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Antimicrobial Drugs
Ch.20
25
Microbiology
Undergraduate 2
04/28/2012

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Term
Spectrum of antimicrobial activity
Definition
the range of microbes affected by an antimicrobial
Term
Narrow spectrum antibiotics
Definition
effective only against a limited variety of microbes
Ex: Bacitracin (G+)
Term
Broad Spectrum antibiotics
Definition
effective against wide variety of G+ and G- bacteria
Ex: 1. Tetracyclines (G+, G-, mycoplasmas, mycobacteria)
2. Trimethoprim (G+, G-, against DNA and nucleic acid synthesis)
Term
Antibiotics kill sensitive bacteria and become favorable for.....
Definition
resistant
Term
Action of antimicrobial drugs:
Definition
1. inhibition of cell wall synthesis
2. inhibition of protein synthesis
3. Injury to the plasma membrane
4. Inhibition of nucleic acid synthesis
5. Inhibition of enzymatic activity
Term
1. Inhibition of cell wall synthesis: (narrow)
Definition
-inhibit transpeptidation enzymes involved in the cross-linking of the polysaccharide chains of the bacterial cell wall peptidoglycan
-cell wall synthesizes at log
- drugs are effective in rapidly growing bacteria
-cell wall prevents from lyses
EX: penicillins, vancomycin, bacitracin, cephalosporins
Term
2. Inhibition of protein synthesis: (broad)
Definition
-translation, no transcription
- the process of protein synthesis is same in prokaryotic and eukaryotic cells
-The difference in ribosomal structure accounts for the selective toxicity of antibiotics that affect protein synthesis.
EX: Chloramphenicol, tetracyclines, erythromycin, streptomycin
Term
***Chloramphenicol
Definition
prevents peptide bond formation between growing polypeptide chain
Term
***Tetracyclines
Definition
interfere with protein synthesis by preventing the attachment of the amino-acid-carrying tRNA (anticodon) to mRNA (codon)-prevents the bonding
Term
***Erythromycin
Definition
jams the ribosome on mRNA, binds to 50S and prevents traslocation
Term
***Gentamycin and streptomycin
Definition
appear to act by changing the shape of 30S subunit of the ribosome, causing misreading of the genetic code
Term
3. Injury to the plasma membrane: (broad)
Definition
the cytoplasmic membrane is selectively permeable structure that controls the transport of many metabolites into and out of the cell. This damage to this structure hampers or destroys its ability to prevent a number of necessary biosynthetic functions (synthesis of ATP or cellular respiration) from taking place in the membrane
EX: polymyxin-B, nystatin, amphotericin-B
Term
*Polymyxin-B (antibacterial)
Definition
attaches to phospholipids of membrane and increase permeability
Term
***Amphotericin-B and nyastatin (antifungal)
Definition
combine with sterols which are not found in bacterial membrane but are present in animal membranes and can be toxic to the host

Fungi--Eukaryote
Animal--" "
Term
4. Inhibition of nucleic acid synthesis: (broad)-->Avoid
Definition
-transcription and translation
-any antibiotic that specifically reacts with DNA to prevent its replication or transcription will inhibit cell growth and division.
-use is limited d/t lack of selectivity
EX: idoxuridine, rifamycin, nalidixic acid, trimethoprim

***trimethoprim-stop synthesis of nucleic acid
Term
5. Inhibition of enzymatic activity:
Definition
-transcription and translations
-an antimetabolite is a substance that prevents a cell from carrying out a metabolic reaction
-(PABA)
EX:sulfonamides
vidarabine (purine analog) A, G
idoxuridine (pyrimidine analog) T, C
-Most useful in tx viral diseases b/c viruses incorporate analogs more rapidly than cells and are more severely damaged.
Term
***Bacteria become drug resistant in what ways?
Definition
1. alternation of receptors to which antimicrobial agents bind
2. a mutation that allowas the organism to bypass the sensitive step inhibited by antibiotic
3. a mutation that changes the nature of proteins in the cell membrane so an antimicrobial agent no longer crosses the membrane, usually acquired by R-factor (R-plasmid)
4. Development of enzymes that can destroy or inactivate antimicrobial agents (most common) acquired by plasmid.
ex: one enzyme of this type is beta-lactamase. Several beta-lactamases exist in various bacteria; they are capable of breaking the beta-lactam ring in penicillins and cephalosporins
Term
The origin of drug resistance:
Definition
1. spontaneous mutations (1 per million to 1 per billion
2. R-factors, a single plasmid may carry genes for resistance to several drugs. Plasmids may be transferred to other cells through conjugation, transduction and transformation
3. Extensive drug treatment destroys normal flora and leads to superinfection. Ex:Clostridium dificile
Term
Synergistic effect (1+1=>2)
Definition
enhance each others effect
Ex: Penicillin and streptomycin
Term
Antagonistic effect (1+1=<2)
Definition
undesirable effect,neutralize
Ex: Penicillin and tetracycline
Term
Additive effect (1+1+2)
Definition
don't enhance nore neutralize
Ex: trimethoprim and tetracycline
Term
Why may combination of drugs be used?
Definition
1. to minimize the development of resistant strains
2. to achieve synergistic effect
3. to provide therapy prior to diagnosis
4. to use small dosage of each drug to lessen toxicity
Term
Mode of action of penicillin?
Definition
eliminate the enzyme that sugar and peptidoglycan, cell wall prevents from lyses
Term
Penicillin resistance
Cephalosporin resistance
Definition
beta- lactamase exist in various bacteria; capable of breaking the beta-lactam ring
Term
Antibiotics that interfere with protein synthesis?
Definition
Chloramphenicol, tetracyclines, erythromycin, streptomycin
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