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Evolution: 10. Contemporary stories of evolution! (a) antibiotic-resistant bacteria and (b) a moth story

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Sub-index of biology notes on all aspects of EVOLUTION


(10) Contemporary stories of evolution! (a) antibiotic-resistant bacteria and (b) a moth

(10a) Antibiotic-resistant bacteria - a contemporary and worrying story of evolution!

and other examples of evolution in practice including moths!

Example 1. Antibiotic resistant bacteria

The rapid growth and genetic development of bacteria provide strong evidence for evolution.

This is a relatively new area of research and is giving scientists for information as to how evolution works at the molecular level.

Introduction - what are antibiotics?

An antibiotic is a type of antimicrobial substance active against bacteria and is the most important type of antibacterial agent for fighting bacterial infections.

Antibiotic medications are widely used in the treatment and prevention of such infections.

They may either kill or inhibit the growth of bacteria.

Antibiotics like penicillin (discovered in 1928) have proved an amazing medical development for treating infectious (sometimes fatal) bacterial diseases.

The action of antibiotics in killing certain bacteria has saved millions of lives over 90 years.

However, they have their limitations and a particular antibiotic cannot provide a permanent long term solution to particular infections - the reasons why are discussed in detail further down.

How and why can bacteria become antibiotic-resistant?

The tale of antimicrobial resistance by pathogens

Random mutations can occur in the DNA of any living organism, and bacteria are no exception.

These mutations can lead to a change in the bacteria's alleles, hence its characteristics (phenotypes), and, unfortunately, from our point of view, can become less susceptible to the destroying action of an antibiotic.

This is evolution in action!

Strains of bacteria can develop with genes-alleles that give the bacteria protection against a specific antibiotic.

The presence of these genetic variants in the genome of the organism leads to the formation of antibiotic-resistant bacteria.

The strains of bacteria that offer antibiotic resistance have an evolutionary advantage,

so are more likely to survive and multiply in the bacterial population (in the environment of the antibiotic),

at the expense of less antibiotic resistant bacteria, that will be killed/inhibited by the antibiotic,

so the gene (allele) for antibiotic resistance becomes more common in the population. This is illustrated in the diagram below.

diagram formation of antibiotic resistant bacteria carrying the antibiotic resistant gene allele multiplication bybinary fission

The ability of a bacterial strain to resist the effects of an administered antibiotic gives it an genetic advantage and these new strains are better able to survive by the process 'natural selection' in the host (your body!).

The situation is made worse because bacteria multiply rapidly, and this speed of reproduction allows the evolution of antibiotic-resistant strains of bacteria to take place more quickly.

This increases the relative population of antibiotic-resistant strains of bacteria.

See cell division of bacteria by binary fission.

This another case of 'natural selection', the antibiotic-resistant bacteria become more common in the population.

i.e. a case of 'survival of the fittest', because the development of antibiotic resistant genes/alleles in the organism, means that these evolved strains of bacteria are better able to survive in the environment of antibiotics!

This evolution is relatively rapid e.g. taking place over a few years (or less?) rather than thousands or millions of years - worrying!

Another contemporary example is the development weeds resistant to a herbicide that once killed them - the scientific explanation is exactly the same as for the development antibiotic-resistant bacteria!

These days research scientist can monitor the changes in the DNA of generations of bacteria and actually genetically plot their evolution into different strains.

What is the problem for us?

Its quite simple! If we become infected with a strain of bacteria that is resistant to the antibiotic treatment, the treatment is ineffective!

The person is not immune to the invasive bacteria AND the infection can spread more easily in the population.

Drug companies are able to develop new antibiotics BUT, unfortunately, they take a lot of time and money to develop AND 'superbugs' are evolving which are resistant to most common antibiotics - overuse is contributing to the problem.

MRSA is one of the most common superbugs that is difficult to treat and get rid of.

MRSA is often contracted by people in hospitals and can be fatal if it gets into the bloodstream.

Superbugs have to be treated with 'cocktail' of the strongest acting antibiotics.

Why is antibiotic resistance by bacteria on the increase?

Overuse and inappropriate use of antibiotics has led to a great increase in the 'evolution' of antibiotic-resistant strains of bacterial infections.

The more antibiotics are used, the greater the chance of antibiotic-resistant strains of bacteria evolving, the greater the problem becomes.

For many years antibiotics have been very successful in treating bacterial infections (NOT viruses).

The death rate from infectious bacterial diseases like pneumonia has fallen quite dramatically.

However, there are several reasons why antibiotic resistance is on the increase:

(i) The overuse of antibiotics

The more antibiotics are prescribed the bigger the problem of antibiotic resistance - you are giving more scope for different strains of rapidly reproducing bacteria to evolve.

The use of antibiotics doesn't cause resistant strains to develop, its just that by extensively using them, you unfortunately create a situation where there is an increased probability of an antibiotic-resistant strain developing, and these strains have a genetic advantage over the bacteria you are trying to treat.

(ii) The inappropriate prescribing of antibiotics

Antibiotics are often prescribed by doctors for viral conditions that antibiotics cannot treat - perhaps under pressure from patients feeling unwell and demanding a treatment? BUT, antibiotics are completely ineffective against viruses e.g. flue or common cold viruses.

(Note: Antiviral drugs are particularly costly to research and develop.)

(iii) Fully complete your prescription instructions

Despite point (i), for a genuine bacterial infection,  it is really important you complete the full course of your prescribed antibiotics.

This ensures all the bacteria are destroyed, not only curing you of the infection, BUT preventing the bacteria form mutating into another antibiotic-resistant form.

(iv) Use of antibiotics in agriculture

Farmers treat animals with antibiotics to protect them against potential bacterial infections - obviously the prevention of illness will increase the yield of milk or meat from the herd.

Again, unfortunately, this mass treatment of farm animals will lead to the development of antibiotic-resistant strains of bacteria.

These could be passed on to humans in meat and milk based products.

The overuse of antibiotics in farming has caused sufficient concern for some countries to restrict the use of antibiotics with farm animals.

(v) Can we keep up with bacterial evolution?

Drug companies are always looking for a new market and, encouraged by both government and the medical profession, to develop new antibiotics that can combat these new deadly strains of antibiotic-resistant bacteria.

However, it takes time and a lot of money to develop new antibiotic products.

Because this process is so slow, it is difficult to produce new antibiotics in time to keep up with the rate of evolution of new strains of antibiotic-resistant mutant bacteria - the stuff you are trying to treat!!!

I'm afraid this is a problem that is not going away, however clever pharmaceutical research chemists are!

 

See also Culturing microorganisms like bacteria - testing antibiotics and antiseptics  gcse biology revision notes

and Keeping healthy: defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

 

(10b) Example 2 of evolution in practice - a famous moth and its colours!

A tale of two moths!

The white peppered moth with black spots (of melanin pigment)

The white peppered moth was, and is, common throughout the UK, but not all of the time!

It is a relatively light coloured insect, which, due to its wing markings, is relatively well camouflaged against predatory birds, particularly in rural areas that are not polluted from industry.

Its colour blended well light coloured tree bark and especially lichen covered surfaces, and was less susceptible to predation by birds.

In pre-industrialised Britain, no dark coloured varities were reported until ~1848, when black peppered moths were observed in grimy polluted areas such as the large and highly industrialised city of Manchester.

The black peppered moth, in increasing numbers, were found resting on any dark surface, whatever the location and the numbers of lighter coloured peppered moths decreased - because they stood out more against the 'dark' background and were likely to caught by predatory birds - so natural selection comes into play!

Due to this predation, the decrease in the less well camouflaged light coloured peppered moth meant the gene pool on average was decreasing in the alleles causing the phenotype of the lighter colour.

The black peppered moth - with lots of the pigment melanin

The dark 'melanic' variety of this moth was rarely observed in rural areas well away from industrialised areas.

It was also noted that the lighter coloured white peppered moth was rarely observed in the darker landscape of Victorian industrial towns - not well camouflaged.

In fact, the black peppered moth became the dominant form in industrial Britain from the mid-19th century to the 1950s.

The white peppered moths stood out more against the 'darker' background of the industrial landscape were much easier prey to catch by predators - mainly birds, less so for the black variety of moth.

It has be shown by scientific research that the dark coloured peppered moth is a mutant with the advantage of blending into the industrial background and less easily seen by predators.

The black peppered moth is much better camouflaged than the white peppered moth in these areas.

Remember - the landscape did NOT change the moth colour, the genes for 'white' and 'black' are already in the gene pool - a single gene mutation is responsible for the wings moths becoming black (melanic).

What environmental change in the industrial revolution produced this natural selection situation?

In the late 18th, 19th and early 20th centuries, many surfaces in towns and cities became dirty from industrial smoke and dust, particularly soot from chimneys.

Also, air pollutants kill lichen on trees and stone walls making them darker, so black peppered moths are better camouflaged when resting on these surfaces as well as industrial buildings and blackened houses.

Lichen are poisoned by air pollutants such as sulfur dioxide, soot and heavy metal compounds.

Natural selection enables the less vulnerable black peppered moth to survive in polluted industrial areas much better than the white peppered moth.

Later in the 20th century, with the decrease in 'dirty' industrial activity and lower air pollution levels, the proportion of white peppered moths has increased considerably over the past few decades.

Less polluted dark surfaces, less black peppered moths.

This tale of 'industrial melanism' has also been observed other species like ladybirds.

This tale of the two moth colours is considered a classic case of natural selection.

Survey statistics showed the emergence and rise dominance of the black peppered moth (over the white peppered moth) and its decline coinciding with the decline of heavy polluting industry in the decades after the Second World War (from the 1950s onwards).

It is now known from molecular genetics that a single gene mutation is responsible for the wings moths becoming black (melanic).

So, its actually the tale of one mutation and two different alleles/genes!


Summary of learning objectives and key words or phrases

Be able to describe and explain the growth of antibiotic resistant strains of bacteria because of the overuse of antibiotics.

Know that antimicrobial resistance happens when pathogens like bacteria and fungi evolve and develop the ability to be unaffected by drugs designed to kill them, so the pathogen organisms are not killed and their population continue to grow.

Know that resistant pathogen infections can be very difficult, and sometimes bacterial are impossible to treat, resulting in death in extreme cases.

Know the story of the black peppered moth and white peppered in terms of mutations favouring one environment (e.g. industrial - black) and another (rural - white) and discuss this in terms of evolution of a species.


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