Antibiotics are chemical substances that kill or inhibit the growth of microorganisms. They can be classified based on their source (natural, semisynthetic, synthetic), spectrum of activity (broad or narrow), or mechanism of action. Common mechanisms include inhibition of cell wall synthesis, protein synthesis, nucleic acid synthesis, and cell membrane function. Examples provided include penicillins, cephalosporins, carbapenems, glycopeptides, aminoglycosides, macrolides, quinolones, sulfonamides, and metronidazole.

1. Dr Karthik
First year post graduate
Department of microbiology
Chengalpattu medical college

2.  The noun “antibiotic” was first used in 1942 by Dr. Selman A Waksman, soil
microbiologist. Dr. Waksman and his colleagues discovered several
actinomycetes derived antibiotics

3. What is an Antibiotic?
Antibiotic is a chemical substance produced by
a microorganism that inhibits the growth of or
kills other microorganisms.
Antimicrobial agent is a chemical substance
derived from a biological source or produced
by chemical synthesis that kills or inhibits the
growth of microorganisms

4. Sources of Antibacterial Agents
 Natural - mainly fungal source
 Semi-synthetic - chemically-altered natural compound
 Synthetic - chemically designed in the lab
 The original antibiotics were derived from fungal sources. These can
be referred to as “natural” antibiotics
 Organisms develop resistance faster to the natural antimicrobials
because they have been pre-exposed to these compounds in
nature. Natural antibiotics are often more toxic than synthetic antibiotics.
 • Benzylpenicillin and Gentamicin are natural antibiotics

5.  Semi-synthetic drugs were developed to decrease toxicity and increase
effectiveness
 Ampicillin and Amikacin are semi-synthetic antibiotics
 Synthetic drugs have an advantage that the bacteria are not exposed to
the compounds until they are released. They are also designed to have
even greater effectiveness and less toxicity.
 Moxifloxacin and Norfloxacin are synthetic antibiotics
 There is an inverse relationship between toxicity and effectiveness as you
move from natural to synthetic antibiotics

6. 
Natural
Semisynthetic
Toxicity Synthetic
Effectiveness

7. CLASSIFICATION
 Antibiotics are classified several ways
On the basis of mechanism of action
On the basis of spectrum of activity
On the basis of mode of action

8. On basis of mechanism of action
I . Cell wall synthesis inhibitor
ii . Protein synthesis inhibitor
iii. DNA synthesis inhibitor
iv. RNA synthesis inhibitor
v. Folic acid inhibitor
vi. Mycolic acid synthesis inhibitor

10. On the basis of spectrum of activity
 Broad spectrum Antibiotics
 The term broad-spectrum antibiotic refers to an antibiotic that acts
against a wide range of disease-causing bacteria
 Tetracycline
 Chloramphenicol
 Amoxicillin
 Cephalosporin
 Erythromycin

12.  Narrow spectrum Antibiotics
The term narrow-spectrum antibiotic refers to an antibiotic that acts
against a narrow range of disease-causing bacteria
 Penicillin-G
 Cloxacillin
 Vancomycin
 Bacitracin

14. On the basis of mode of action
 Bacteriostatic
A bacteriostatic agent is a biological or chemical agent that
stops bacteria from reproducing, while not necessarily killing them
 Bactericidal
A bactericidal agent is a biological or chemical agent
that kills the bacteria

17. Inhibitors of Cell Wall Synthesis
 Beta-lactams:
Penicillins
Cephalosporins
Monobactams [Aztreonam]
Carbapenems [Imipenam, Meropenam, Ertapenam]
 Glycopeptides [ Vancomycin]
 Lipopeptides [Daptomycin]
 Polypeptides [Bacitracin, Polymyxin]

18. The Penicillins
 1928 - Alexander Fleming
 Bread mold (Penicillin notatum) growing on petri dish
 1939 - Florey, Chain, and Associates Began work on isolating and synthesizing
large amounts of Penicillin.

19. •The beta-lactam nucleus itself is
the chief structural
requirement for biological activity;
• Metabolic transformation or
chemical alteration of this
portion of the molecule causes loss
of all significant
antibacterial activity

21. Mechanism of Actions of Beta
lactams
 All penicillin derivatives produce their bacteriocidal effects by inhibition
of bacterial cell wall synthesis.
 Specifically, the cross linking of peptides on the mucosaccharide chains
is prevented. If cell walls are improperly made cell walls allow water to
flow into the cell causing it to burst.

22. Bacteria Cell Wall Synthesis
 The cell walls of bacteria are essential for their normal growth and
development.
 The peptidoglycan
Polysaccharide (repeating disaccharides of Nacetylglucosamine and N-
acetylmuramic acid) + cross-linked pentapeptide
Pentapeptide with terminal D-alanyl-D-alanine unit - required for cross-
linking

23.  Peptide cross-link formed between the free amine of the amino acid in the 3rd
position of the peptide & the D-alanine in the 4th position of another chain
 In gram-positive microorganisms, the cell wall is 50 to 100 molecules thick,
but it is only 1 or 2 molecules thick in gram-negative bacteria

24. The PBPs and Binding of Penicillins
 Related targets of penicillins and cephalosporins collectively termed penicillinbinding
proteins (PBPs)
 PBPs functions are diverse:
1. Catalyze the transpeptidase[cross-linking] reaction,
2 . Maintain shape, forms septums during division,
3 . Inhibit autolytic enzymes.

25.  Binding to PBPs results in:
 Inhibition of transpeptidase: transpeptidase catalyzes the cross-
linking
of the pentaglycine bridge with the fourth residue (D-Ala) of
the pentapeptide. The fifth reside (also D-Ala) is released during this
reaction. Spheroblasts are formed.
 Structural irregularities: binding to PBPs may result in abnormal
elongation, abnormal shape, cell wall defects

26. Lysis of bacterial cell
Isotonic environment - cell swelling --rupture of
bacterial cell
Hypertonic environment – microbes change to
protoplasts (gram +) or spheroplasts (gram -) covered
by cell membrane – swell and rupture if placed in
isotonic environment

27. Comparison of the structure and composition of
gram-positive and gram-negative cell walls

29. Cephalosporins
 1st generation: cephalexin/cefazolin (mostly GP, some GN)
 2nd generation: cefuroxime(some GP and some GN, *anaerobes)
 3rd generation: cefixime/cefotaxime, ceftriaxone (good Streptococcal
coverage, mostly GN) and ceftazidime (no GP, mostly GN, Pseudomonas)
 4th generation: --/cefepime (most GP, most GN, Pseudomonas)

30. CEPHALOSPORINS
 Similar structure and mechanism of action as penicillin
 Most are products of molds of the genus Cephalosporium

32. Carbapenems
 (broad coverage: GP, GN and anaerobes)
 Imipenem (+ Pseudomonas)
 Meropenem (+ Pseudomonas)
 Ertapenem
 Structurally different from penicillin and cephalosporin with widest spectrum
of activity of the b-lactam drugs
 Bactericidal vs. many gram (+), gram (-) and anaerobic bacteria
 Not inactivated by b-lactamases

34. Glycopeptides
 Include two compounds with similar structures;
 Vancomycin and Teicoplanin
 Both are of high molecular weight (1500-2000 daltons)
 Glycopeptides have a complex chemical structure
 Inhibit cell wall synthesis at a site different than the beta-lactams
 All are bactericidal
 All used for Gram-positive infections. (No Gram negative activity)

35. VANCOMYCIN
 Source: Streptomyces orientalis
 In Gram-Positives: The drugs enter without any problem because
peptidoglycan does not act as a barrier for the diffusion of these
molecules.
 In Gram-Negatives: Glycopeptides are of high molecular weight
(1500-2000 daltons), stopping them from passing through the porins of
gram-negative bacteria (i.e., glycopeptides have no activity against
Gram-negatives)

37. Lipopeptides
 Daptomycin
 Naturally occuring cyclic lipopeptide- Streptomyces roseosporus
 Binds irreversiblly to cytoplasmic membrane- membrane depolarisation –
disruption of ionic gradient- leads to cell death
 Active against Gram positive oraganisms
 Gram negative organisms are resistant

38. CYCLOSERINE
 Inhibit 2 enzymes –
 D-alanine-D-alanine synthetase and
 Alanine racemase catalyze cell wall synthesis
 Inhibit 1st stage of peptidoglycan synthesis
 Structural analogue of D-alanine –inhibit synthesis of D-alanyl-D-alanine
dipeptide
 second line drug in the treatment of TB

39. Other Cell Wall Inhibitors
 ISONIAZID & ETHIONAMIDE
 Isonicotinic acid hydrazine (INH)
 Inhibit mycolic acid synthesis-unknown reason[may be
elongation of fatty acids & hydroxy lipids are disrupted]
 ETHAMBUTOL
 Interferes with synthesis of arabinogalactan in the cell wall

40. Other Cell Wall Inhibitors
 BACITRACIN
 Source: Bacillus licheniformis
 Prevent dephosphorylation of the phospholipid that carries the peptidoglycan
subunit across the membrane –block regeneration of the lipid carrier & inhibit
cell wall synthesis
 Too toxic for systemic use -treatment of superficial skin infections

41. Inhibition of cell membrane function
 POLYMYXINS
 Source: Bacillus polymyxa
 With positively charged free amino group - act like a cationic detergent -
interact with lipopolysaccharides & phospholipid in outer membrane -
increased cell permeability
 Activity: gram negative rods, especially Pseudomonas aeruginosa

42. Inhibition of cell membrane function
 POLYENES (Anti-fungal)
 Require binding to a sterol (ergosterol) -change permeability of fungal cell
membrane
 AMPHOTERICIN B
 Preferentially binds to ergosterol
 With series of 7 unsaturated double bonds in macrolide ring structure
 Activity: disseminated mycoses

43. Inhibition of cell membrane function
 NYSTATIN
 Structural analogue of amphotericin B
 AZOLES (Anti-fungal)
 Block cyt P450-dependent demethylation of lanosterol - inhibit ergosterol
synthesis
 Ketoconazole, Fluconazole, Itraconazole, Miconazole, Clotrimazole

44. Inhibition of protein synthesis
 Binds the ribosomes - result in:
 1. Failure to initiate protein synthesis
 2. No elongation of protein
 3. Misreading of tRNA-deformed protein

45. Overview of Protein Synthesis

46. Overview of Protein Synthesis

48. Drugs that act on the 30S subunit
 AMINOGLYCOSIDES (Streptomycin)
 Mechanism of bacterial killing involves the following steps:
 1. Attachment to a specific receptor protein (e.g. P 12 for
Streptomycin)
 2. Blockage of activity of initiation complex of peptide formation
(mRNA + formylmethionine + tRNA)
 3. Misreading of mRNA on recognition region --wrong amino acid
inserted into the peptide

49. Drugs that act on the 30S subunit
 TETRACYCLINES
 Source: Streptomyces rimosus
 Bacteriostatic vs. gram (+) and gram (-) bacteria, mycoplasmas,
Chlamydiae & Rickettsiae
 Block the aminoacyl transfer RNA from entering the acceptor
site -prevent introduction of new amino acid to nascent peptide
chain

51. Drugs that act on the 30S subunit
OXAZOLIDINONES (LINEZOLID)
Interfere with formation of initiation complex --block
initiation of protein synthesis
Activity: Vancomycin-resistant Enterococci, Methicillin-
resistant S. aureus (MRSA) & S. epidermidis and
Penicillin-resistant Pneumococci

52. Drugs that act on the 50S subunit
 CHLORAMPHENICOL
 Inhibit peptidyltransferase [Chain elongation] –prevent
synthesis of new peptide bonds
 Mainly bacteriostatic; DOC for treatment of typhoid fever

53. Drugs that act on the 50S subunit
 MACROLIDES (Erythromycin, Azithromycin & Clarithromycin)
 Erythromycin derived from Streptomyces erythreus
 Binding site: 23S rRNA of 50S subunit
 Mechanism:
1. Interfere with formation of initiation complexes for peptide chain
synthesis
2. Interfere with aminoacyl translocation reactions-- prevent release of
uncharged tRNA from donor site after peptide bond is formed – chain
elongation prevented

54. Drugs that act on the 50S subunit
 LINCOSAMIDES (Clindamycin)
 Source: Streptomyces lincolnensis
 resembles macrolides in binding site, antibacterial activity and
mode of action
 Bacteriostatic vs. anaerobes, gram + bacteria (C. perfringens)
and gram – bacteria (Bacteroides fragilis)

56. Drugs that act on both the 30S and 50S
subunit
 GENTAMICIN, TOBRAMYCIN, NETILMICIN
 Treatment of systemic infections by susceptible gram (-) bacteria including
Enterobacteriaceae & Pseudomonas
 AMIKACIN
 Treatment of infection by gram (-) bacteria resistant to other aminoglycosides
 KANAMYCIN
 Broad activity vs. gram (-) bacteria except Pseudomonas

58. Inhibition of nucleic acid synthesis
 Inhibition of precursor synthesis
Inhibit synthesis of essential metabolites for synthesis of nucleic acid

60. Antimetabolites
 Folate Pathway Inhibitors:
Sulfonamides, Trimethoprim/Sulfamethoxazole
 The drug resembles a microbial substrate and competes with that --substrate for
the limited microbial enzyme

62. SULFONAMIDES
Structure analogue of PABA (precursor of tetrahydrofolate) --inhibit
tetrahydrofolate --methyl donor in synthesis of A, G and T
 Bacteriostatic vs. bacterial diseases (UTI, otitis media to S. pneumoniae or H.
influenzae, Shigellosis, etc.)
 DOC for Toxoplasmosis & Pneumocystis pneumonia

64. Inhibition of nucleic acid synthesis
 TRIMETHOPRIM
 Inhibit dihydrofolate reductase (reduce dihydrofolic to tetrahydrofolic acid) --
inhibit purine synthesis
 TRIMETHOPRIM + SULFAMETHOXAZOLE
 Produce sequential blocking - marked synergism of activity
 Bacterial mutants resistant to one drug will be inhibited by the other

65. Quinolones inhibit DNA synthesis
 Bactericidal; not recommended for children & pregnant women since damages
growing cartilage
 Fluoroquinolones -Ciprofloxacin, Norfloxacin, Ofloxacin, etc.
 All topoisomerases ( which are involved in DNA replication, transcription and
recombination) can relax DNA but only gyrase which carry out DNA
supercoiling.
 The main quinolone target is the DNA gyrase which is responsible for cutting
one of the chromosomal DNA strands at the beginning of the supercoiling
process. The nick is only introduced temporarily and later the two ends are
joined back together (i.e., repaired).
 The quinolone molecule forms a stable complex with DNA gyrase thereby
inhibiting its activity and preventing the repair of DNA cuts

66. Inhibition of DNA synthesis
 METRONIDAZOLE
 Anti-protozoal , anaerobes incl. C. difficile; Trichomonas, Entamoeba
 MOA
 Step 1- Entry into cell by diffusion across cell membrane
 Srep 2- Reduction of its nitro group by bacterial nitroreductase- concentration
gradient formed- influx of more drug- production of cytotoxic compounds
 Step 3- Breakage & destabilization of host DNA by cytotoxic compounds

68. Inhibit RNA synthesis
 RIFAMPICIN
 Semisynthetic derivative of rifamycin B (produced by Streptomyces
mediterranei)
 Binds to DNA-dependent RNA polymerase- block initiation of bacterial RNA
synthesis
 Bactericidal vs. M. tuberculosis and aerobic gram (+) cocci

69. Anti-mycobacterials

70. Sulfones

71. Antibiotics for Selected Bacteria

73. DRUG RESISTANCE
 Resistance
If the concentration of drug requires to inhibit or kill the microorganism is
greater than the normal use then the microorganism is considered to be
resistant to that drug
 Cross-resistance
Cross-resistance to a particular antibiotic that often results in resistance to
other antibiotic, usually from a similar chemical class, to which the bacteria
may no have been exposed.
For-example …Clindamycin and lincomycin

74. ACQUISITION OF BACTERIAL RESISTANCE
INTRINSIC RESISTANCE
 Stable genetic property encoded in the chromosome and shared by all strains of
the species
 Usually related to structural features (e.g. permeability of the cell wall) -e.g.
Pseudomonas cell wall limits penetration of antibiotics

75. INTRINSIC RESISTANCE – EXAMPLES
1. Mutation affecting specific binding protein of the 30S subunit -
Streptomycin-resistant M. tuberculosis & S. faecalis
2. Mutation in porin proteins – impaired antibiotic transport into the
cell - lead to multiple resistance - P. aeruginosa
3. Mutation in PBPs - Strep pneumoniae
4. Altered DNA gyrase - quinolone-resistant E. coli

76. ACQUIRED RESISTANCE
 Species develop ability to resist an antimicrobial drug to which it is as a whole
naturally susceptible
 Two mechanisms:
 1. Mutational – chromosomal
 2. Genetic exchange – transformation, transduction, conjugation

77. ACQUIRED RESISTANCE – EXAMPLES
 1. Resistance (R) plasmids –
Transmitted by conjugation
 2. mecA gene –
Codes for a PBP with low affinity for b- lactam antibiotics
Methicillin-resistant S. aureus

78. ORIGIN OF DRUG RESISTANCE
 NON-GENETIC
1. Metabolically inactive organisms may be phenotypically resistant to
drugs – M. tuberculosis
2. Loss of specific target structure for a drug for several generations
3. Organism infects host at sites where antimicrobials are excluded or are
not active – aminoglycosides (e.g. Gentamicin) vs. Salmonella enteric fevers

79.  GENETIC
1. Chromosomal-
Spontaneous mutation in a locus that controls susceptibility to a given
drug - due to mutation in gene that codes for either:
a. drug target
b. transport system in the membrane that controls drug uptake

80. 2. Extrachromosomal
 Plasmid-mediated
 Occurs in many different species, esp. gram (-) rods
 Mediate resistance to multiple drugs
 Can replicate independently of bacteria chromosome - many copies
 Can be transferred not only to cells of the same species but also to other
species and genera

81. MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
 Production of enzymes that inactivate the drug
 b-lactamase
S. aureus, Enterobacteriaceae, Pseudomonas, H. influenzae
 Chloramphenicol acetyltransferase
S. aureus, Enterobacteriaceae
 Adenylating, phosphorylating or acetylating enzymes (aminoglycosides)
S. aureus, Strep, Enterobacteriaceae, Pseudomonas

82. MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
 Altered permeability to the drug - result to decreased effective
intracellular concentration
 Tetracycline, Penicillin, Polymixins, Aminoglycosides, Sulfonamide

83. MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
 Synthesis of altered structural targets for the drug
a. Streptomycin resistance – mutant protein in 30S ribosomal subunit -
delete binding site - Enterobacteriaceae
b. Erythromycin resistance – altered receptor on 50S subunit due to
methylation of a 23S rRNA- S. aureus
 Altered metabolic pathway that bypasses the reaction inhibited by the
drug
Sulfonamide resistance – utilize preformed folic acid instead of
extracellular PABA - S. aureus, Enterobacteriaceae

84. MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
 Multi-drug resistance pump
Bacteria actively export substances including drugs in exchange for
protons. Eg.. Quinolone resistance

85. LIMITATION OF DRUG RESISTANCE
 Maintain sufficiently high levels of the drug in the tissues - inhibit original
population and first-step mutants.
 Simultaneous administration of two drugs that do not give cross-resistance -
delay emergence of mutants resistant to the drug (e.g. INH + Rifampicin)

86. Limit the use of a valuable
drug - avoid
exposure of the organism to the
drug