"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.