We all know that we should take our full course of antibiotics because if we don't, it can encourage the growth of naturally antibiotic resistant bacteria.

 

My question is: if there are indeed naturally antibiotic resistant bacteria that get selected for during antibiotic use, wouldn't they get selected for anyway even if the full course of antibiotics were taken? If they really are resistant, no quantity of that particular antibiotic would kill them would they?

"Resistant" does not mean the same as "invulnerable".

 

There's one thing to think about

Also, the antibiotic is not the only thing fighting the bacterial infection. Your immune system is fighting it as well. If you beat the bacteria infection to almost oblivion by taking your full course of antibiotics then you immune system can finish it off. If you don't take the full course then a larger population of bacteria will be left that is enriched in antibiotic resistance. This larger population will have a better chance of repopulating before the immune system can finish it off.

Well, some recent studies have actually shown that a short dose of antibiotics can increase the number of bacteria resistant to that antibiotic, regardless of confounding factors like the immune system. The stress presented by the antibiotic triggers the bacterial SOS response, which ups the mutation rate. Therefore, if you were to add an antibiotic to a petri dish with bacteria, then wash it away, there could be more antibiotic-resistant bacteria than in an untreated plate.

"Well, some recent studies have actually shown that a short dose of antibiotics can increase the number of bacteria resistant to that antibiotic, regardless of confounding factors like the immune system. The stress presented by the antibiotic triggers the bacterial SOS response, which ups the mutation rate. Therefore, if you were to add an antibiotic to a petri dish with bacteria, then wash it away, there could be more antibiotic-resistant bacteria than in an untreated plate."

 

 

Ok so, I take a short dose of antibiotics (600mg clindamycin) 1 hour before dental procedures, to be used as a prophylactic against endocarditis. Now in theory over time wouldn't the antibiotic I'm taking be rendered useless as a prophylactic, and making me more likely to contract endocarditis.

No, the prophylatic dose would not be useless, but it would, in theory, make you more likely to contract an antibiotic-resistant strain of endocarditis. To go back to the petri dish, taking a prophylactic dose is the equivalent of spreading an antibiotic across a plate with a few very small colonies, trying to kill only a few million bacteria. Even a very small amount of antibiotic would suffice, and there would likely be no remaining antibiotic-resistant bacteria. Similarly, because you are not trying to clear an infection, but only prevent one, for prophylaxis, you don't need an entire course of antibiotic. But, in stressing the bacteria, the antibiotic might increase the chances of an antibiotic-resistant strain appearing, but very slightly.

 

But if you were to take a plate containing a lawn of bacteria (trillions), spreading the same amount of antibiotic would not kill all the bacteria and there would likely be a few antibiotic-resistant bacteria that would result. This would be equivalent to trying to clear an infection with a single antibiotic dose.