Ex 1: Principles 1-4 Biological basis for antibacterial action Flashcards
What is a chemotheraputic agent?
chemical compounds used to treat disease
What is an antimicrobial?
destroy pathogenic microbes or
inhibit their growth within host
What is an antibiotic?
- destroy or inhibit bacteria
- a class of chemotherapeutic agents
What are most antibiotics made of?
made of microbial products or their derivatives
What is selective toxicity?
ability of drug to kill or inhibit pathogen while damaging host as little as possible
What is the therapeutic dose?
drug level required for clinical treatment
What is the toxic dose?
drug level at which drug becomes too toxic for patient
What is the therapeutic index?
ratio of toxic dose to therapeutic dose
What do bacteriocidal antibiotics do?
kill bacteria
What do bacteriostatic antibiotics do?
inhibit growth of bacteria
What do broad-spectrum antibiotics do?
attack many different bacteria (Gram + and Gram -)
What do narrow-spectrum antibiotics do?
attack only a few different bacteria
What is the minimal inhibitory concentration (MIC)?
lowest concentration of drug that inhibits growth of pathogen
What is the minimal bacteriocidal concentration (MBC)?
lowest concentration of drug that kills pathogen
What are the two ways to determine the level of antimicrobial activity?
– minimal inhibitory concentration (MIC)
– minimal bacteriocidal concentration (MBC)
What are the two techniques routinely used to determine MIC and MBC?
- dilution susceptibility tests
- disk diffusion test
What is the dilution susceptibility test?
inoculating media containing different concentrations of drug
– broth or agar with lowest concentration showing no
growth is MIC
* broth from which microbe cannot be recovered is MBC
What is the disk diffusion test?
- disks impregnated with specific drugs are placed on agar plates inoculated with test microbe
- drug diffuses from disk into agar, establishing concentration gradient
- observe clear zones (no growth) around disks
What is the kirby-bauer method?
- standardized method for carrying out disk diffusion test
- sensitivity and resistance determined using tables that relate zone diameter to degree of microbial resistance
- table values plotted and used to determine if concentration of drug reached in body will be effective
What are the zone diameters based on for disk diffusion tests?
based on studies to establish the levels a drug can safely reach in the human body
concentration of drug at infection site must be _______ MIC to be effective
> greater than
What are factors that influence the effectivenss of antimicrobial drugs?
- ability of drug to reach site of infection
- ability of drug to reach concentrations in body that exceed MIC of pathogen
- susceptibility of pathogen to drug
What factors affect the ability of drug to reach site of infection?
- mode of administration
– oral
– topical
– parenteral routes - drug can be excluded by blood clots or necrotic tissue
What factors influence the ability of the drug to reach concentrations above MIC?
- amount administered
- route of administration
- speed of uptake
- rate of clearance (elimination) from body
What are the main mechanisms of action of antimicrobial agents?
can impact pathogen by targeting some function necessary for its reproduction or survival
- disrupt bacterial cell wall
- inhibit protein synthesis
- inhibit nucleic acid synthesis
- target folic acid pathways
What are the cateogories of antimicrobial agents that disrupt bacterial cell walls?
B-lactams
Glycopeptides
Polypeptides
Other (cycloserine, isoniazid & ethionamide, ethambutol)
What are the antimicrobial agents in the B-lactams group?
penicillins
cephalosporins
carbapenems & monobactams
(+ b-lactamase inhibitors)
What are the antimicrobial agents in the glycoprotein group?
vancomycin & teichoplanin
What are the antimicrobial agents in the polyprotein group?
bacitracin & polymixins
What are the steps to peptidoglycan synthesis?
– peptidoglycan repeat unit forms in
cytoplasm
* involves use of uridine diphosphate (UDP) as a carrier
– then transported across membrane by bactoprenol (“lipid”)
– attached to growing peptidoglycan chain
– cross-links formed by transpeptidation
What process creates the crosslinks in peptidoglycan?
Transpeptidation
- the exchange of one peptide bond for another
What is the crosslink in gram negative bacteria?
between D-ala and DAP
What is the crosslink in gram positive bacteria?
peptide interbridge of gly
What do B-lactam antibiotics inhibit?
transpeptidation (formation of crosslinking)
What type of bacteria does ampicillin target?
gram-positive and gram-negative
What type of bacteria does methicillin target?
penicillinase-resistant bacteria
What is the function of B-lactamase inhibitors?
help b-lactam antibiotics by preventing their degradation by b-lactamases
- b-lactamases are enzymes produced by some bacteria that are resistant to b-lactam antibiotics
What are examples of B-lactamase inhibitors?
clavulanic acid
sulbactam
tazobactam
How do you use B-lactamase inhibitors?
use in combination with B-lactam antibiotics
ex: amoxicillin + clavulanic acid
How does vancomycin work?
- binds terminal D-Ala-D-Ala and sterically inhibits addition of peptidoglycan subunits to the cell wall
- inhibits the transpeptidation reaction that crosslinks the chains
Why are vancomycin and teicoplanin important antimicrobial agents?
important for treatment of antibiotic resistant staphylococcal and enterococcal infections
What do polypeptide antimicrobial agents do?
- Prevent recycling of lipid carrier
- Binds phospholipids and disrupts outer and inner membranes of gram negative bacteria
How are polypeptide antimicrobial agents given?
topically because of more general mode of action = more toxic to humans
What is cycloserine used for?
Second line treatment for
Mycobacterium tuberculosis
Why is cycloserine used only as a second line of treatment?
crosses blood brain barrier and is an NMDA receptor agonist (with uses and side effects)
What do isoniazid and ethionamide do?
Inhibits Mycobacteria by affecting synthesis of mycolic acid
What does ethambutol do?
Inhibits Mycobacteria by affecting attachment of mycolic acid in the cell wall
What groups of drugs inhibit protein synthesis?
- Oxazolidinones (linezolid)
- Tetracyclines
- Aminoglycosides (streptomycin, amikacin, gentamycin, tobramycin)
- Chloramphenicol
- Lincosamides
- Macrolides (Erythromycin, azithromycin, clarithromycin)
What is the cycle of protein synthesis?
- 30S ribosomal subunit and mRNA
- forms 30S initiation complex
- 50S ribosomal subunit becomes the 70S initiation complex
- peptidyltransferase reaction
- translocation reaction
- restart
How do oxazolidinones (linezolid) work?
Binds 23S rRNA and prevents formation of 70S initiation complex
How do tetracyclines work?
Bind 16S rRNA of 30S subunit and prevent binding of aa-tRNA to A site
How do aminoglycosides (streptomycin, amikacin, gentamycin, tobramycin) work?
Bind to 30S subunit and distort A site, causing translation misreading, which inhibits protein synthesis
How do chloramphenicols work?
Bind to 50S subunit and inhibit
peptidyltransferase activity
How do lincosamides work?
blocks the peptidyltransferase
reaction
How do macrolides (Erythromycin, azithromycin, clarithromycin) work?
- Bind 23S rRNA in the 50S subunit and block the translocation reaction
- also prevent formation of the 50S subunit
What are the drugs that inhibit nucleic acid synthesis?
- quinolones (ciprofloxacin)
- rifampin and rifabutin
- metronidazole
How do quinolones (ciprofloxacin) work?
Interfere with type II topoisomerases and stabilizes DNA double strand breaks
How do rifampin and rifabutin work?
Bind to RNA polymerase and prevent the initiation of transcription
How do metronidazoles work?
- a prodrug with no inherent antimicrobial activity
- produces DNA-damaging radicals under anaerobic conditions via enzymes functioning in anaerobes and microaerophiles
What do antimetabolites (Sulfonamides, trimethoprim, dapsone, p-aminosalicylic acid) do?
disrupt the bacterial folic acid synthesis pathways
What are the four main ways of drug resistance?
- modified cell wall protein
- modified drug target
- drug-inactivating enzyme
- efflux pump
What mechanism are bacteria resistant to penicillins and cephalosporins?
hydrolysis of b-lactam ring by b-lactamase
What mechanism are bacteria resistant to methicillin?
change in penicillin-binding protein
What mechanism are bacteria resistant to tetracyclines?
efflux pump pushes drug out of cell
What mechanism are bacteria resistant to oxazolidinones?
mutations in 23S rRNA
What mechanism are bacteria resistant to quinolones?
mutations in genes encoding DNA gyrase and topoisomerase IV
What are the two ways bacteria become drug resistant
- New mutations of bacterial genes that encode the targets of antibiotics
- Pre-existing resistance genes that are transmitted from one bacterium to another
What are the five genetic elements involved in drug resistance?
- Plasmids: conjugation
- Transducing bacteriophage: transduction
- Bacterial chromosomal genes: mutations/transformation
- Transposons
- Integrons
What is a superinfection?
development and spread of drug-resistant pathogens caused by drug treatment, which destroys drug sensitive strains
How do superinfections occur?
Killing of normal flora removes the inhibitory effect of the normal flora. This allows for uninhibited growth of potentially pathogenic bacteria & fungi
What are the common organisms in superinfections?
- Clostridium difficile
- MDR (multi-drug-resistant) gram-negative rods
- MRSA (methicillin-resistant Staphylococcus aureus)
- Candida or other fungi
What are the ways to prevent emergence of drug resistance?
- give drug in high concentrations
- give two or more drugs at same time
- use drugs only when necessary
- possible future solutions
– continued development of new drugs
– use of bacteriophages to treat bacterial disease
What are the upcoming major antibiotic resistant threats?
- clostridium difficile
- carbapenem-resistant enterobacteriaceae
- neisseria gonorrhoeae
- fluconazole-resistant candida
