions'.

Attachment and Penetration

Generally speaking, In attachment bacteriophages bind to specific receptors on the surface of the host cell, this can include things like lipopolysaccharide, proteins and even things like flagella. Because bacteriophages bind to a specific receptor it means that there is a very limited host range they can replicate within.

As part of the initial infection process, once the bacteriophage has attached to the host receptor, it may secrete a range of enzymes to degrade the outer layer of the host. they can then often use a syringe-like motion to inject their genetic material into the host cell.

Synthesis of proteins and nucleic acid.

Once the genetic information has been injected into the host cell, the ribosomes (these produce proteins) start to translate this genetic information (mRNA) into proteins. Furthermore, the genetic information is also replicated, interestingly proteins from the phage modify some of the proteins within the host cell so that it will preferentially transcribe the virus genetic information and subsequently bacteriophage proteins.

Viron assembly

In this stage, the construction of new virus particles takes place. In some cases, this is done through helper proteins (i.e., with the T4 phage). When we consider the T4 phage, the base plate is built first, and the tails are then built on them afterwards. Finally, the head capsid (which is assembled separately) is joined to the base plate and tail.

Depending on the virus, it can be fairly important to ensure that there is a balance between the lower part of the virus (base and tail) and the head capsid.

Essentially in this whole stage, the components of the bacteriophage are basically brought together and the overall bacteriophage is constructed/assembled.

Release of Virions

Once the bacteriophages are constructed the end goal is for them to spread and for this to take place they need to be released from the host cell. In the case of bacteriophages, this can occur through cell lysis (bursting the cell), through extrusion, or in a small number of cases through budding.

Where the bacteriophage bursts the cell (lysis) this often occurs through an enzyme called endolysin which breaks down the host cell's – cell wall (peptidoglycan).

In other cases, the host cell can in effect be instructed to continually secrete new bacteriophages which means that the cell wall is not destroyed and the host is not killed, often this means that the bacteriophage's genetic information is incorporated into the host genome (prophase)

How do Bacteriophages spread?

Interestingly Bacteriophages don't have any real movement capability, they rely on random encounters between the bacteriophage and the host receptor such as when in solution such as blood, irrigation, soil water and so on.

Are Bacteriophages used industrially?

Bacteriophages are used industrially and their use varies across industries, in some cases, these can be used to infect specific bacteria in food (i.e., Listeria monocytogenes) to make food safer and to reduce food spoilage bacteria. Furthermore, in some cases, bacteriophage-based products have been used for diagnostic use (such as identifying specific bacteria and methicillin resistance).

microscopic shot of a virus
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