Pharmacology – ANTIBIOTICS – CELL WALL & MEMBRANE INHIBITORS (MADE EASY)

Pharmacology – ANTIBIOTICS – CELL WALL & MEMBRANE INHIBITORS (MADE EASY)


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

54 Replies to “Pharmacology – ANTIBIOTICS – CELL WALL & MEMBRANE INHIBITORS (MADE EASY)”

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  2. Don't forget to watch Antibiotics part 2 ——-> https://www.youtube.com/watch?v=5HQmvQJWzNY

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