By Dr. Rakesh Prasad Sah, Associate Professor, Microbiology

 Antimicrobials/anti-infectives:
These are umbrella terms for drugs with activity against
microorganisms.
They include antibacterials, antivirals, antifungals, and
antiparasitic agents.
Antibiotic:
Chemical Substance  m.os./chemical synthesis
kill or inhibit growth of m.os. low concn
Chloramphenicol  Streptomyces venezuelae
chemical methods.

Classification of
Antibiotics

 Broad spectrum:
For example: tetracyclines are active against many gram-
negative rods, chlamydiae, mycoplasmas, and rickettsiae.
 Narrow spectrum:
For example: vancomycin is active against certain gram-
positive cocci, namely, Staphylococci and Streptococci.
Classification of
Antibiotics

 Antibiotics have one of two effects on the growth and viability
of microorganisms:
 Bacteriostatic - slows growth or prevents multiplication; not
the cure; cure results from combined action of drug and host's
defense mechanisms, like phagocytosis.
 Bacteriocidal - usually effective only for growing microbes;
ineffective dormant cells.
Classification of
Antibiotics

Penicillins produced by mold Penicillium
Cephalosporins produced by mold Cephalosporium
Bacitracin produced by Bacillus licheniformis
Polymyxins produced by Bacillus polymyxa
Aminoglycosides produced by Streptomyces griseus
Tetracyclines produced by Streptomyces
Chloramphenicol produced by Streptomyces venezuelae
Macrolides - (Erythromycin) produced by Streptomyces erythreus
Rifamycins produced by Streptomyces mediterrani

1. Selective toxicity: against target pathogen but not against host.
 LD50 vs. MIC
 Therapeutic index: (the lowest dose toxic to the patient divided by the dose
typically used for therapy).
 High therapeutic index  less toxic
2. Bactericidal vs. Bacteriostatic. (Static rely on normal host defences to kill
or eliminate the patogen after its growth has been inhibited. (UTIs) CIDAL
given when host defenses cannot be relied on to remove or destroy pathogen.)

1. Favorable pharmacokinetics: reach target site in body with effective
concentration.
2. Spectrum of activity: broad vs. narrow.
3. Little resistance development.
4. Lack of “side effects” allergic, toxic side effects, suppress normal flora.
5. There is no perfect drug

 Can be classified as:-
1) Cell wall synthesis inhibition
2) Acting on Cytoplasmic Membrane
3) Protein synthesis inhibition
4) Nucleic acid synthesis inhibition
5) Antimetabolites

DRUGS MECHANISM OF ACTION
CellWall Synthesis
Inhibition
β-lactam antibiotics
(Penicillin,
cephalosporins)
Bind to receptors (penicillin binding proteins
present on the inner layer of cytoplasmic
membrane) and leads to interference with
the synthesis of peptidoglycan of cell wall.
Cell membrane vulnerable to damage by
solutes of the plasma.
Acting on
Cytoplasmic
Membrane
Binds to plasma membrane & disrupts its
structure and permeability properties.

DRUGS MECHANISM OF ACTION
Protein Synthesis Inhibition
A. Inhibitors of
Transcription
(Rifampicin)
Inactivates DNA-dependent RNA polymerase
thus inhibiting transcription.
B. Inhibitors of
Translation
Inhibit 30S ribosome
Inhibit 50S ribosome
Combine with 30S and 50S components of
ribosomes and lead to malfunctioning of
ribosomes. Affects initiation, elongation or
termination of peptide chain leading to
inhibition of protein synthesis and cell dies.
(Streptomycin, kanamycin, amikacin,
tobramycin, tetracycline, doxycycline)
(Erythromycin, Azithromycin, Chloramphenicol
and lincomycin.)

DRUGS MECHANISM OF ACTION
Nucelic Acid Synthesis
Inhibition (Quinolones &
Fluroquinolones)
Norfloxacin, Ciprofloxacin,
Ofloxacin Etc
Inhibit DNA gyrase and thus blocking DNA
synthesis.
Antimetabolites
Sulfonamides
Trimethoprim
Dapson
Inhibit folic acid synthesis by competing with
p-aminobenzoic acid (PABA).
Blocks folic acid synthesis by inhibiting the
enzyme tetrahydrofolate reductase .
Thought to interfere with folic acid synthesis.

Mechanisms of action of antibiotics
Interference of
cell wall
synthesis
Inhibition of
cytoplasmic
membrane
function
Inhibition of
protein
synthesis
Inhibition of
DNA function
Metabolic
antagonists
Penicillins Polymyxin Streptomycin Nalidixic acid Sulphonamide
Cephalosporins Nystatin Kanamycin Norfloxacin Dapsone
Bacitracin Amphotericin B Amikacin Ciprofloxacin PAS
Vancomycin Neomycin Novobiocin Isoniazid
Cycloserine Tobramycin Metrozidazole Trimethoprim
Doxycycline
Minocycline
Erythromycin
Chloramphenicol

 Penicillins:
 Bacteriocidal and inhibits
synthesis of cell wall.
 Toxicity to humans - has least of
any antimicrobial drug for host
cells, most serious side-effect is
allergic reactions.
 Both natural and semi-synthetic
forms.
Common β-lactam ring nucleus for
all penicillins; side groups vary.

Naturals are very narrow spectrum and susceptible to penicillinases, which cleave
the β-lactam ring rendering the drug inactive.
PenicillinV is preferred for oral administration as it is resistant to acid hydrolysis.

Semi-synthetic penicillin’s are designed to:
* increase range of action to include effectiveness against Gram-
negative bacteria (e.g. Ampicillin); or
* resistance to Penicillinases (e.g. Methicillin).

 Related to penicillins.
 Bacteriocidal and their MOA is similar to
penicillins
 Broad spectrum (active against both Gram
positive and Gram negative).
 E.g.

Cephalosporins Antibacterial spectrum
First generation
Cephalexin, Cephaloridine, Cefadroxil,
Cepharadine, Cephalothin, Cephazolin
Staph. aureus, Streptococci (other than
enterococci), E. coli, Klebsiella, Proteus mirabilis
and H. influenzae
Sencond generation
Cefamandole, Cefoxitin, Cefuroxime,
Cefonicid, Ceforanide, Cefaclor, Cefprozil,
Cefmetazole, Cefotetan etc.
First generation spectrum and Proteus,
Enterobacter, Citrobacter, Serratia and Gram
negative anaerobes.
Third generation
Cefotaxime, cefoperazone, ceftizome,
ceftazidime, Ceftriaxone, Cefixime,
Ceftibuten, Cefpodoxime etc.
Second generation spectrum and N. gorrhoeae
including beta lactamase producing strains, Ps.
aeruginosa.
Fourth generation
Cefepime, Cefpriome, Ceftaroline,
Ceftobiprole etc
Third generation spectrum including enhanced
activity against Enterobacter and Citrobacter
spp. that are resistant to third generation
cephalosporins.

Polymyxin :
 Act as cationic detergents; integrate
within & disrupt outer membrane.
 Causes loss of osmotic function &
selective membrane permeability.
 Unique in being Bacteriocidal in absence
of cell growth.
 More effective against gram negatives
than gram positives (due to more LPS in
gram negatives)
 Toxicity - damages kidneys

Aminoglycosides
Macrolides

 Inhibit Transcription
 Toxicity - occasional rashes,
platelet decrease & some
decline in liver function may
occur.
 Imparts an orange color to
urine and sweat.
 Example: Rifampicin

 MAO is by inhibiting protein synthesis of
bacteria.
 Broad-spectrum; Bactericidal.
 Side effects: nephron- and oto-toxicity
 Example:
 Streptomycin
 Gentamycin
 Amikacin
 Kanamycin

 MAO is by inhibiting protein synthesis of bacteria.
 Broad-spectrum; inhibits only rapidly multiplying bacteria
(Bacteriostatic).
 Highly effective against rickettsial and chlamydial
infections.
 Problems with tetracyclines are:
1. inhibition of normal flora leads to "superinfection“
2. weakening of bone structure (esp. true in growing
children);
3. photosensitivity in some hosts.
 Example: Tetracycline, Doxycycline, Minocycline

 MAO is by inhibiting protein
synthesis of bacteria.
 Primarily Bacteriostatic
 Less effective than penicillins,
yet good alternative in cases of
penicillin allergy.
 Problem - numbers of resistant
mutants arise with use.
 Example: Erythromycin,
Azithromycin

Quinolones:
 Binds and inhibits activity of DNA gyrase.
 Bacteriostatic & Bacteriocidal.
 Example:
 Nalidixic acid
 Norfloxacin
 Ciprofloxacin
 Ofloxacin
 Levofloxacin.

Sulfonamides:
 p-aminobenzoic acid (PABA) is the substrate in the essential synthesis of
folic acid in most bacteria.
 Sulfonamides are structural analogs of PABA; act as competitive
inhibitors.
 No effect on human cells; we cannot synthesize folic acid, but rather
obtain folic acid in diet.
 Bacteriostatic
 Example: Sulphamethoxazole
Trimethoprim:
 Inhibits conversion of dihydrofolic acid to tetrahydrofolic acid.
 Bacteriocidal

 Refers to development of resistance to an antimicrobial agent by
a microorganism.
 Two types
 Acquired
 Intrinsic

 Emergence of resistance in bacteria  ordinarily susceptible
to antimicrobial agents  by acquiring the genes coding for
resistance.
 Most of AMR  shown by bacteria  belongs to this group.
 Infection caused by resistant microorganisms often fail to
respond to the standard treatment, resulting in
 Prolonged illness
 Higher healthcare expenditures
 Greater risk of death

 Overuse and misuse of antimicrobial agents  single most
important cause of development of acquired resistance. (natural
phenomenon )
 Resistant bacterial populations flourish in areas of high
antimicrobial use  where they enjoy a selective advantage over
susceptible populations.
 Resistant strains then spread in the environment and transfer the
genes coding for resistance to other unrelated bacteria.

 Other factors favouring AMR
 Poor infection control practices in hospitals
▪ e.g. Poor hand hygiene practices  facilitate transmission of resistant
strains.
 Inadequate sanitary conditions
 Inappropriate food-handling
 Irrational use of antibiotics by doctors not following AST
report.
 Uncontrolled sale of antibiotics over the counters without
prescription.

 Refers to the innate ability of a bacterium to resist a class of
antimicrobial agents due to its inherent structural or functional
characteristics, (e.g. Gram negative bacteria  resistant to
Vancomycin).
 Is non-transferable.
 In +ce of selective antibiotic pressure, bacteria acquire new genes
by two methods:-
 Mutational Resistance
 Transferrable drug Resistance

 Resistance developed due to mutation of the
resident genes.
 E.g. Mycobacterium tuberculosis – ATT
 Usually, Low level resistance, developed to one drug
at a time-overcome by using combination of different
classes of drugs.

 is plasmid coded - transferred by
 conjugation or rarely transduction, transformation.
 Resistance coded plasmid (called R plasmid) –
 carry multiple genes,
 each coding for resistance to one class of antibiotic.
 Results in high degree of resistance to multiple drugs, which
cannot be overcome by using combination of drugs.


S.No.
Mutational drug resistance Transferable drug resistance
1 Resistance to one drug at a
time
Multiple drug resistance at the
same time
2 Low-degree resistance High-degree resistance
3 Resistance can be overcome by
combination of drugs
Cannot be overcome by drug
combinations
4 Virulence of resistance mutants
may be lowered
Virulence not decreased
5 Resistance is not transferable
to other organisms but
spread to off- springs by
vertical spread only
Resistance is transferable to
other organisms.
Spread by: Horizontal spread
(conjugation, or rarely by
transduction/transformation)

Decreased Permeability across the Cell Wall :
 Bacteria modify their cell membrane porin channels - either in
frequency, size, or selectivity Preventing the antimicrobials from
entering into the cell.
 Seen in Pseudomonas, Enterobacter and Klebsiella species against drugs,
such as imipenem, aminoglycosides and quinolones.

Efflux Pumps:
 Mediate expulsion of the drugs from the cell - thereby preventing the
intracellular accumulation of drugs.
 Escherichia coli and other Enterobacteriaceae  against tetracyclines,
chloramphenicol
 Staphylococci  against macrolides and streptogramins
 Staph aureus and Strept pneumoniae  against fluoroquinolones.

By Enzymatic Inactivation :
 β lactamase enzyme production - It breaks down the β lactam rings 
inactivating the β lactam antibiotics.
 Aminoglycoside modifying enzymes - destroy the structure of
aminoglycosides
 Chloramphenicol acetyl transferase - destroys the structure of
chloramphenicol.

By Modifying target sites:
 MRSA -Target site of penicillin i.e. penicillin binding protein (PBP)
gets altered to PBP-2a.
 Streptomycin resistance in Mycobacterium tuberculosis- due to
modification of ribosomal proteins or 16S rRNA.
 Rifampicin resistance in Mycobacterium tuberculosis- due to
mutations in RNA polymerase.