An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals are used against fungi. Antivirals: used against virus

1. Antimicrobial agents:
Dr Smitha Vijayan
Associate Professor
Mar Athanasios College For Advanced Studies Tiruvalla (MACFAST)

2. Definition: Antibacterial agents are a
group of materials that selectively
destroy bacteria by interfering
with bacterial growth or survival.
An antimicrobial is an agent that kills
microorganisms or stops their growth.
Antimicrobial medicines can be grouped according
to the microorganisms they act primarily against.
For example, antibiotics are used against
bacteria, and antifungals are used against fungi.
Antivirals: used against virus

3. “
◎ What is the difference between
antibiotics and other antimicrobial
agents?
Antibiotics specifically target bacteria and are used to
treat bacterial infections.
On the other hand, antimicrobials encompass a broader
range of products that act on microbes in general.
Ethyl alcohol, n-propanol and isopropyl alcohol
are the most commonly used antimicrobial agents

4. Antimicrobials as therapeutic agents!!!!!!
The modern era of chemotherapy
began with the work of the German
physician Paul Ehrlich (1854–
1915).
“magic bullet”
Ehrlich and a Sahachiro Hata
(1873–1938) developed a
treatment for syphilis by testing a
variety of arsenic-based chemicals
on infected rabbits.
They found that arsphenamine
was active against the syphilis
spirochete.

5. Pencillin: Alexander Fleming (1881–
1955) :1928
Ernest Duchesne: first discovered in
1896
Drawbacks of Flemings study: Fleming
could not demonstrate that penicillin
remained active in vivo long enough to
destroy pathogens and he dropped the
research

6. Howard Florey, Norman Heatley and Ernst
Chain: Professors at Oxford University
Obtained the Penicillium culture from Fleming
and set about purifying the antibiotic.
They devised the original assay, culture, and
purification techniques needed to produce
crude penicillin.
When purified penicillin was injected into mice
infected with Streptococci or staphylococci,
almost all the mice survived.
This success was reported in 1940, and
subsequent human trials were equally
successful.
Fleming, Florey, and Chain received the Nobel
Prize in 1945 for the discovery and production
of penicillin

7. Selman Waksman had found a new antibiotic, streptomycin,
produced by Streptomyces griseus.
This discovery arose from the careful screening of about
10,000 strains of soil bacteria and fungi.
The importance of streptomycin : it was the first drug to
successfully treat tuberculosis.
Nobel Prize in 1952, and his success led to a worldwide
search for other antibiotic-producing soil microorganisms.
Chloramphenicol, neomycin, oxytetracycline, and
tetracycline were isolated from other Streptomyces spp. by
1953
Golden era of antibiotics

9. Successful chemotherapeutic agent
◎ Successful chemotherapeutic agent has selective toxicity,
meaning it kills or inhibits the microbial pathogen while
damaging the host as little as possible.
◎ The degree of selective toxicity may be expressed in terms
of (1) the therapeutic dose—the drug level required for
treatment of a particular infection, and (2) the toxic dose—
the drug level at which the agent becomes too toxic for the
host.
◎ The therapeutic index is the ratio of the toxic dose to the
therapeutic dose.
◎ The larger the therapeutic index, the better the
chemotherapeutic agent in general

10. ◎ A drug that disrupts a microbial structure or function not found in
host cells often has greater selective toxicity, fewer side effects, and a
higher therapeutic index.
◎ Example: penicillin inhibits bacterial peptidoglycan synthesis but has
little effect on host cells because they lack cell walls; therefore
penicillin’s therapeutic index is high.
◎ A drug may have a low therapeutic index because it inhibits the same
process in host cells or damages the host in other ways. This can lead
to a diverse range of side effects that may involve almost any organ
system.
◎ Because side effects can be severe, chemotherapeutic agents must be
administered with great care.

11. Classification of antimicrobial drugs
◎ Narrow spectrum drugs: that is, they are effective only against
a limited variety of pathogens
◎ Broad-spectrum drugs :that attack many different kinds of
bacteria
◎ Drugs may also be classified based on the general microbial
group they act against: antibacterial, antifungal, antiprotozoan,
and antiviral.
◎ cidal or static: Static agents reversibly inhibit growth; if the
agent is removed, the microorganisms will recover and grow
again.
◎ A cidal agent kills the target pathogen, it may be static at low

12. ◎ The effectiveness of a chemotherapeutic agent against a
pathogen can be obtained from the minimal inhibitory
concentration (MIC)
◎ The MIC is the lowest concentration of a drug that prevents
growth of a particular pathogen.
◎ The minimal lethal concentration (MLC) is the lowest drug
concentration that kills the pathogen.
◎ A cidal drug generally kills pathogens at levels only two to
four times more than the MIC, whereas a static agent kills at
much higher concentrations

17. Specific test for Antimicrobial drug
1. Dilution Susceptibility Tests:
Procedure: done in both agar and broth.
Media called Mueller-Hinton broth or agar is used because all conditions must
be standardized.
In the broth dilution test, a series of tubes containing broth with antibiotic
concentrations in the range of 0.1 to 128 mg per milliliter (twofold dilutions) is
inoculated with a standard density of the test organism.
The lowest concentration of the antibiotic resulting in no growth after 16 to 20 hours
of incubation is the MIC
MLC can be ascertained if the tubes showing no growth are then cultured into fresh
medium lacking antibiotic.
The agar dilution test is very similar to the broth dilution test.
Plates containing Mueller-Hinton agar and various amounts of antibiotic are
inoculated and examined for growth

18. 1. Once the MIC is calculated, it can
be compared to known values
for a given bacterium and
antimicrobial agent and is
interpreted as susceptible,
susceptible-dose dependent
(SSD), intermediate and
resistant.
• extensive research that correlates
MIC with serum achievable levels
for each antimicrobial agent,
• particular resistance mechanisms,
and
• successful therapeutic outcomes

19. New instrument for automated antimicrobial susceptibility testing
provides rapid, accurate and
reliable detection of known
and emerging antimicrobial
resistance. It also enables
workflow efficiency by
utilizing automated
nephelometry, which results
in a standardized isolate
inoculum and a reduction in
potential technologist error.
Additionally, state-of-the-art
data management monitors,
analyzes and communicates
actionable results directly to
laboratories and clinicians.
Robotic instruments inoculate
the microdilution plates,
incubate the cultures, collect
data, and interpret the results.

20. ■ fairly simple.
■ Mueller-Hinton agar is inoculated with the bacterium, isolated from the
clinical sample
■ Small paper disks, each impregnated with a different antibiotic, are placed
on the inoculated agar.
■ When the antibiotic diffuses radially outward through the agar, it produces
a concentration gradient.
■ The antibiotic is present at high concentrations near the disk and affects
even minimally susceptible microorganisms.
■ Resistant organisms will grow close to the disk.
■ As the distance from the disk increases, the antibiotic concentration
decreases and only more susceptible pathogens are harmed.
■ A clear zone or ring forms around an antibiotic disk after incubation if the
agent inhibits bacterial growth.
■ The wider the zone surrounding a disk, the more susceptible the pathogen
is.
■ Zone width also is a function of the antibiotic’s initial concentration, its
solubility, and its diffusion rate through agar.
■ Thus zone width cannot be used to compare directly the effectiveness of
different antibiotics
Disk Diffusion Tests
Kirby-Bauer method:
William Kirby, A. W.
Bauer

22. Inhibition Zone Diameter of Selected Chemotherapeutic Drugs

23. ■ Each bacterial isolate to be
tested for antimicrobial
sensitivities is inoculated
on the surface of an agar
medium and then Etest®
strips are placed on the
surface
■ Each strip contains a
gradient of an antibiotic
and is labeled with a scale
of MIC values. After 24 to
48 hours of incubation, an
elliptical zone of
inhibition appears.
■ MICs are determined from the
point of intersection
between the inhibition zone
and the strip’s scale of MIC
The Etest®

24. ■ Most selective antibiotics are
those that interfere with bacterial
cell wall synthesis.
■ Drugs such as penicillins,
cephalosporins, and vancomycin have
a high therapeutic index because
they target structures and
functions not found in eukaryotic
cells
Inhibitors of Cell Wall Synthesis
Mode of Action: Penicillin pass through
porins of gram negative bacterial cell
wall.
The penicillin then binds to penicillin
binding protein linked the cell membrane to
be activated. Active penicillin binds to
and inactivate transpeptidase enzyme.
As a result peptidoglycans NAM and NAG
sugars are not cross-linked and the cell
wall collapse
Because cell wall formation is blocked,
osmotic lysis occurs.
Penicillins also destroy bacteria by

25. Transpeptidase enzyme
Porins
Penicillin Binding
Proteins

26. Penicillin /Cephalosporin

27. ■ ᵝ-Lactamase/penicillinase enzyme:
Bind with penicillin and inactivates
it by removing the ᵝ lactam ring
■ Modification of porins
■ Modification of PBP binding site
Resistance mechanisms developed by Bacteria towards Pencillin

28. ■ The two naturally occurring penicillins, penicillin G and
penicillin V, are narrow-spectrum drugs
■ Penicillin G is effective against many Gram-positive pathogens.
■ Administered by injection (parenterally) because sensitive to
stomach acid.
■ Penicillin V is similar to penicillin G in spectrum of activity
but can be given orally because it is more resistant to stomach
acid.
■ Almost immediately after penicillin was introduced, bacterial
resistance was discovered.
■ To fight this and broaden the spectrum of activity,
semisynthetic penicillins were developed.
■ Among the first semisynthetic penicillins were the
antistaphylococcal drugs nafcillin, oxacillin, dicloxacillin,
and methicillin.
■ These compounds have bulkier side chains than natural
penicillin, making them more difficult for β-lactamase enzymes
to degrade
Penicillins

29. ■ Aminopenicillins, which are more hydrophilic allowing
passage through porins in Gram-negative outer membranes.
■ While the antistaphylococcal penicillins retain the
narrow Gram-positive spectrum of the parent compound,
aminopenicillins like ampicillin and amoxicillin have
broader coverage that includes many Gram negatives
■ carbenicillin :it specifically targets Gram-negative
bacteria.
■ Extended-spectrum penicillins, such as piperacillin
■ penem—penicillin (penam) and cephalosporin (cephem)
hybrid—class of drugs, were synthesized to specifically
resist β-lactamase-producing Gram-negative and Gram-
positive bacteria. Unfortunately, but not surprising,
these newer drugs are not immune from bacterial
resistance

30. Carbapenems and Monobactam
Two new classes of β-lactam drugs, the
carbapenems and monobactams
There are four members of the carbapenem
family: imipenem, ertapenem, meropenem,
and doripenem
Taken orally
monobactam, aztreonam: clinical usage
Resistant bacteria: carbapenemases
Vancomycin: intravenous
Bactericidal only for Gram-positive bacteria.
Targets members of the genus Staphylococcus and
some species in the genera Bacillus (food
poisoning), Streptococcus (“strep” throat),
Enterococcus (urinary tract infections), and
Clostridium
drugs of last resort

31. Protein Synthesis Inhibitors
By binding bacterial ribosomal proteins or
rRNA.
These drugs discriminate between bacterial
and eukaryotic ribosomes, their therapeutic
index is fairly high but not as high as that of
cell wall synthesis inhibitors.
Several different steps in protein synthesis can
be affected by drugs in this category
Aminoglycosides: contain a cyclohexane
ring and amino sugars
Streptomycin, kanamycin, neomycin, and
tobramycin are synthesized by different species
Streptomyces
Gentamicin bacterium, Micromonospora
purpurea

32. Aminoglycosides: quite toxic, causing
hearing and renal damage, loss of balance,
nausea, and allergic reactions, they are
used sparingly
Used along with other antibiotics as
combination
All aminoglycoside antibiotics disrupt peptide elongation
during translation.
This occurs as aminoglycosides bind to ribosomal RNA of the
bacterial 30S ribosomal subunit, interfering with mRNA
reading and/or causing early termination of peptide
synthesis.
Aminoglycoside side effects are thought to be due to their
ability to bind to host mitochondrial ribosomes, which
share the same binding site as their bacterial ancestors

33. Aminoglycosides are not effective in
anaerobes
Streptomycin: 30 s subunit but other
aminoglycosides binds to 50s or 50s-30s
interface
Tetracyclines

34. Macrolide antibiotics
 Inhibit protein synthesis
 Bind to 50 s subunit of proteins
 Example Erythromycin, Azithromycin, Ketolides
 inhibit bacterial protein elongation.
 Chloramphenicol
 Lincosamides
 Oxazolidinones: synthetic drugs
 bind to a site on the 23S rRNA within the 50S
ribosomal subunit before protein synthesis has
begun and prevent the assembly of the 70S
initiation complex

35. Metabolic Antagonists
Antimetabolites: they antagonize, or block, the functioning of
metabolic pathways.
structurally similar to substrates of key enzymes
compete with the metabolites for the binding site of these enzymes
they are broad spectrum but bacteriostatic
their removal reestablishes the metabolic activity
Sulfonamides or Sulfa Drugs
structural analogues of p-aminobenzoic acid, or
PABAPABA
for folic acid (folate) synthesis.
Folic acid is required for the synthesis of
purines.
Cause depletion of folate in the cell,
sulfonamides prevent synthesis of DNA, RNA,
proteins, and
other important cell constituents (e.g., ATP).
When a sulfa drug enters a bacterial cell, it
competes with PABA for the active site of an
nzyme (dihydropteroate synthase) involved in one

36. Trimethoprim
Trimethoprim binds to dihydrofolate reductase (DHFR)
enzyme responsible for converting dihydrofolic acid to
tetrahydrofolic acid,
competing with the dihydrofolic acid substrate
Usually used in combination with sulfa drugs to increase efficacy
of treatment by blocking two key steps in the folic acid pathway.
The inhibition of two successive steps in a single biochemical
pathway means that less of each drug is needed in combination than
when used alone.
This is termed a synergistic interaction
In the synergistic response, the applied antibiotics work together
to produce an effect more potent than if each antibiotic were
applied singly.

37. Nucleic Acid Synthesis Inhibition
Commonly antibacterial drugs that inhibit nucleic acid synthesis function
inhibits
(1)DNA polymerase and topoisomerases (fluoroquinolones)
(2)RNA polymerase (rifamycins).
Neither are as selectively toxic as other antibiotics
Bacteria and eukaryotes do not differ greatly with respect to nucleic
acid synthesis.
The most commonly used drugs in this category are the fluoroquinolones.
Fluoroquinolones
Synthetic drugs that feature a fluorinated four quinolone ring
They act by binding to the bacterial topoisomerases
DNA gyrase and topoisomerase II.
DNA gyrase : negative twist in DNA and helps separate its strands.
Decrease tortional strain
Inhibition of DNA gyrase disrupts replication and repair
Only used in case of severe infections
Cause serious side effects: disabling damage to tendons and nerves

38. “
◎ Rifamycins
The most commonly used member of the rifamycin :
semisynthetic derivative rifampin.
◎ They block bacterial transcription by binding to the β-subunit
the β-subunit of RNA polymerase.
◎ Rifampin is an important member of the multidrug regimen
used
to treat tuberculosis and other mycobacterial infections.
infections.
◎ It can also be used to protect those who have come into
come into contact with an
individual diagnosed with meningitis caused by Neisseria
Neisseria meningitidis or Haemophilus influenzae.