Microbio Exam 4

What application must be filed before human clinical trials of a new antibiotic can begin?

An investigational New Drug (IND) application, which includes pharmacology, dosing, and toxicity studies.

What is the purpose of Phase I clinical trials for antibiotics?

To evaluate safety and initial pharmacokinetics in a small group of healthy people

What is the focus of Phase II antibiotic trials?

Efficacy and safety in several hundred patients with the target infection

What distinguishes Phase III antibiotic trials?

Large-scale trials (300-3000) assessing comparative efficacy, safety, and dosing

What is Phase IV FDA approval?

Post-marketing surveillance to identify rare adverse effects and long term outcomes

Why is antibiotic development less financially attractive than non-microbial drugs?

Antibiotics generate about 65$ million annually compared to ~490 billion for top non-antimicrobials and stewardship limits use

What is pharmacodynamics(PD)?

The interaction of a drug with the target bacterium

What does pharmacokinetics(PK) describe?

The interaction of the drug with the human body, including absorption, distribution, metabolism, and excretion

Define bacteriostatic activity

Reversible inhibition of bacterial growth without killing bacteria

Define bactericidal activity

irreversible killing of bacteria

What is the minimal inhibitory concentration?

AKA the MIC. The lowest antibiotic concentration that prevents visible bacterial growth under standardized conditions

What is the post antibiotic effect?

AKA PAE. It is a persistent suppression of bacterial growth even when drug isn’t being used or when drug levels fall below MIC.

What does the bacterial growth curve demonstrate about drug effects?

Bacteriostatic agents halt growth, while bactericidal agents reduce viable bacterial counts

can bacteriostatic drugs still be effective clinically?

yes, especially in immunocompetent patients and for infections where bacterial eradication is not immediately required.

what is meant by “rate of killing”

How quickly an antibiotic reduces bacterial colonly-forming units(CFUs)

What is the inoculum effect?

increased bacterial burden can raise the MIC, especially for enzyme-labile drugs like β‑lactams

Why might MICs measured in the lab differ from those in patients?

Due to inoculum size, protein binding, site of infection, and immune factors

What is the primary goal of pharmacodynamic optimization?

To maximize bacterial killing and minimize resistance selection

What are the three major PK/PD idices used to guide antibiotic dosing?

• T>MIC

• Cmax/MIC

• AUC/MIC

what does monte carlo simulation contribute to antibiotic dosing?

Uses simulated PK and MIC data to predice probability of target attainment and clinical success

What does susceptible-dose dependent (SDD) mean?

clinical success depends on using higher doses or more frequent dosing than standard regimens

How does maintaining adequate PK/PD targets reduce resistance emergence?

Prevents bacterial survival at sub-therapeutic concentrations that select for resistant subpopulations

Which antibiotic classes produce prolonged PAE?

• Aminoglycosides(protein synthesis inhibition)

• Fluroquinolones(DNA synthesis inhibitors)

Why do cell-wall active agents have minimal PAE?

Cell wall components are quickly regenerated once drug pressure is removed

What factors influence the magnitude of PAE?

• Drug class

• bacterial species

• duration of exposure

• host immune status

How does PAE effect dosing strategies?

Allows less frequent dosing without loss of efficacy

What PK/PD index best predicts efficacy of β‑lactam antibiotics?

T>MIC (time above MIC)

Which antibiotic classed are concentration-dependent killers?

Aminoglycosides

Fluoroquinolones

Which PK/PD index governs aminoglycosides efficacy?

Cmax/MIC

Which antibiotic classes rely on AUC/MIC for efficacy?

• Fluoroquinolones

• Macrolides

• Glycopeptides

• Tetracyclines

• Oxazolidinones

• Some β‑lactam/β‑lactamase inhibitor combinations

Why is protein binding important in pharmacodynamics?

Only unbound(free) drug can interact with bacteria

What is the key pharmacodynamic feature of β‑lactams?

Time-dependent killing with minimal post-antibiotic effect

What is the antimicrobial susceptibility testing(AST)?

Laboratory testing that determines whether a bacterium is susceptible, intermediate, or resistant to a specific antibiotic under standardized conditions

What does MIC stand for and what does it measure?

MIC: Minimum Inhibitory Concentration. It measures the lowest concentration of antibiotic that prevents visible bacterial growth

What bacterial concentration defines visiblle growth in susceptibility testing?

Approx. 10^7 CFU/mL

What is a breakpoint in susceptibility testing?

A defined antibiotic concentration used to categorize isolates as susceptible intermediate, resistant or SDD

What data are used to establish antimicrobial breakpoints

• microbiologic data

• PK/PD

• Clinical outcomes data

Which organizations establish antibiotic breakpoints

• FDA
  CLSI(american)

• EUCAST(european)

What does “susceptible” mean in AST reporting?

The organism is likely inhibited by achievable drug concentrations but success is not guaranteed

Why does susceptibility not guarantee clinical success?

factors such as drug penetration, immune status, and need for surgery affect outcomes

What are the three meanings of “Intermediate” susceptibility?

• MIC uncertainty

• Efficacy with higher doses

• Efficacy at sites where drug concentrates well(ex; urine)

What is susceptible dose dependent(SDD)?

Susceptibility depend on using higher or optimized dosing regimens

How is macrobroth dilution performed?

Serial 2-fold antibiotic dilutions in broth tubes inoculated with ~5 × 10⁵ CFU/mL, incubated 18–24 hours.

What are advantages of macrobroth dilution?

It is the gold standard, fully quantitative, and allows MBC determination

What are disadvantages of macrobroth dilution?

• labor intensive

• low inoculum, may miss resistant mutants

• not practical for routine clinical labs

How does microbroth differ from macrobroth?

inoculum size is smaller than macrobroth

Why is smaller inoculum a disadvantage of microbroth testing?

smaller samples may miss resistant populations(sampling error)

What does MBC stand for/what is its purpose?

MBC; Minimum Bactericidal Concentration

MBC is a quantitative assay

step 1 is to take inoculum from tube that doesnt have visible bacteria then place on agar to determine MBC

what is agar dilution susceptibility testing?

Bacteria are spotted onto agar plates containing fixed antibiotic concentrations to determine MIC

What is an advantage of agar dilution?

up to 32 isolates per drug plate

easier than macrobroth

quantitative

What are disadvantages of agar dilution?

• Cannot measure MBC

• Lower inoculum

• MICs are lower than macrobroth

• Most time consuming-only one hospital uses this—> mayo clinic

How does the E-test work?

An antibiotic gradient strip diffuses into agar, creating an eclipse of inhibition where MIC is read

What are advantages of E-test?

• quantitative MIC

• easy to perform

• multiple drugs per plate

What are disadvantages of E-test?

• costly

• No MBC determination

• MIC endpoints can be difficult to interpret

What type of result foes disk diffusion provide?

Qualitiative(S,I,R, only)-no exact MIC

What influences zone size in disk diffusion testing?

• drug concentration in disk

• inoculum size

• temperature

• organism susceptibility

What are advantages of disk diffusion?

• Fast

• inexpensive

• many drugs per organism

• can show drug interactions

What is the main limitation of disk diffusion?

cannot determine how sensitive an organism is(no MIC)

What are the goals of automates susceptibility testing(AST)

rapid, accurate results with reduced laboratory workload

What is a key limitation of AST systems?

some organism-drug combinations may be inaccurate due to algorithms or low inoculum

What is the error of MIC testing?

A + or - 1 two fold error (ex; 2ug/mL could be 1, 2, or 4)

Why is the inherent MIC error clinically important?

A small MIC change can shift an isolate from susceptible to resistant

What is the inoculum effect?

Increased bacterial burden increases MIC, especially for enzyme-labile antibiotics

why is the inoculum effect clinically relevant?

Laboratory MICs may underestimate resistance in real infections

What are “trailing endpoints” in susceptibility testing

fuzzy or unclear growth inhibition endpoints that complicate MIC interpretation

Why can mixed bacterial populations cause AST errors?

minor resistant subpopulations may not be detected (heteroressitance)

Why does detection of resistance gene not always equal resistance?

the gene may be unexpressed or nonfunctional

Why does failure to detect a resistance gene not guarantee susceptibility?

Resistance may be due to unknown or undetected mechanisms

What are the major classes of b-lactam antibiotics?

• Penicillins

• Cephalosporins + Cephamycins

• Carbapenems

• Monobactams

• β‑lactam / β‑lactamase inhibitor combinations

What structural feature defines all β‑lactam antibiotics?

A β‑lactam ring, which is essential for antibacterial activity

Which drugs are considered 1st generation(natural) penicillins?

Penicillin G(IV) and Penicillin V (oral)

Which penicillins are anti-staphylococcal (β‑lactam resistant)

Methicillin,nafcillin, oxacillin, dicloxacillin(MSSA only)

Which penicillins are aminopenicillins(extended spectrum)?

Amoxicillin, ampicillin, pivmecillinam

Which penicillins have activity against pseudomonas?

-piperacillin(ureidopenicillin) and ticarcillin

Name a 1st generation cephalosporin

cefazolin and cephalexin

Name a 2nd generation cephalosporin or cephamycin

• Cefuroxime, ceftriaxone, cefoxitin, cofotetan

Name a 3rd generation cephalosporin

ceftazidine,ceftriaxone, cefotaxime

Which 3rd generation cephalosporin covers pseudomonas?

Ceftazidime

Name the 4th generation cephalosporin

cefepime

Name two 5th generation cephalosporins

ceftaroline and ceftobiprole

Which cephalosporin has MRSA activity?

Ceftaroline—> binds PBP2a

Name four carbapenems

• Imipenem, meropenem,doripenem, and ertapenem

Which carbapenem has poor activity against pseudomonas and acinetobacter?

ertapenem

Which β‑lactam class contains only one ring?

• monobactams

Name the monobactam antibiotic

• Aztreonam

Name three common β‑lactam/β‑lactamase inhibitor combos

• Amoxicillin/Clauvate

• Piperacillin/taxobactam

• Ceftazidime/avibactam

What is the molecular target of β‑lactam antibiotics?

Penicillin-binding proteins(PBPs)

What bacterial function do PBPs perform?

Cross-linking peptidoclycan during cell wall synthesis

Why are β‑lactams called D-Ala-D-Ala analogs

They structurally mimic the D-alanine-D-alanine terminus of peptidoglycan precursors

How do β‑lactams kill bacteria?

By inhibiting cell wall synthesis—> osmotic instability—> lysis

Are β‑lactams bactericidal or bacteriostatic?

• bactericidal

Which β‑lactams are best for Gram + infections?

• Penicillins and early generation cephalosporins

Which β‑lactams have the broadest Gram - coverage?

carbapenems and advanced cepahlosporins

Which β‑lactams are most reliable against anaerobes?

• Carbapenems

• Piperacillin/tazobactam

• cephamycins

Which β‑lactams cover pseudomonas aeruginosa?

• Piperacillin

• Ceftazidime

• Cefepime

• Carbapenems(not ertapenem)

Which β‑lactams are NOT active against MRSA?

All except 5th generation cephalosporins

Why are β‑lactamase inhibitors combined with β‑lactams?

To protect β‑lactam from enzymatic degradation

What 4 factors influence activity of β‑lactam/inhibitor combos?

1. Potency of β‑lactam

2. Potency of inhibitor

3. Pharmacokinetics

4. Inducer potential of the inhibitor

Why is amoxicillin/clavulanate ineffective against inducible AmpC producers?

• clavulanate induced AmpC expression, worsening resistance

Which organisms commonly have inducible AmpC β‑lactamase?

Enterobacter,citrobacter,serratia