⚡ Welcome to Catalyst University! I am Kevin Tokoph, PT, DPT.
Welcome back to microbiology lab my name is kevin tow cough please make sure to like this video and subscribe to my channel for future videos and notifications alright in the previous video we discussed a little bit about the basics of antibiotics and then we talked about the bacitracin an oppotunity interpreted the exact same way and we had two major results
We either had susceptible or resistant now we’re gonna deal with something called the kerby bauer method alright so i mentioned in the previous video that a method very similar to this can be used in medicine if you want to test to see whether or not somebody’s infection is susceptible to a particular antibiotic because it wouldn’t make sense to give somebody an
Antibiotic for which their particular infection was resistant that’s not gonna do any good and it can propagate further resistance okay you would only want to give them an antibiotic for which their infection was susceptible but you’re only dealing with really one antibiotic here and that’s not very efficient we want a very efficient method so what we’re gonna do
Is something called the curvy bower method this method involves a plate like this it has one species for example here the species is escherichia coli ecoli but notice instead of using one antibiotic we’ve got a hana nana not gonna count it but you can clearly see there’s a bunch of different antibiotics here alright and what the kerby bauer method does it involves
The same rules we have resistant and susceptible there is an added intermediate level of susceptibility that we’ll talk about but you’re able to classify each of these antibiotics whether or not the bacteria is resistant to them intermediate susceptibility or full susceptibility and what it’s all going to depend on is the diameter of the zone of inhibition but
For now it suffice us to say that the zone of inhibition is this clearing area that manifests as a circle around the disc and we’re gonna want to measure the diameter of that zone in the kirby bower method now this was a picture taken in the lab real kirby bower plade and we have first of all several antibiotics here for which we can automatically say e coli is
Resistant to which ones do you think those are well first of all bacitracin there’s absolutely no zone of clearing around bacitracin so e coli in this case is resistant to bacitracin ecole i would also be resistant to op the chin penicillin and this one called vancomycin all right again it i mean you can take a ruler right i mean you can easily tell that there’s
No zone of inhibition so therefore the diameter is zero and for any antibiotic where the diameter is zero that’s automatic resistance you don’t even even to measure it but for all of these others some of the small ones that might still count as resistant you know this big one looks like susceptible but for these ones in the middle is it intermediate susceptibility
Is it susceptibility well we have to measure it and so what we’re gonna do is we’re going to take a bunch of measurements using actual rulers in the lab and then we’re gonna look at a table now this table doesn’t actually go with this figure but you’re gonna have a table in your lab nail it’s gonna look very similar to this you’re gonna have a list of antibiotics
And then you’re gonna have values and you have to match the diameter that you measure and see where it falls for example let’s look at this ampicillin comma row okay let’s suppose i had ampicillin on a kirby bower plate and i measured the diameter of the zone of inhibition was 15 millimeters notice we’re doing this in millimeters so the diameter of the zone of
An inhibition was 15 millimeters is that resistant intermediate susceptibility or susceptible well for ampicillin 15 falls right in the middle of 14 and 16 so therefore that species would have intermediate susceptibility to ampicillin okay now if i had another one on here let’s say i had tetracycline that’s another antibiotic that’s commonly used and let’s say i
Had a tetracycline zone of inhibition that was 25 millimeters well i go to the row of tetracycline where does 25 fall well it’s not here it’s bigger than 18 okay so early in this case twenty fives bigger than nineteen so that species would have complete susceptibility to tetracycline and so that’s what you have to do in the kirby bower method after you incubated
And all that stuff and you pull it out the next day of class you’re gonna measure all of these zones of diameters write them all down and then classify them using a table very similar to this all right we’re actually gonna do an example of this using hypothetical antibiotics we’re not going to give actual names here but our antibiotics are gonna be antibiotic
A antibiotic b c and d and here are the values for which we’re gonna be determining resistant intermediate susceptibility and susceptibility the very important thing here to remember is we are using millimeters you do not use inches your ruler does have inches on it you are not using inches you’re using millimeters okay and so the question asks us to determine
Whether or not the bacteria plated in this kirby bower test are resistant our intermediate gets an i or susceptible s and so the first thing we’re gonna have to do is determine the diameter of the zone of inhibition so let’s actually do the one for a first so i’m going to take this ruler and i’m gonna put it right across this okay and i think that looks pretty
Good and i’m just gonna kind of estimate this to me that looks like a diameter of around 19 millimeters so 19 millimeters so let’s write that down first so a has a diameter of 19 millimeters right now we can then take this diameter for a and determine whether or not it has if it’s resistant intermediate susceptibility or susceptible and 19 looks like it falls
Right in the middle of 15 and 25 right here okay so this antibiotic the bacteria has intermediate susceptibility to a so i’m gonna put the eye there all right so that one is intermediate susceptibility okay now let’s do b well b doesn’t have a zone of inhibition at all okay it’s therefore zero millimeters and it’s automatically resistant okay you don’t even need
To measure this there’s nothing to measure there’s no inhibition so it’s automatically resistant all right now let’s look at si all right si has a zone of inhibition it looks pretty small but let’s go ahead and put this up here so i’m measuring it that looks to be about let’s say nine millimeters so nine millimeters all right so si nine millimeters all right so
Now that we have this we’re gonna go to the table see so nine millimeters that’s less than 14 fact it’s less than equal to 14 so this antibiotic the bacteria is going to be resistant to it so this is going to be resistant again all right and again make sure you’re looking at the right antibiotic you can make a really bad mistake if you’re looking at antibiotic c
But you’re looking at row b okay so make sure it’s the right antibiotic and last let’s do d now i’m just looking at this it’s a very large zone of inhibition it looks susceptible okay in some cases it may be but it can be a mistake that you make if you automatically look at it and say it’s pretty big i’m just gonna put susceptible make sure you measure it because
Sometimes it can be intermediate susceptibility i don’t think it’s gonna be here but we’re gonna measure it just to be safe all right so measure the diameter right here that’s probably close enough and it looks like the diameter is ill it’s over 30 it’s around 33 so the diameter of the zone of inhibition for d is 33 millimeters now in d that’s clearly over the
Maximum limit for susceptibility so it is susceptible but let’s say in one case a different case you just looked at it it looked pretty big and you said that’s acceptable but let’s say it actually was 25 24 millimeters that’s almost acceptable but it’s still in the intermediate susceptibility category so don’t just assume things you actually have to measure
Them okay so hopefully this process makes sense the first thing you do after you fb pull it out of the incubator so it has to incubate for two days obviously but you measure the diameters of the zone of inhibition right um and then you use a table like this that has your corresponding antibiotics and you determine which area which range your value or your length
Diameter whatever falls in and that allows you to classify that bacteria as being resistant having intermediate susceptibility or being susceptible to that particular antibiotic and that’s the kerby bauer method and again you can hopefully see that the the advantage of using the kerby bauer method and this is the one that’s particularly used in hospitals because
You can screen a very large amount of antibiotics for a given species at the same time okay so hopefully it makes sense to you please make sure to like this video and subscribe to my channel for future videos and notifications thank you
Transcribed from video
The Kirby-Bauer Method for Antibiotic Susceptibility (with examples) By Catalyst University
