Saturday, 18 September 2021

How to tell the difference between bacteria and virus like covid-19

DURING March last year, the graph depicting UK Google searches of the phrase “what is a virus” showed a spectacular spike.

This, of course, coincided with the early stages of the covid-19 outbreak.

But viruses are nothing new so it is interesting that such levels of curiosity only developed with the pandemic.

An official definition of a virus might read “a sub-microscopic infectious agent that replicates only inside the cells of a living organism”.

There are lots of microscopic and sub-microscopic agents around and typically a virus is compared most with bacteria. Both are numerous and present within our environment and in our bodies. Both have the potential to cause infectious disease.

Often the names we give to diseases give little or no indication as to whether it is a virus or bacterium that is responsible. (Don’t forget, things like fungi and protozoa can also be responsible).

The difference between viruses and bacteria though is, in microbiological terms, like the difference between chalk and cheese.

Bacteria consist of all sorts of components, including their genetic material, glycogen for energy and ribosomes, which facilitate translation and reproduction of genetic material, all bundled up within a relatively complex cell wall.

A virus, on the other hand, is simply genetic material bundled within a shell of protein.

While bacteria are living organisms in their own right; viruses are technically not living at all. Outside a body, they are just an inert bundle of proteins and chemicals.

What brings viruses closer to life than other inanimate objects is their ability to reproduce but for this they need, as their definition states, another living organism, such as bacteria or a human cell, in order to reproduce.

Hence they could be described as intracellular parasites.

Viruses are very small. This is part of the reason why they were only discovered relatively recently.

In 1892, a man called Dmitri Ivanosky noticed an infection in tobacco plants that appeared to him to be spread by something smaller than a bacterium. It was from this observation that the first virus was described later that decade.

Now we know of a plethora of viruses and, more to the point, we know there are far more out there undiscovered.

Generally speaking, they are less than 200 nanometres in diameter. The coronavirus that causes covid-19 (Sars-Cov-19) is thought to be between 80 and 160nm in diameter.

Poliovirus is around 30nm, which is roughly 10,000 times smaller than a grain of sand.

Considering that the wavelength for visible light ranges between 300 and 800nm, they are pretty tricky to see without the use of ultra-powerful electron microscopes.

In some samples of fresh water, around 10 billion viruses per millilitre have been discovered, giving an idea of just how commonplace they are.

More to the point, they are incredibly diverse. They use all sorts of different methods to infect hosts, which makes them very difficult to treat.

There are various classes of antiviral medications that can inhibit viruses to some extent but we are largely helpless against them a lot of the time. And, no, antibiotics do not work against viruses.

One might often hear talk of the human biome or microbiome. This consists of all the microorganisms living within our body at any one time.

Typically, you think of the bacteria that live harmlessly and frequently helpfully within our gut and on our skin. For each individual, there are thought to be around 38 trillion of them.

We tend to think of viruses as inherently harmful. They cause countless diseases, including everything from Ebola to measles and influenza to the common cold. Herpes viruses cause cold sores and skin rashes. The coxsackie virus causes hand, foot and mouth disease.

HIV became a global issue at the end of the last century and, of course, we know all about coronaviruses now. These are just a handful of the many ailments we suffer from hosting a virus. But while there are around 38 trillion bacteria in our bodies, it is thought that there are ten times as many virus particles. Most of these are just minding their own business.

A significant proportion of these, named bacteriophages, focus their attentions on the bacteria in our guts and use the machinery from these bacteria to reproduce.

Before the advent of antibiotics, the earlier part of the 20th century saw a lot of interest in bacteriophages as potential therapeutic agents to target bacterial infections.

Fast forward 100 years or so and they are once again being looked at as one potential solution to bacterial infections in an age of ever-increasing bacterial antibiotic resistance. Not always harmful then.

In fact, viruses integrate with humans far more than we might think. The Human Genome Project found that the sequence of genes within human DNA contains ghost fragments of many different viruses collected and passed down by our ancestors.

About eight per cent of our genome is made up of these viral echoes and it is thought that some may even have sparked evolutionary changes in our predecessors as a result. For example, about 60 million years ago, one gene found its way into the genome from a viral infection which produced a protein now thought to have sparked the evolution of the mammalian placenta.

Prior to that, eggs were the only option creatures had to separate a child from its mother’s immune system.

Recently there has been much talk of variants to the original covid-19 virus. It has been reported that the emerging Delta variant is more transmissible than previous ones. This sort of variation is par for the course for any organism, living or otherwise, that reproduces.

As a virus replicates, it will frequently develop mutations, many of which are inconsequential. Others will be detrimental to its survival and these versions will quickly disappear.

Just occasionally a mutation gives a virus an advantage, for example, greater transmissibility, and these are the ones that become more successful.

There is, of course, the theory that viruses (and indeed bacteria) become less harmful the more they evolve simply because if they kill too many victims and too quickly, there is less chance for them to transfer and pass on their genes. It is possible the coronaviruses and rhinoviruses that cause the common cold today may once have been far more devastating to early humans.

At present there are five covid-19 variants of concern and a further eight that are under investigation. The Delta variant that first emerged in India is the most prevalent currently. For that reason, we must remain cautious.

Incidentally, it is often asked how long a virus might “live” outside the body on surfaces or in the air. Knowing what we know about their status as living things, this is technically the wrong terminology. Rather than asking how long a virus might survive, we should probably be asking instead how long a virus remains infectious or viable.

Norovirus (causing vomiting and diarrhoea) can remain infectious for weeks on a surface provided that conditions are stable. More amazingly, back in 2014, French scientists discovered a virus buried deep within the Siberian permafrost that, when thawed out, still maintained its ability to infect. They think it had been there for 30,000 years.

Thankfully, it targets only amoeba but the possibility that viruses could be in some way immortal is a little concerning.

As I mentioned, viruses are just tiny bits of genetic material and the protein shells surrounding them are the keys they use to get to the replication machinery living creatures possess.

What “kills” a virus therefore is the disruption of its protein structures. Disinfectants can disrupt the chemical bonds that give viruses structure so cleaning and hand washing are really important in preventing spread and, considering the significant rise of the new covid variants, precautions which we should all be taking.

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