This document discusses antimicrobial agents and antibiotic resistance. It defines antimicrobial agents as chemicals that treat infectious diseases by inhibiting or killing pathogens. Ideal antimicrobial agents kill or inhibit pathogens without harming the host. The document then discusses different classes of antibiotics including their sources, mechanisms of action, and examples. It covers antibiotics that inhibit cell wall synthesis, cell membrane function, protein synthesis, and nucleic acid synthesis. The document concludes by discussing intrinsic and acquired antibiotic resistance in bacteria.

1. Maria Ellery Mendez, MD,
DPASMAP,FPAMS,FPAAAM
Department of Microbiology
Our Lady of Fatima University

2. ANTIMICROBIAL AGENT
• any chemical or drug used to treat an
infectious disease, either by inhibiting or
killing the pathogens in vivo

3. ANTIMICROBIAL AGENT
Ideal Qualities:
1. kill or inhibit the growth of pathogens
2. cause no damage to the host
3. cause no allergic reaction to the host
4. stable when stored in solid or liquid form
5. remain in specific tissues in the body long
enough to be effective
6. kill the pathogens before they mutate and
become resistant to it

4. ANTIBIOTICS
 Substances derived from a microorganism
or produced synthetically, that destroys or
limits the growth of a living organism

5. ANTIBIOTICS – Sources
1. Natural
a.Fungi – penicillin, griseofulvin
b.Bacteria – Bacillus sp. (polymixin,
bacitracin) ; Actinomycetes
(tetracycline, chloramphenicol,
streptomycin)
2. Synthetic

6. ANTIBIOTICS – Classification
I. Accdg to antimicrobial activity
1. Bactericidal
2. Bacteriostatic
I. Accdg to bacterial spectrum of activity
1. Narrow spectrum
2. Broad spectrum

7. ANTIBIOTICS – Classification
III.Accdg to absorbability from the site of
administration to attain significant
concentration for the treatment of
systemic infection
1. Locally acting
2. Systemic

8. ANTIBIOTICS – Classification
IV.Accdg to mechanism of action
1. Inhibit bacterial cell wall synthesis
2. Alter the function and permeability
of the cell membrane
3. Inhibit protein synthesis (translation
and transcription)
4. Inhibit nucleic acid synthesis

10. Inhibition of cell wall synthesis
Target: block peptidoglycan (murein) synthesis
Peptidoglycan
 Polysaccharide (repeating disaccharides of N-
acetylglucosamine and N-acetylmuramic acid)
+ cross-linked pentapeptide
 Pentapeptide with terminal D-alanyl-D-alanine
unit  required for cross-linking
 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

11. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
 inhibit transpeptidation reaction (3rd stage)
to block peptidoglycan synthesis  involves
loss of a D-alanine from the pentapeptide
 Steps:
a. binding of drug to PBPs
b. activation of autolytic enzymes (murein
hydrolases) in the cell wall
c. degradation of peptidoglycan
d. lysis of bacterial cell

12. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
Penicillin binding proteins (PBPs)
 enzymes responsible for:
a. cross-linking (transpeptidase)
b. elongation (carboxypeptidase)
c. autolysis

13. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
Lysis of bacterial cell
o Isotonic environment  cell swelling 
rupture of bacterial cell
o Hypertonic environment – microbes change
to protoplasts (gram +) or spheroplasts
(gram -) covered by cell membrane  swell
and rupture if placed in isotonic environment

14. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
o intact ring structure essential for
antibacterial activity
o inhibition of transpeptidation enzyme due
to structural similarity of drugs (penicillin
and cephalosporin) to acyl-D-alanyl-D-
alanine

15. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
PENICILLIN
 Source: Penicillium spp (molds)
 inhibit final cross-linking step
 bind to active site of the transpeptidase &
inhibit its activity
 bactericidal but kills only when bacteria
are actively growing
 inactivated by β-lactamases

16. Inhibition of cell wall synthesis
Α. β-lactam antibiotics
CEPHALOSPORINS
 similar structure and mechanism of action
as penicillin
 most are products of molds of the genus
Cephalosporium

17. Inhibition of cell wall synthesis
B. Other β-lactam antibiotics
CARBAPENEMS
 structurally different from penicillin and
cephalosporin
 Imipenem
 with widest spectrum of activity of the
β-lactam drugs
 Bactericidal vs. many gram (+), gram
(-) and anaerobic bacteria
 not inactivated by β-lactamases

18. Inhibition of cell wall synthesis
A. Other β-lactam antibiotics
MONOBACTAMS (Aztreonam)
 activity vs. gram negative rods
 useful in patients hypersensitive to
penicillin

19. Inhibition of cell wall synthesis
C. Other Cell Wall Inhibitors
 Inhibit precursor for bacterial cell wall synthesis
VANCOMYCIN
 Source: Streptomyces orientalis
 Inhibit 2nd stage of peptidoglycan synthesis
by:
a. binding directly to D-alanyl-D-alanine 
block transpeptidase binding
b. inhibiting bacterial transglycosylase
 S. aureus & S. epidermidis infection
resistant to penicillinase-resistant PEN

20. Inhibition of cell wall synthesis
C. Other Cell Wall Inhibitors
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

21. Inhibition of cell wall synthesis
C. Other Cell Wall Inhibitors
ISONIAZID & ETHIONAMIDE
 Isonicotinic acid hydrazine (INH)
 Inhibit mycolic acid synthesis
ETHAMBUTOL
 Interferes with synthesis of arabinogalactan
in the cell wall

22. Inhibition of cell wall synthesis
C. 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

23. Inhibition of cell membrane function
A. 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

24. Inhibition of cell membrane function
B. 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

25. Inhibition of cell membrane function
B. POLYENES (Anti-fungal)
NYSTATIN
 Structural analogue of amphotericin B
 Topical vs. Candida
C. AZOLES (Anti-fungal)
 Block cytP450-dependent demethylation of
lanosterol  inhibit ergosterol synthesis
 Ketoconazole, Fluconazole, Itraconazole,
Miconazole, Clotrimazole

26. 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

27. Inhibition of protein synthesis
A. Drugs that act on the 30S subunit
AMINOGLYCOSIDES (Streptomycin)
 Mechanism of bacterial killing involves the ff.
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

28. Inhibition of protein synthesis
A. 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

29. Inhibition of protein synthesis
A. 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

30. Inhibition of protein synthesis
B. Drugs that act on the 50S subunit
CHLORAMPHENICOL
 Inhibit peptidyltransferase  prevent
synthesis of new peptide bonds
 Mainly bacteriostatic; DOC for
treatment of typhoid fever

31. Inhibition of protein synthesis
B. Drugs that act on the 50S subunit
MACROLIDES (Erythromycin, Azithromycin &
Clarithromycin)
 Binding site: 23S rRNA
 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
(Erytnromycin)

32. Inhibition of protein synthesis
B. Drugs that act on the 50S subunit
LINCOSAMIDES (Clindamycin)
 Source: Streptomyces lincolnensis
 resembles macrolides in binding site, anti-
bacterial activity and mode of action
 Bacteriostatic vs. anaerobes, gram + bacteria
(C. perfringens) and gram – bacteria
(Bacteroides fragilis)

33. Inhibition of protein synthesis
C. 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

34. Inhibition of nucleic acid synthesis
A. Inhibition of precursor synthesis
 Inhibit synthesis of essential metabolites
for synthesis of nucleic acid
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 20 to S. pneumoniae or H. influenzae,
Shigellosis, etc.)
 DOC for Toxoplasmosis & Pneumocystis
pneumonia

35. Inhibition of nucleic acid synthesis
A. Inhibition of precursor 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

36. Inhibition of nucleic acid synthesis
B. Inhibition of DNA synthesis
QUINOLONES
 Inhibit α subunit of DNA gyrase  (+)
supercoiling  (-) DNA synthesis
 Bactericidal; not recommended for children &
pregnant women since damages growing
cartilage
 Fluoroquinolones (Ciprofloxacin),
Norfloxacin, Ofloxacin, etc.

37. Inhibition of nucleic acid synthesis
B. Inhibition of DNA synthesis
NOVOBIOCIN
 Inhibit β subunit of DNA gyrase
FLUCYTOSINE (Anti-fungal)
 Nucleoside analogue  inhibit thymidylate
synthetase  limit supply of thymidine

38. Inhibition of nucleic acid synthesis
B. Inhibition of DNA synthesis
METRONIDAZOLE
 Anti-protozoal; anaerobic infections
 Antimicrobial property due to reduction of its
nitro group by bacterial nitroreductase  (+)
production of cytotoxic compounds  disrupt
host DNA

39. Inhibition of nucleic acid synthesis
C. 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

41. RESISTANCE
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

42. RESISTANCE
ACQUISITION OF BACTERIAL RESISTANCE
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

43. RESISTANCE
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

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

45. RESISTANCE
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 (intracellular)

46. RESISTANCE
GENETIC
1. Chromosomal
 Occurs at a frequency of 10-12 to 10-7
 20 to 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

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

48. RESISTANCE
MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
1. Production of enzymes that inactivate the drug
α. β-lactamase
 S. aureus, Enterobacteriaceae, Pseudomonas,
H. influenzae
a. Chloramphenicol acetyltransferase
 S. aureus, Enterobacteriaceae
a. Adenylating, phosphorylating or acetylating
enzymes (aminoglycosides)
 S. aureus, Strep, Enterobacteriaceae,
Pseudomonas

49. RESISTANCE
MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
2. Altered permeability to the drug  result to
decreased effective intracellular concentration
 Tetracycline, Penicillin, Polymixins,
Aminoglycosides, Sulfonamides

50. RESISTANCE
MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
3. 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

51. RESISTANCE
MECHANISMS THAT MEDIATE BACTERIAL
RESISTANCE TO DRUGS
4. Altered metabolic pathway that bypasses the
reaction inhibited by the drug
Sulfonamide resistance – utilize preformed folic
acid instead of extracellular PABA  S.
aureus, Enterobacteriaceae

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

54. RESISTANCE
LIMITATION OF DRUG RESISTANCE
1. Maintain sufficiently high levels of the drug in
the tissues  inhibit original population and
first-step mutants.
2. Simultaneous administration of two drugs that
do not give cross-resistance  delay
emergence of mutants resistant to the drug
(e.g. INH + Rifampicin)
3. Limit the use of a valuable drug  avoid
exposure of the organism to the drug

55. THE END