Antimicrobial Therapy
"Magic bullet" - kills bacteria, but has no effect on host cells.
pharmaceutical industry - Goal is to find agents with the ability to inhibit microbes at concentrations that can be tolerated by the host.
HISTORY: 1929, London - Fleming first observed that a contaminating mold colony of the fungus Penicillium notatum lysed adjacent colonies of staphylococci.
10 years later - others were able to produce a stable penicillin product in sufficient quantities for use.
Antibiotic - small molecules made by microbes (Penicillium, Streptomyces, Cephalosporium, Bacillus, etc.)
Chemotherapeutic - synthetic antimicrobial agents
However, overlap exists - antibiotics are now chemically modified to improve their properties and some antibiotics, e.g., chloramphenicol (produced by Streptomyces), are now chemically synthesized.
Bacteriostatic v. bactericidal; immunocompetent v. immunocompromised patient.
Bactericidal agents irreparably damage certain sites:
a. cell wall biosynthesis
b. membrane function
c. DNA
d. protein-synthesizing apparatus (including the ribosome)
e. key enzymatic reactions (e.g. folic acid biosynthesis)
A. CELL WALL ACTIVE
1. Penicillin
bacteriocidal
interferes with PG synthesis
nontoxic, except in allergic patients.
key feature - four-membered beta-lactam ring
analogue of the D-ala-D-ala bond in the transpeptidation reaction in peptidoglycan synthesis
covalently binds to the transpeptidase enzymes (penicillin binding proteins) that are responsible for catalyzing the cross-linking of peptidoglycan. By binding the PBPs, penicillin inactivates them.
also inhibits carboxypeptidases
effective against GP cocci, spirochetes, and a few GN's like Neisseria.
Penicillin semi-synthetic derivatives: altered in side chain of molecule that affects absorption from gut and/or protects beta-lactam ring.
a. Oxacillin, nafcillin, and methicillin - resistant to plasmid-coded staphylococcal beta-lactamases. Bind and inactivate PBP’s as normal beta-lactam antibiotics.
Methicillin resistance by staphylococci is not mediated by a beta-lactamase enzyme. Methicillin-resistant strains make all the normal PBPs and, in addition, a modified PBP2 called PBP2a or PBP2’. It does not bind methicillin, and thus the organisms are resistant. PBP2a is able to take over the functions of 3 of the high MW PBPs inactivated by methicillin.
- transposable element responsible; one clone that has spread
b. Ampicillin - amino group on the side chain – aminopenicillin
- improved penetration through the negatively charged OM of GN's
- susceptible to GN plasmid-encoded beta-lactamases.
Amoxicillin is an ampicillin derivative with better absorption characteristics after oral administration.
c. Carbenicillin - carboxy penicillin
- first with antibacterial activity against Pseudomonas aeruginosa
- penetrates the bacterial envelope better than other penicillins
- stable to the chromosomal beta-lactamase produced by all Pseudomonas
- must be given intra venous (IV) - not absorbed by the oral route
2. Cephalosporins
from mold Cephalosporium
Original cephalosporin, called cephalothin, had an antimicrobial spectrum somewhat broader than that of penicillin. intra muscular (IM) or IV only.
Recent derivatives (2nd and 3rd generation cephalosporins) - broad spectrum
- resistant to beta-lactamase enzymes of GN's
- expensive, not as active against GP's
- Examples: cephuroxamine, cefaclor, cefoxitin, cefotaxime.
Resistance
Beta-lactamase inhibitors: substrates with little bactericidal action but with high affinity for beta lactamases; irreversibly bind beta-lactamases.
Examples: clavulanic acid or sulbactam
e.g. Augmentin = clavulanic acid + amoxicillin.
3. Other inhibitors of PG synthesis:
a. Vancomycin - glycopeptide isolated from Nocardia
effective against GP bacteria
especially useful against methicillin-resistant staphylococci
GN's are resistant because vancomycin does not penetrate the OM
Resistance mechanisms of enterococci
Newest problem: S. aureus with reduced susceptibility to vancomycin; unknown mechanism
b. Bacitracin - polypeptide produced by Bacillus subtilis
C55 isoprenyl-PP <-----> C55 isoprenyl-P + Pi
ANTIBIOTICS THAT INHIBIT PROTEIN SYNTHESIS
bacterial ribosome - made up of two subunits of unequal size, designated 30S and 50S. Each subunit contains many protein molecules and some RNA molecules. Some antibiotics bind to the small and others to the large ribosomal subunits.
- must enter the bacterial cytoplasm, so they must either induce membrane damage or have hydrophobic regions that promote entry by diffusion through the membrane lipid.
1. Aminoglycosides
- aminosugars
- produced by Streptomyces
- highly polar, polycationic compounds (derivatives of inositol)
e.g. streptomycin, kanamycin, tobramycin, gentamicin, amikacin - differing little in antimicrobial spectrum
Mechanism- adsorbs to anionically charged exterior of cell. A few molecules enter the cell through transient imperfections in the growing membrane. Aerobes transport some across the membrane in an O2-dependent fashion.
These molecules bind to the 30S ribosomal subunit, and cause the ribosome to misread the genetic code, yielding proteins that are aberrant or nonfunctional.
some of these proteins enter the membrane, where their misfolding creates aqueous channels. Further entry of the antibiotic then increases, leading to more misreading and more membrane damage. Protein synthesis halted.
a. Streptomycin
first member to be discovered
from Streptomyces griseus
effective against GP and GN bacteria
binds to a single protein of the ribosomal 30S subunit
Resistance to Sm is due to alterations in a single amino acid in a ribosomal protein in the 30S subunit - prevents binding
The other aminoglycosides also bind to 30S ribosomal subunit but with at least two binding sites, so resistance is less easily accomplished
b. Kanamycin - from streptomyces, effective against GN rods.
c. Gentamicin
not isolated from Streptomyces
effective against P. aeruginosa
parenteral administration only (not oral).
d. Tobramycin - resistant to inactivating enzymes produced by
P. aeruginosa that attack gentamicin.
e. Amikacin
semisynthetic derivative of Kanamycin
a poor substrate for most aminoglycoside-inactivating enzymes
Resistance mechanisms effective against aminoglycosides:
a. mutation of ribosome binding site. Resistance to Sm is due to alterations in a single amino acid in a ribosomal protein in the 30S subunit.
b. decreased antibiotic uptake into the cell
c. enzyme modification by:
acetyl transferases
adenyl transferases
phosphotransferases.
These enzymes transfer acetyl, adenyl or phosphate residues to specific amino and hydroxyl groups on aminoglycoside molecules.
Modified aminoglycosides are unable to enter the cell or to bind to the ribosomal target site.
2. Tetracycline
binds to the 30S subunit
interferes with binding of aminoacyl tRNA to the acceptor (A) site of the ribosome, thus inhibiting protein synthesis
bacteriostatic
broad antibacterial spectrum: both GP and GNs, aerobic and anaerobic
bacteria accumulate Tc by an energy-dependent transport system not present in eukarotic cells
Resistance: usually increased efflux of the antibiotic from the cell
3. Chloramphenicol
synthesized by streptomyces but now synthesized chemically
binds to the 50S ribosomal subunit
blocks peptide bond formation – binds to peptidyl transferase enzyme
bacteriostatic
broad spectrum
readily absorbed from the GI tract
Resistance - production of an acetyltransferase - acetylates Cm, inactivating it. acetylated form of Cm binds less well to 50S subunit
4. Erythromycin
macrolide characterized by large lactone ring
binds to the 50S subunit
bacteriostatic – inhibits formation of new peptide chains
Blocks translocation step in protein synthesis; interferes with the release of charged tRNA bound to the donor (P) site of the ribosome after peptide bond formation. Interferes with translocation of peptidyl tRNA from acceptor (A) to donor site (P).
Lincomycin and clindamycin similar. Structurally these antibiotics (abics) are different, but functionally the same. These abics compete with Cm for binding to 50S ribosomal subunit.
Resistance:
GP: due to modification (dimethylation) of adenine residue of ribosomal 23S RNA so that binding by the antibiotic is prevented
GN: altered protein of 50S subunit so that binding is prevented
5. Mupirocin
topical antibiotic unrelated structurally to any of the others
produced by Pseudomonas fluorescens
used for eradication of nasal and skin carriage of S. aureus
acts by arresting protein synthesis via competitive inhibition of bacterial isoleucyl tRNA synthetase
active against GP cocci - particularly effective in the treatment of Me-
resistant S. aureus colonization of hospital staff.
MEMBRANE-ACTIVE ANTIBIOTICS
Polymyxins, e.g. Polymyxin B and Colistin
cyclic polypeptides produced by Bacillus spp.
resemble cationic detergents with multiple basic groups
not absorbed after oral administration; parenteral injection only
only antibiotics that are bactericidal to nongrowing cells
toxicity limits their use to topical application.
INHIBITORS OF TRANSCRIPTION
The RNA polymerases of bacteria differ from mammalian cells, so their inhibitors may be selective.
Rifampin - semisynthetic derivative of the antibiotic rifamycin
bacteriocidal, interferes with initiation of transcription in bacteria by selective inactivation of the beta subunit of DNA-dependent RNA polymerase
Resistant mutants are altered in the beta subunit of the polymerase (one aa change causes reduced binding of Rif).
INHIBITOR OF DNA SYNTHESIS
Quinolones - block DNA replication by inactivating the A subunit of bacterial DNA gyrase. DNA gyrase unwinds the negative supercoiling in the closed-circular duplex DNA of bacteria preparatory to DNA replication.
Quinolones are difficult to inactivate because they are chemically unreactive.
Resistance is due to chromosomal mutation causing an alteration in the A subunit of DNA gyrase or a permeability change or both.
Oral administration. Most effective against Enterobacteriaceae.
a. Nalidixic acid (1962). Synthetic; narrow antimicrobial spectrum (GN rods), but good for urinary tract infections.
b. Fluoroquinolones - Synthetic; greater potency, broader spectrum. Example: ciprofloxacin, 1000X more potent than nalidixic acid.
c. Novobiocin (an antibiotic) - also inhibits DNA gyrase; bacteriostatic.
Agents that act on key enzymatic reactions
1. Sulfonamides - simple chemicals; many related types. Act on folate metabolism; blocks synthesis of tetrahydrofolate.
Confusing nomenclature - there are a number of different sulfonamides (sulfamethoxazole, sulfisoxazole, sulfanilamide, sulfadiazine).
structural analogs of para-aminobenzoic acid (PABA). PABA is a precursor of folic acid (a cofactor of one-C metabolism in cells)
bacterial growth is inhibited because the sulfonamide has affinity for pteridine synthetase and competes with PABA for binding of the dihdropteroate synthetase enzyme
Mammals obtain folic acid in their diet, but bacteria lack a transport system for folic acid and so make their folate from PABA. A few bacteria can utilize exogenous folate; they are resistant to sulfonamides.
Resistance mechanisms:
a. bacteria use exogenous folate (instead of synthesizing their own)
b. excessive synthesis of PABA
c. synthesis of dihydropteroate synthetase enzyme with a lower affinity for sulphonamide (single aa change).
2. Trimethoprim
analogue of dihydrofolate
binds to and blocks another enzyme in the folic acid pathway: dihydrofolate reductase.
has a much lower (10-4 to 10-5 fold) affinity for the mammalian than for the bacterial enzyme
Resistance to trimethoprim: variant dihydrofolate reductase enzyme that is resistant to drug (reduced affinity).
Combine sulfonamides and trimethoprim and get synergism because they are both inhibitors of the same pathway.
MECHANISMS of DRUG RESISTANCE
Whether inherent or acquired, resistance can be ascribed to one of several mechanisms:
1. Production of a drug-inactivating enzyme:
a. beta-lactamases that hydrolyze penicillins and cephalosporins
b. aminoglycosides are inactivated by phosphorylation, adenylation, or acetylation. Interferes with transport of the antibiotic into the cell.
c. chloramphenicol acetyltransferase (CAT)
2. Target alterations:
a. ribosomal modifications
erythromycin resistance is due to an inducible enzyme that methylates the 23S ribosomal RNA to prevent antibiotic binding.
Sm resistance is due to alterations in a single amino acid in a ribosomal protein in the 30S subunit.
b. alternate PBPs (PBP2a) of methicillin-resistant Staphylococcus and pen-res pneumococcus
c. different enzymes in folic acid biosynthesis
d. altered PG structure - vancomycin
3. Impermeability to the drug, especially in GN bacteria.
a. porin loss can cause resistance to agents like beta lactams, chloramphenicol, and quinolones.
b. resistant bacteria actively extrude Tc by a plasmid-coded protein in the membrane; same for Cd resistance.
c. Some species are intrinsically resistance to aminoglycosides because of alterations in cell permeability.
To prevent antibiotic resistance from developing, one effective strategy is to use combination therapy.
Rationale: There is a low probability that resistance through spontaneous, chromosomal mutations to both antibiotics will develop. This does not apply to plasmid mediated resistance where multiple resistances can be selected by a single antibiotic.
Other solutions to increasing development of antibiotic resistance: