What is the Minimal Inhibitory Concentration (MIC)
= the lowest concentration of the
antibiotic required to inhibit growth
What is the breakpoint?
Breakpoint = an estimate of the reasonable concentration that might be achieved clinically
Any organism that can grow at a concentration of the breakpoint or
greater than the breakpoint is resistant
•Give some gram negative MAJOR ANTIBIOTIC RESISTANT BACTERIAL PATHOGENS
Pseudomonas aeruginosa E. coli (ESBL) Klebsiella spp. (NDM-1) Salmonella spp. (MDR) Acinetobacter baumannii (MDRAB) Neisseria gonorrhoeae
Give some grampositive MAJOR ANTIBIOTIC RESISTANT BACTERIAL PATHOGENS
Staphylococcus aureus (MRSA, VISA) Streptococcus pneumoniae Clostridium difficile Enterococcus spp (VRE) Mycobacterium tuberculosis (MDRTB, XDRTB)
How do beta lactams work?
Interfere with synthesis of the PEPTIDOGLYCAN component of the bacterial cell wall
• Examples: Penicillin and methicillin
• Binds to Penicillin Binding Proteins (PBP) - they catalyse a number of steps in a large peptidoglycan synthesis
• The beta-lactam ring is similar in structure to a precursor of peptidoglycan - interferes with the biosynthetic pathway of peptidoglycan
NOTE: MRSA has a different PBP (PBP2a) which doesn’t bind with high affinity to beta-lactams
How does tetracycline work?
Bacteriostatic • Inhibits PROTEIN SYNTHESIS • Binds to the 16S component of the 30S ribosomal subunit thus preventing charged aminoacyl tRNAs from binding to the mRNA/ribosome complex • Prevents elongation of the polypeptide
How does chloramphenicol work?
Bacteriostatic
• Inhibits PROTEIN SYNTHESIS
• Binds to 50S subunit and blocks peptidyl transfer step
• Often used topically due to toxicity
How do quinolones work?
Targets DNA gyrase (in Gram NEGATIVE) and topoisomerase (in Gram POSITIVE)
• DNA gyrase and topoisomerase is responsible for unravelling DNA
How do sulphonamides work?
Bacteriostatic
• Used to treat UTI, Reproductive Tract Infection (RTI) and bacteraemia
• Interferes with the folate pathway
How do aminoglycocides work?
• Bactericidal
• EXAMPLES: Gentamycin, Streptomycin
• Affect PROTEIN SYNTHESIS
• Affects RNA PROOFREADING which leads to misfolded proteins
• Some of these proteins are incorporated into the membrane and allow leakage
so the cells rupture.
• Has toxicity issues
How do macrolides work?
EXAMPLE: Erythromycin
• Gram POSITIVE infections
Targets 50S ribosomal subunit preventing aminoacyl transfer
• Causes truncation of polypeptides
What are the mechanisms of antibiotic resistance?
Altered Target Site
• Inactivation of Antibiotic
• Altered Metabolism
• Decreased Drug Accumulation
Describe altered target site
Can arise from acquisition of an alternative gene or a gene that encodes a target-modifying enzyme
• You can acquire a gene, which performs the same function but has a different structure and hence is not susceptible to the AB.
• EXAMPLE: MRSA acquired a gene, which produces an alternative penicillin binding protein - it performs the same function but has lower affinity to beta- lactams so methicillin is ineffective.
• Streptococcus pneumoniae is resistant to erythromycin because it has acquired a gene which encodes an enzyme that methylates the AB target site in the 50S ribosomal subunit - this changes its structure so erythromycin can no longer act
Describe inactivation of an antibiotic
Acquire gene for an enzyme which breaks down the antibiotic
• EXAMPLES: beta-lactamase (bla) and chloramphenicol acetyl-transferase (cat)
• ESBL and NDM-1 are examples of broad spectrum beta-lactamases
Describe altered metabolism
Re-engineer the metabolic pathways so you bypass the step that the antibiotic interferes with.
• Increased production of enzyme substrate can be used to outcompete antibiotic inhibitor
EXAMPLE: increased production of PABA confers resistance to sulphonamides
Describe decreased drug accumulation
Reduced permeability of AB into bacterial cell
• Increase EFFLUX of AB out of cell or reduce penetrance
• Drug does not reach sufficient concentration to be effective
What are 3 Sources of AB resistance genes:
Plasmids Transposons Naked DNA
Why also might an antibiotic not work?
Inappropriate choice of organism
• Poor penetration of AB into target site
• Inappropriate dose
Inappropriate administration
• Presence of AB resistance within commensal flora e.g. secretion of beta-lactamase
What are risk factors for HAI?
High number of ill people! (immunosuppression)
Crowded wards
Presence of pathogens
Broken skin – surgical wound/IV catheter Indwelling devices - intubation
AB therapy may suppress normal flora
Transmission by staff – contact with multiple patients
How can AB resistance be addressed?
Tighter controls on prescription
• Reduce use of broad spectrum antibiotics
• Quicker identification of infections caused by resistant strains
• Combination therapy
• Knowledge of local strains/resistance patterns
How do different AB become resistant
Sulfonamide resistance is conferred by blocking uptake/decreasing influx.
Tetracycline resistance is mediated by efflux/membrane pumps.
Penicillin resistance works via drug inactivation (beta-lactamase) or altered target site (alternative penicillin-binding proteins as found in MRSA).
Metronidazole resistance is associated with with target amplification.
Quinolone by target site modification
What are transposons
Transposons are sequences of DNA that can move around to different parts of a cell’s genome, carrying with them resistance genes.