Genetic Variation in Bacteria

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(1) Genetic - why? All of the characteristics of bacterial organisms are a result of the blueprint for the various proteins coded in their DNA, the same DNA that all life has on Earth (except for the specific base sequence). Genetic simply means arising from DNA.

(2) Variation - bacteria have variation? Who would have thought? I mean, I always thought they were just a bunch of tiny hot dogs without the sausage in the middle, hanging around causing trouble. Well, I found out, no, they're not hot dogs. They're beautiful organisms in their own right, and if it wasn't for our inherent bias of being on the receiving end of their infection, they wouldn't be baddies. They could be heroes. In fact, some of them are heroes! Some bacteria do contribute positively to our life. The undeniable evidence for that is pickles.

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(1+2) So, the variation is genetic.

(3) Bacteria - why bacteria? Due to bacteria being a common cause of disease, as well as their fast life cycle, they are a good case study for explaining selection and resistance. This is done in the light of antibiotic resistance. Things are complicated on Earth; in the Antibiotics topic I point out that antibiotics literally murder bacteria in a very efficient way. What could go wrong?

As generations of bacteria come to life, their DNA doesn't stay completely identical. Random mutations sometimes arise. Mutations are changes in DNA which result in different characteristics. No, not bacteria with fangs, but subtle changes in, say, the shape of a certain protein which sits on the cell wall. These mutations and the bacteria don't "know" which, or if, these changes will turn out favourable or unfavourable. This depends on their environment. Some mutations may even be irrelevant or neutral.

The key point is that sometimes, some bacteria develop mutations which just so happen to give them resistance to an antibiotic. 

This resistance, being genetic, is passed on to the offspring by vertical transmission (bacteria dividing; called binary fission - which literally means splitting in two). It's called vertical because it happens from top to bottom, as multiple generations arise.


As you can see, this happens "vertically". In reality, there is no such thing as vertical. Bacteria divide any which way, clearly. But silly humans can't understand the concept of no direction... A way of looking at it is that the transmission only occurs if the given bacterium divides.

In horizontal transmission, the given bacterium doesn't divide. It exchanges genetic material from its plasmid (circular bit of DNA) by replicating the plasmid and passing it along via a tube to a different bacterium, even of another species. This process is called conjugation, and many a cheeky teachers have compared it to sex.




No worries about the F factor (it's the factor which enables the receiving bacterium to initiate conjugation).

In the above picture think of the plasmid - the black circle being copied and transferred - as a bit of DNA which contains the allele responsible for antibiotic resistance. Now the other fellow has it. Damn.

If out of a million bacteria, just 1 lone bacterium happens by chance to have an allele that confers it resistance to the antibiotic used, then that bacterium will survive. Replicate. To a million antibiotic-resistant bacteria. See the problem now?

It's not even started. Now all of the million bacteria are resistant, and can pass on the resistance to bacteria of other species by conjugation. Now... see the catastrophe? Sooner rather than later, that particular antibiotic will be rendered useless. This is a real world problem, resulting in many strains of bacteria being untreatable, for example in tuberculosis and MRSA. MRSA stands for Methicillin-Resistant Staphylococcus Aureus. Methicillin is a sort of antibiotic from the penicillin class. Staphylococcus Aureus is a species of bacteria. And they're resistant. MRSA is a big issue in hospitals.

This issue can be managed, yet irresponsible use of antibiotics makes it worse. This includes:

1. Doctors over-prescribe antibiotics, when they are not necessary, or in case of viral infections (viruses aren't killed by antibiotics).

2. Patients don't finish their full course of antibiotics. This leaves the strongest bacteria alive, and gives them a chance to breed, resulting in a population of bacteria which is more antibiotic resistant than before.

3. Antibiotics are given to animals as a preventive measure. This creates a strong selective pressure for resistant bacteria, as well as leakages (of antibiotic) into the subsequent meat which people eat.

(3) is also an ethical issue.
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