HELP with this question!!!!

Ok so I'm really confused about the sequence of events here. Does mutation happen after or before the antibiotic is used? If it happens before then why is completing the course of medication significant in antibiotic resistance arising? Doesn't the fact that resistant bacteria are selected for imply that they are there originally and the presence of antibiotics gives them a selective advantage as they are not killed as the mark scheme states? How can those two answers be used together then?

A) Patient does not complete course of medication so some bacteria remain. These may then mutate to develop a resistance gene, which is passed on through vertical and horizontal transmission to produce a large population of resistant bacteria.
B) In a given population of bacteria, some may have acquired genes that code for antibiotic resistance through spontaneous mutation. In the presence of antibiotics, the resistant bacteria are selected for and survive and reproduce, whilst non resistant bacteria die. The resistant bacteria then pass on the resistance gene through vertical and horizontal transmission.

I'm struggling to answer this without a clear sight of the original question(s). But going from what you have posted:

In most cases, the resistant mutation will occur/be present before treatment. When this population is treated with the full course of the antibiotic, the individual bacteria with that mutation will survive regardless of whether or not the treatment course is completed. In that case, the whole population of bacteria will be the progeny of these resistant individuals, and have the resistance mutation through vertical transmission.

However, completing the course may still be important. If the resistant mutation occurs only rarely, completing the course may reduce the bacterial population to such a low level (of mutants only) that the infection is no longer viable i.e. it is now so reduced that the body's own defences can eliminate it. But if the course is stopped early, enough non-mutant bacteria may also remain that the infection continues to be viable. Not only might the mutant bacteria continue to spread the mutation via vertical transmission, but they may also pass on the mutation to non-mutant bacteria via horizontal transmission. So overall, the mutation spreads through the population and the infection remains viable.

It is also possible that resistance could appear as a result of a mutation occurring after incomplete treatment, in a population of bacteria that did not have that mutation at the time of treatment. i.e. the incomplete treatment left a reduced but viable population surviving, in which the mutation for resistance appeared later.

Does that make sense?

Many many thanks for your answer and sorry about the late reply. I have attached the question earlier on but here it is again. Do tell me if it doesn't show up. It is a four mark question and I believe everything you said makes a lot of sense but I'm struggling to structure one complete answer according to what is said in the marks scheme since there is a mention of antibiotics acting as selection pressure as well. If you can kindly give me a little guidance on writing a logically sequenced answer to this, I will be very grateful.

Sorry I havent time to answer properly, but the abx act as a selection pressure when they are administered to the original bacterial population - tey select for those bacteria with the e th resistance mutation

So the process is something like:

- mutation occurs spontaneously
- abx added ac as selection pressure, selecting for abx resistance
- if course completed, only bacteria selected for remain. transmission of resistance only vertical and there may not be enough to maintain a viable infection. Immune system may be able to kill this small number off.
- if course incomplete, selection pressure is only partial/light so mixture of resistant plus some surviving non-resistant bacteria remain. Further resistance may spread ny both vertical and horizontal transmission

Hi again Maryam,

The answers by OxFossil are complete, precise and detailed, and they are brilliant for understanding the principles and mechanisms involved.

Just a little comment about answering Qs in A level biology exams. At A level these days (and matters were very different several years ago when we used to write 15 A4 pages in a 3 hour exam!), you are expected to only write brief key points/ideas to earn your marks [apart from in the synoptic Q]; therefore you do not need to put the 4-5 points you need for your full 4 marks in a particular "sequence", nor do you need to write prose in sentences - in fact the latter is counter-productive from the time point of view.

As I stress to my students repeatedly, "Mark, mark, mark!" - just like what matters at the end of the day in hockey (my main sport) is goals, so what matters in an exam is marks!

M