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What is the minimal inhibitory concentration (MIC)?
Concentration of antibiotic which prevents bacteria from growing enough to make the fluid turbid (usually good enough for patient unless they have a weak immune system)
What is the minimal bactericidal concentration? (MBC)
- The concentration of antibiotic that kills all the bacteria.
- (this is the concentration of antibiotic that must be reached if the patient is immunocompromised or has a life threatening condition)
What is the MIC 90?
- It is the best way to compare antibiotic potency
- An analysis of hundreds of isolates of a species of bacteria.
- It is the MIC value that inhibits 90% of the bacteria tested
- Low is better
What is the MPC?
- Mutant Prevention Concentration = antibiotic concentration that when added to 10^10 CFU allows no bacteria to survive
- Reaching this concentration in patients slows the spread of antibiotic resistance
What is peptidoglycan made of?
- Chains of NAG and NAMA
- Attached to NAMA is AA, AA, AA, D-Ala, D-Ala
- These peptides crosslink with other peptidoglycan strands
What does Fosfomycin do?
- Antibiotic that inhibits the formation of UDP-NAMA by binding to PEPA transferase, inhibiting muramic acid
- Broad spectrum both gram + and –
- Bacteria rapidly become resistant to
What does cycloserine do?
- Blocks the addition of both D-ala D-ala’s
- Good for drug resistant TB when used in conjunction with other drugs
What does bacitracin do?
- Blocks lipid dephosphorylation so that lipid cannot be added to the forming peptidoglycan
- If lipid is not added it cannot flip across membrane
- Only works on gram + by itself
How does triple antibiotic ointment work?
- Bacitracin – disrupts cell walls of gram +
- Polymyxin – disrupts cell walls of gram –
- Neomycin – broad spectrum aminoglycoside
What is the target of vancomycin?
The terminal D-ala on a peptidoglycan peptide
What are the two main mechanisms for vancomycin resistance?
- Method 1:
- VanX destroys d-ala-d-ala (plasmid coded)
- VanH converts pyruvate to d-lactate (plasmid coded)
- VanA links d-ala to d-lac
- Method 2:
- Staphylococcus have thick cell wall that binds vancomycin on outer surface
- This strain of staph is called “VIRSA”
What is the only target of Nafcillin?
What two conditions merit dental prophylaxis?
- Previous endocarditis
- Prosthetic heart valve
What type of beta-lactamase best kills pseudomonas and bacteroides?
- Carbenicillin and Ticarcillin
- They have an extra carboxy group which allows penetration
- They resist intrinsic Bla
What can first generation cephalosporins be used for?
- Staphylococcus and Streptococcus (NOT enterococcus)
- Susceptible gram – like Klebsiella
- Prophylaxis of clean surgery
- Prophylaxis of dental patient with implant
What can second generation cephalosporins be used for?
- Haemophilus respiratory infections
- Gram negative beta-lactamase producers like bacteroides
- Prophylaxis for surgery involving mucous membranes
What is Ceftaroline?
Blocks PBP2a of MRSA
Explain how Extended Spectrum Beta-Lactamases (ESB) have achieved such a broad spectrum of activity
- The gene for beta lactamase activity is found on a small plasmid with a high copy number
- The high copy number allows for the production of more mRNA and thus more enzyme
- The high copy number also allows for more chance of mutations per cell which leads to the broad spectrum
The Monobactam Aztreonam resists Bla, but only for one subset of bacteria. What is that?
Aerobic Gram-negative bacteria