Pharmacology

it is 5 33 a.m thursday april 1st here in japan and i just wrote this microbiology question because this has prevalence significance on the nbme exams for step one i'd reckon the chance is 20 to 33 ish that you would get a similar question on your usmle same concept okay so we're just going to slightly increase the chance of you augmenting your score one or two points because you decided to watch this youtube clip that got maybe eight views if i'm lucky so we have this 32 year old chick she's got uh clearly a uti e coli most likely cause organism obviously there can be exceptions where it's not e coli but we're not going to digress down that direction at the moment culturing shows resistance to ampicillin sensitivity cetraxone and we're merely asked why this is the case what's the mechanism now when we think of ampicillin resistance ampicillin amoxicillin penicillin are classic beta-lactams that bear sensitivity to beta-lactamases penicillinases okay that's something we think about initially and then we see that there's uh sensitivity to ceftriaxone ceftraxone is resistant to beta-lactamases so this is not complicated the organism is producing a penicillinase or a beta-lactamase and we look at the answer choices here correct answer is d production of cleavage enzyme okay it's like wow like it's worded slightly differently but like that i mean that's it methylation of beta-lactamaring wrong [ __ ] answer it's not what the usmle wants i just made that up i'm going to give you some other high yield value here apart from just giving you that quick answer usmle wants you to know that the other time beta-lactamase production will be the answer is when we're dealing with resistance of staph aureus not mrsa okay community staff mssa methicillin sensitive staphys when we're dealing with resistance of community staff mssa to ampicillin amoxicillin penicillin 90 plus of community staff produces beta-lactamase which is why if you have eg cellulitis non-bolus or bolus impetigo you can't give amoxicillin ampicillin or penicillin they're wrong [ __ ] answers okay they're not going to work against staph over 90 of the time so aerosyphilis group a strep streptogenesis eclipses staph aureus in rare circumstances penicillin is not a not a wrong selection as per the literature but on usmle it's the wrong [ __ ] answer okay so the point is you never give amoxicillin penicillin or ampicillin for skin because if it's staph aureus mssa there's likely resistance to it because the organism produces a betalactamase so usmle likes you to know the resistance mechanism when you go from mssa to mrsa this is where students get [ __ ] up now if you know the point already that mssa produces beta-lactamase it already makes lactamase so if it's becoming mrsa you know that the production of beta-lactamase can't be the answer it's already doing that so the answer is altered penicillin binding protein or production of novel penicillin binding protein sometimes you'll see pbb2 doesn't matter now in this in this uh in the answer choices i wrote here i have synthesis of novel penicillin binding protein that would be the answer if the question said why is staff why do we have mrsa okay how did it become resistant to uh methicillin okay and it's the production of a novel penicillin binding protein so most of the time staph aureus is going to be already sensitive to methicillin class antibiotics so methicillin dicloxacillin flucloxacillin et cetera nav silinoxillin and it's because the methicillin class of beta-lactams are beta-lactamase resistant okay so you can use those against community staff you can't use penicillin oxalate or ampicillin they're sensitive to beta-lactamase but you can use methicillin-class beta-lactams because they're heavily steric they're resistant to beta-lactamase they will work against mssa so if someone has a skin infection cellulitis non-bolus or bolus and pythagoras or aerosophists as we mentioned before you can give dicloxacillin oral flu clocks are still an iv a lot we can talk about okay but i'm just reiterating this because this is good medicine for you to know so the concise short recapitulation here for usmeli is that if you get a question where they ask about why you have resistance to ampicillin amoxicillin or penicillin but you have sensitivity to a third generation cephalosporin such as ceftriaxone or even a fourth generation cephalosporin like cephepime the answer is production of betalactamase okay a lot we can talk about uh it just depends on how much i want to go off on different tangents but i'd say one final point is i've also seen an nbme questions where they'll say why is mycoplasma why can you not use amoxicillin against that and it's because mycoplasma not mycobacterium mycoplasma does not have peptidoglycan in its cell wall okay so beta lactams rely on disrupting the crosslinking of the peptidoglycan cell wall okay by interacting with penicillin binding proteins so you can't use beta-lactams to treat mycoplasma we use classically macrolides like azithromycin okay or sometimes tetracyclines like doxy to treat microplasma as i said a lot we can [ __ ] talk about right why do we make this a 47 minute meandering clip so i'm obviously going to make more content if you liked this subscribe to my channel and i appreciate your time that's it