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.
Maria Ellery Mendez, MD, DPASMAP,FPAMS,FPAAAM Department of Microbiology Our Lady of Fatima University ANTIMICROBIAL AGENTS
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
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
9.
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
40.
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
53.
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