in this first part of the lecture covering pharmacology of antibiotics we
are going to discuss cell wall synthesis inhibitors and cell membrane integrity
disruptors but first things first let's define antibiotics and let's classify
them so antibiotics are broadly defined as chemical agents that kill or inhibit
the growth of microorganisms antibiotics may be classified as broad-spectrum
meaning they act against wide range of microorganisms or as narrow-spectrum
meaning they act against very few types of microorganisms furthermore
antibiotics may be categorized as bactericidal meaning they kill bacteria
or as bacteriostatic meaning they only stop bacteria from growing selection of
antibiotic depends largely on clinical manifestation of the infection as well as
the patient profile and it's often guided by the culture sensitivity
results the Kirby-Bauer method is one of the most commonly performed tests that
helps to guide selection of an effective antibiotic while the dilution test is
commonly performed to determine the lowest concentration of antibiotic that
inhibits visible bacterial growth known as minimum inhibitory concentration or
MIC and the lowest concentration of antibiotic that kills at least 99.9% of
bacteria known as the minimum bactericidal concentration or MBC now let's
dive a little deeper and take a closer look at how antibiotics work so based
on their mechanism of action antibiotics can be divided into five broad
categories number one cell wall synthesis inhibitors number two cell
membrane integrity disruptors number three nucleic acid synthesis inhibitors
number four protein synthesis inhibitors and number five metabolic pathway
inhibitors now let's discuss these one by one in
detail starting with cell wall synthesis inhibitors so the cell wall is essential
for the growth and survival of bacteria it gives the bacterial cell its shape
and protects it against spontaneous cell lysis due to the high internal
osmotic pressure that results from high concentration of proteins within the
bacterial cytoplasm the vast majority of bacteria have one of two different types
of cell wall the first one is called gram-negative and it is composed of the
outer membrane linked by lipoproteins to a thin inner layer of peptidoglycan the
second one is called gram-positive it is composed of many interconnected layers
of peptidoglycan and it lacks the outer membrane now peptidoglycan is what gives
both cell wall types their rigid and protective qualities it consists of
glycan chains of alternating n-acetylglucosamine NAG for short and
n-acetylmuramic acid NAM with short peptide chain attached to it the
biosynthesis of peptidoglycan is mediated by transpeptidase enzymes also
known as penicillin-binding proteins specifically penicillin-binding proteins
catalyze the final transpeptidation reaction that results in formation of
bond between the last lysine residue of one peptidoglycan and the terminal
alanine on the other strand now in order for bacteria to grow and divide a new
cell wall must be continuously built this way so this is where inhibitors of cell wall
synthesis come into play most of antibiotics belonging to this
group are characterized by beta-lactam ring at the core of their structure
which resembles substrates for penicillin-binding protein when
penicillin-binding protein binds to beta-lactam ring portion of the drug covalent bond
is formed resulting in permanently blocked active site this makes the
enzyme unable to perform their role in cell wall synthesis which in turn leads
to death of bacteria due to osmotic instability or autolysis now the
beta-lactam ring is part of the structure of several antibiotic families
namely penicillins cephalosporins carbapenems and monobactams
collectively we call them beta-lactam antibiotics
here are some examples of beta lactams due to the large number of antibiotics
that belonged to this family keep in mind that this diagram is not meant to
be exhaustive so all looks good up to this point we have all these powerful
antibiotics that can easily kill all harmful bacteria right well not so fast
unfortunately over the years exposure to antibiotics provided bacteria with
selective pressures which led to emergence of different resistance
mechanisms the most common mechanism for drug resistance to beta-lactam
antibiotics is bacterial synthesis of beta-lactamases beta-lactamases are
enzymes produced by certain types of bacteria that simply break the
beta-lactam ring and thus destroy antibacterial activity in an effort to fight
back against this resistance scientists were able to develop beta
lactamase inhibitors that irreversibly bind to and inhibit beta-lactamase enzymes the
use of beta-lactamase inhibitors in combination with beta-lactam antibiotics
is currently the most successful strategy that we have to combat
this specific mechanism of resistance however I would like to point out two
exceptions here which are carbapenems and monobactams unlike penicillins and
cephalosporins they don't need to be combined with beta-lactamase inhibitors
because they have modified beta-lactam rings in their structures that provide
them with significant resistance to beta-lactamases examples of beta-lactamase
inhibitors are avibactam clavulanic acid sulbactam and tazobactam now
when it comes to side-effects all beta-lactam antibiotics are likely to
cause nausea vomiting and diarrhea in addition to that small number of
patients may experience allergic reactions ranging from mild rashes to
life-threatening anaphylaxis now beta-lactams are not the only antibiotics
that interfere with synthesis of the bacterial cell wall four other
antibiotics that you may frequently encounter namely fosfomycin
cycloserine vancomycin and bacitracin also disrupt cell wall
synthesis however they accomplish that through a different mechanisms in order
to gain better understanding of how these antibiotics work first we need to
take a closer look at the enzymatic steps involved in cell wall synthesis so
the first major step of the cell wall synthesis involves the cytoplasmic
enzyme enolpyruvate transferase abbreviated as MurA MurA catalyzes
the addition of phosphoenolpyruvate abbreviated PEP to UDP-n-acetyl-glucosamine
to form UDP-n-acetyl-muramic acid to which then three amino
acids are sequentially added the next crucial step involves two enzymes first
d-alanine racemase that converts l-alanine into d-alanine and the second
d-alanine:d-alanine ligase that joins to d-alanine molecules which are then
incorporated into the growing peptidoglycan precursor next with the
help of translocase enzyme peptidoglycan precursor is transferred to the lipid
carrier called undecaprenyl-pyrophosphate also
known as bactoprenol this is followed by sequential addition of n-acetylglucosamine
along with five amino acid molecules now once this cell wall
building block is transported across the inner membrane penicillin-binding
proteins catalyze the final step of polymerization of n-acetylmuramic acid
and n-acetylglucosamine complexes via transglycosylation and cross-linking of chains
via transpeptidation also at the very end the bactoprenol lipid carrier
gets dephosphorylated which enables it to perform another round of transfer so
now let's go back to our four antibiotics and let's examine how they
disrupt this cell wall synthesis machinery starting with fosfomycin so
fosfomycin acts in the first cytoplasmic step of the cell wall synthesis by
irreversibly inhibiting MurA enzyme this in turn prevents the formation of
peptidoglycan precursor and eventually leads to bacterial cell death
our second antibiotic cycloserine comes into play at the next crucial step of
the synthesis because of its chemical resemblance to d-alanine cycloserine
competitively inhibits both d-alanine racemase and d-alanine:d-alanine ligase
when both of these enzymes are inhibited then d-alanine residues cannot be formed and
previously formed d-alanine molecules cannot be joined together again the
formation of peptidoglycan precursor is disrupted which eventually leads to
death of bacteria now let's move on to our third antibiotic that is vancomycin
vancomycin belongs to a small family of antibiotics called glycopeptides that
includes few other drugs that work in similar way
so unlike fosfomycin and cycloserine vancomycin works in the late stages of
cell wall synthesis specifically vancomycin interferes with both
transpeptidation and transglycosylation during peptidoglycan
assembly by binding to two d-alanine residues at the top of the peptide chains this
binding in turn prevents linking of long polymers of n-acetylmuramic acid and
n-acetylglucosamine that form the peptidoglycan backbone and it prevents
cross-linking between amino acid residues in the peptidoglycan chain
again this brings cell construction to a halt which ultimately results in
bacterial cell death now let's move on to our last antibiotic that is
bacitracin which comes into play at the very end of the synthetic process
specifically bacitracin works by binding to bactoprenol after it inserts the
peptidoglycan into the growing cell wall this in turn prevents the
dephosphorylation of the transport protein
thus making it unable to regenerate and perform its job in construction of the
cell wall now that we discussed mechanism of action of these antibiotics let's
briefly discuss their side-effects so the first one fosfomycin
is most likely to cause nausea dizziness headache and diarrhea the second one
cycloserine has been associated with neurologic and psychiatric
disturbances such as peripheral neuropathy depression and psychosis
the third one vancomycin when administered intravenously may cause
hypotension along with flushing of the upper body condition known as redman
syndrome in rare instances vancomycin may also cause nephrotoxicity
ototoxicity and blood disorders including neutropenia
lastly bacitracin when used topically rarely causes side-effects other than
minor skin irritation however when administered intravenously
nausea vomiting allergic reactions and nephrotoxicity may occur so the
antibiotics that we discussed thus far typically are capable of disrupting
cell wall synthesis in species of bacteria that are gram-positive or gram-negative or
both however there is one more type of bacterial cell that presents a
significant challenge to our antibiotic arsenal that is mycobacterial cell wall
mycobacteria are highly pathogenic organisms that are
responsible for deadly diseases such as tuberculosis and leprosy furthermore
mycobacteria are notorious for their ability to resist most antibiotics one
of the main reasons for their toughness is their exceptionally impermeable
cell wall the mycobacterial cell wall is made up of five major components
linked together the inner plasma membrane thin layer of peptidoglycan and
arabinogalactan surrounded by a thick layer of mycolic acid
with a lipid containing outer membrane now because this cell wall is
essential to the survival of mycobacteria couple of antibiotics has been
developed to disrupt its integrity the two agents that are thought to primarily
target mycobacterial cell wall synthesis are well-known
antituberculosis drugs isoniazid and ethambutol now let's
briefly discuss their primary mechanism of action so the first one isoniazid
is a prodrug which upon gaining entry into the cell it must be first
activated by bacterial catalase-peroxidase enzyme called KatG once
activated in the presence of NADH isoniazid forms adduct which then binds to
and thereby inhibits the enoyl-acyl carrier protein reductase
abbreviated as InhA now InhA is a member of the type 2 fatty acid system
which elongates long-chain fatty acids for the synthesis
of mycolic acid inhibition of mycolic acid synthesis in turn leads
to a loss of cell structural integrity and physiologic function that ultimately
results in bacterial cell death now let's talk about our second drug that is
ethambutol so the primary mode of action of ethambutol appears to be the
inhibition of membrane associate enzyme called arabinosyl transferase EmbB
this is the enzyme that mediates polymerization of arabinose into
arabinogalactan an essential component of the mycobacterial cell wall so as a
result of this enzyme inhibition the cell wall permeability increases
allowing toxic substances to enter the cell now when it comes to major side
effects isoniazid may cause hepatotoxicity and peripheral neuropathy
while ethambutol optic neuritis that can lead to vision loss now before
we end this lecture i would like to briefly discuss one more category of
antibiotics that is cell membrane integrity disruptors
unlike cell wall synthesis inhibitors cell membrane integrity disruptors
target primarily the cell membrane of bacteria prominent members of this
category include daptomycin and small family of antibiotics known as polymyxins
so now let's take a closer look at their
mechanism of action the first antibiotic daptomycin works by forming a complex
with calcium that facilitates its insertion into the bacterial cell
membrane next daptomycin-calcium complexes
aggregate within the membrane to form pore-like structures that allow ions
such as potassium to leak through causing depolarization of membrane
potential and eventually cell death now in contrast to daptomycin which targets
cytoplasmic membrane of gram-positive bacteria polymyxins target the outer
membrane of gram-negative bacteria specifically polymyxins initially bind
to the lipopolysaccharides in the outer membrane
causing structural changes that increase membrane permeability this in
turn allows polymyxins to enter in and disrupt the inner cytoplasmic membrane
which then leads to leakage of the cell contents and eventually death of the
bacteria now when it comes to major side-effects daptomycin may cause skeletal muscle
toxicity while polymyxins may cause nephrotoxicity and neurotoxicity and
with that i wanted to thank you for watching i hope you enjoyed this video
and as always stay tuned for more