Welcome to Pharmacology by Lecturio. I'm Dr. Pravin Shukle
and we're going to talk today about anti-bacterial agents.
Antibacterial agents fall in several categories.
We've divided them up here into bacterial cell wall inhibitors,
bacterial protein synthesis inhibitors, agents acting against DNA and folic acid, and the anti-mycobacterial agents.
To start off with let's look at the bacterial cell wall synthesis inhibitors.
We divide these up into several classes
we have the oldest of the group which are the penicillins, cephalosporins, the penems and other miscellaneous agents.
How is it that we build the cell wall? Well, cell wall synthesis in the bacteria starts with two major proteins.
The first one is N-acetylmuramic acid, the second is N-acetylglucosamine
we'll call them NAM and NAG just to make things a little simple.
Now they get join together like cars on a train,
so there's a very long chain of these NAM and NAG particles that make up a train.
In order to make a wall, we need two trains bound together, so what we do is we take two chains,
chains of amino acids and we lash them together.
Now right now, when you look at them in the picture here, it's just a loose knot,
but what we're going to do is we're gonna use a thing called a penicillin binding protein.
Now this protein comes along cleaves off two of the end units, binds them together tightly
and keeps on doing that over and over again until you have a very long set of two trains hook together
to make a strong cell wall for the bacteria.
How do we stop the production of this wall?
We use something called a beta-lactam antibiotic.
Now this beta lactam ring binds to the penicillin binding protein and prevents it from doing its job.
What ends up happening is you have a wall that isn't being built.
The beta-lactam antibiotic binds to the penicillin binding protein
and it prevents the cross linking of NAM and NAG chains to each other.
Well, what does that do to an existing bacteria? Nothing the cell wall was already built.
What really matters though is when the cell wall, pardon me
when the cell wants to start replicating itself, so as the cell stretches into sides to divide,
the dividing cell can't build new cell wall.
What you end up with is the existing bacteria and something else called a spheroplast.
Well, what's a spheroplast? Essentially a spheroplast is a bacteria without a cell wall.
Now with these spheroplast are essentially useless they can't do what they're supposed to do
which is infect the body so bacteria that attempt to grow
and divide in the presence of penicillin end up shutting their cell walls and they stop dividing.
The remaining spheroplast autocatalyze.
They break down on their own and they die.
How is it that we get resistance to these antibiotics? There's three major ways.
First, there's beta lactamase-mediated resistance. Now what is beta-lactamase resistance?
Beta-lactamases are enzymes within the bacteria itself that actually break down that beta lactam ring.
If you think about it it's kind of like warfare.
The mechanism of most types of resistance occurs through beta lactamases,
they break down the very antibiotic that's supposed to be killing them.
This will affect many antibiotics that have a beta-lactam ring so this includes the penicillins,
the cephalosporins, some of the cephamycins and other carbapenems.
So, what is a beta-lactamase?
Well you have here a picture of a beta lactamase, it's a beautiful illustration.
You can see how complex the structure this actually is.
They're also called penicillinases or cephalosporinases
but I would just like you to use the term beta-lactamase because it really reminds us of where these drugs are acting.
Now they're produced by gram positive organisms but they also can be produced by gram negative organism.
They are usually secreted and maybe secreted in response to the presence of an antibiotic
so sometimes beta-lactamase resistance isn't really obvious until you actually exposed the organism
to an antibiotic and then all of a sudden you realize, oh, my gosh.
This particular organism is resistant.
This has become a huge problem now with cephalosporins
and of course, you all probably know already that we use cephalosporins
much more than we have been using penicillins.
Now sometimes the resistance with cephalosporins is a little bit different.
This is a new chromosomal-mediated mechanism. It's a new threat that we're starting to see.
We started to see it around 2016 and over the last several years it's become more and more prominent.
We're also seeing now beta-lactamase resistant with some of the cephamycins
so this is something that we're seeing more and more overtime,
it's going to be more and more important as practice goes on.
Now what do we do about these beta-lactamases?
Well specifically with the penicillin-based beta lactamases
we can counter it with certain types of inhibitors of these enzymes.
One of them and probably the most commonly known is clavulanic acid.
So for example, we will pair clavulanic acid with amoxicillin
so that we have a combination of medication that's relatively beta-lactamases resistant.
Sulfabactam is another one, we put it up together with ampicillin or tazobactam
we'll put together with piperacillin as combination products
and they're often sold as combination products on the market.
The other way that we develop resistance to the beta lactamase antibiotics
are penicillin binding protein mediated resistance mechanisms.
So, this is actually a lot simpler than it sounds.
Basically, we have a penicillin-binding protein that is resistant to the effects of the beta-lactam antibiotics.
Now how does that work?
Now, if you look here, we have a picture of naked DNA from the resistant bacteria.
That naked DNA actually gets incorporated into the cell.
The host DNA is now changed, okay?
When that host DNA produces a new penicillin binding protein, it's slightly different
and it's different enough that it is resistant to the beta-lactam antibiotics
but it still able to produce a cell wall so you can actually have transmission of DNA from one bacteria to another
that provides the resistance and you produce new penicillin binding proteins.
This gives you a reduced affinity to the new beta-lactam antibiotic
or to the old beta-lactam antibiotic and that's how these types of resistance is spread.
The third type of resistance is called porin-mediated resistance and it sounds exactly like it is.
So porins are basically pores, they are water filled channels.
Here's a beautiful illustration of a porin.
Now a porin is a tubular structure that is seen in the cell walls.
The antibiotics travel through porins to get inside the bacteria
but if you have a bacteria for example that adapts
and makes fewer porins you'll have less ability for the antibiotic to get in
and we see that actually in pseudomonas all the time.
Pseudomonas is very commonly recognized as one of the agents that has porin-mediated resistance.
Now I'll give you trick for the exams.
Porin-mediated pseudomonas both start with P
and this is something that got me through at least one of the questions on my exams
so try to remember porin, pseudomonas makes things a little bit easier to remember.