Exam 1

Question 1

What is the role of fever in the diagnosis of an infection? What are some reasons for the absence of a fever in the presence of infection (and vice versa)?

Answer 1

Fever is the hallmark of infection (Oral temp is > 38ºC)
*rectal temp is ~0.6º C higher, axillary temp is ~0.6º lower

Non-infectious causes of fever: (false-pos)
- Malignancy, autoimmune disease
- Drug fever (ex. β-lactams, amphotericin, anticonvulsants, hydralazine, etc.)
- Blood transfusions

Absence of fever w/ s/sx of infeciton: (false-neg)
- Antipyretics
- Corticosteroid therapy
- Antimicrobial therapy
- Overwhleming infection (may be hypothermic)

Question 2

What are the typical signs and symptoms, lab tests, and radiologic findings that may be utilized to identify the presence of an infection? What changes in white count and differential are indicative of an infection caused by various pathogens?

Answer 2

Systemic S/Sx of infection:
- Fever (>38º or <36ºC)
- Increased WBC count (normal WBC is 4,500-10,500/mm^3, normal bands is 0-5%)
- Decreased WBC count (may be a sign of overwhelming infection)
- Chills, rigors
- Tachycardia (>90bpm)
- Tachypnea (>20breaths/min)
- Hypotension (SBP <90mmHg or MAP <70)
- Malaise
- Mental status changes
- Lymphocytosis (increased lymphocytes associated w/ viral, tuberculosis, or fungal infections)

Localized s/sx:
- pain/inflammation (ex. flank pain -> pyelonephritis (E. coli) most common

Tests:
- WBC differential (shows granulocytes & agranulocytes)
- ESR and CRP (elevated in inflammatory process, but doesn’t confirm presence of infection; can be helpful to see response to treatment)
- Procalcitonin (PCT): more specific, when elevated, suggestive of infection
- X-rays, CTs, MRI, nuclear imaging, EKG

Question 3

What are common factors that can predispose a person to develop an infection?

Answer 3

• Alteration in normal flora
• Disruption of natural barriers (skin/mucus membranes, cilia of respiratory tract, pH, motility, integrity of GI tract)
• Age (very young & very old people)
• Immunosuppression (malnutrition, underlying diseases, hormones, drugs)

Question 4

What do sensitivity and specificity mean in the context of antibody/antigen detection?

Answer 4

Antibody and/or antigen detection: (ex. immunofluorescence, ELISA)
- Sensitivity: pos. test in the presence of disease (chance of false negative)
- Specificity: neg. test in the absence of disease (chance of false pos.

Question 5

What is the difference between empiric and directed antimicrobial therapy?

Answer 5

Empiric - Initiation of broad-spectrum antimicrobial therapy before we identify the pathogen and susceptibility results are known. It usually involves multiple antibiotics.
- Use an antibiogram to decide treatment. It’s a summary of susceptibility for organisms at an institution.

Direct/Targeted therapy - Used after the pathogen and susceptibility results are known. Now, we will de-escalate to the agent(s) with the narrowest effective spectrum of activity. We will also try to move from IV to PO.

Question 6

What is the difference between bactericidal and bacteriostatic?

Answer 6

Bactericidal - Kills the organism by acting on the cell wall, cell membrane, or bacterial DNA, etc.

Bacteriostatic - Inhibit bacterial replication without killing the organism by inhibiting protein synthesis. So you need the host defenses to kill the organism.

Question 7

Why may combo antimicrobial therapy be utilized (3) and what are the potential disadvantages (4) of combo therapy?

Answer 7

1. Broaden spectrum coverage for empiric therapy if the organism isn’t adequately covered by a single agent
2. Achieve synergistic bactericidal activity against a pathogen to improve the outcome
3. Prevent emergence of resistance (TB, HIV)

Disadvantages: Increased cost, greater risk of drug toxicity, superinfection with resistant bacteria, antagonism

Question 8

What are some drug-specific and host factors to be evaluated when choosing antimicrobial therapy? (9 main groups)

Answer 8

Host factors:
- Drug allergy/hx of ADR
- Pt age, weight, sex
- Pregnancy, lactation
- Genetic/metabolic abnormalities
- Renal/hepatic dysfunction
- Site of infection
- Concomitant drug therapy/nutritional supplements
- Concomitant disease states (CF, HIV, cancer)
- Prior antibiotic exposure (inc. risk of resistance)

Question 9

How can you design a monitoring plan to assess the clinical response of a patient with an infection after initiation of antimicrobial therapy? What is the criteria to switch to PO therapy?

Answer 9

• “How’s the patient doing?”
• Lab tests: culture/susceptibility results, WBC/differential, therapeutic drug monitoring, ESR/CRP/PCT

Criteria to switch to PO therapy: overall clinical improvement, resolution of fever for 8-24 hours, decreasing WBC count, functioning GI tract, acceptable oral agent w/ good bioavailability

Question 10

What factors should be assessed when a patient fails to clinically respond to antimicrobial therapy?

Answer 10

Patient should be better after 2-3 days.

If not:
- Incorrect diagnosis?
- Wrong abx selection?
- Host factors: immunosuppression, foreign bodies, abscess needing drainage, source control
- Due to microorganism: resistance, mixed infection (polymicrobial)
- Lab errors?

Question 11

What is the structure of bacteria? Where are β-lactamases in gram pos. and gram neg. bacteria? (bacterial DNA, plasmids, cytoplasmic membrane, cell wall, outer membrane, porins, periplasmic space)

Answer 11

Bacterial DNA - single circular, double-stranded chromosome.

Plasmids - some bacteria have plasmids (mainly gram neg.). This is extrachromosomal, double-stranded DNA that encodes for genes whose products are not essential for cell survival, but offer certain benefits. These are transferable to other bacteria.

Cytoplasmic membrane - Acts as selective permeability barrier.

Cell wall - Gram pos. has thick cell wall, gram neg. has a thin cell wall. The cell wall acts as a permeability barrier for large molecules.

Outer membrane - In gram neg. bacteria. This contains lipopolysaccharide (LPS, endotoxin), which is an important mediator in gram neg. sepsis

Porins - In gram neg. bacteria, the outer membrane has porins. These are hydrophilic channels that permit the diffusion of essential nutrients and other small hydrophilic molecules.

Periplasmic space - In between the cytoplasmic membrane and the outer membrane of gram neg. bacteria. This is where β-lactamases are in gram neg. bacteria.

Question 12

What are penicillin-binding proteins? What are their roles in bacterial replication, porins, and periplasmic space?

Answer 12

Penicillin-binding proteins (PBPs) - enzymes that are vital for cell wall synthesis, cell shape, and structural integrity. Inhibiting this enzyme will cause the cell to lyse.
- ex. transpeptidases (most important), carboxypeptidases, endopeptidases

If PBP is altered, there will be decreased binding affinity. This is what causes Methicillin resistance in S. aureus and penicillin and cephalosporin resistance in S. pneumoniae. In S. aureus, the mecA gene produces a new PBP (PBP2a or 2’), which aids in its resistance to methicillin.
- In this scenario, adding a β-lactamase inhibitor won’t do anything.

Question 13

What is the difference between intrinsic resistance and acquired resistance?

Answer 13

Intrinsic resistance - Organisms are always resistance to a given antibiotic. This happens if theres an absence of target site, bacterial cell impermeability, etc.
- Ex. β-lactams vs. mycoplasma (doesn’t have a cell wall, which is the target of β-lactams)

Acquired resistance - Organisms initially susceptible to a drug become resistant. This happens when there is a change in bacterial DNA or acquisition of new DNA.
- Ex. fluoroquinoloes vs. P. aeruginosa

Question 14

What are the 3 ways of transferring genetic information? What are the 3 specific mechanisms of resistance?

Answer 14

Ways to transfer genetic info:
- Conjugation - direct contact or mating via sex pili
- Transduction - genes are transferred between bacteria by bacteriophage
- Transformation - transfer or uptake of “free-floating” DNA from the environment; then DNA is integrated into the host DNA

Specific mechanisms of resistance:
1. Enzymatic inactivation (most common): β-lactamases, aminoglycoside-modiyfing enzymes
2. Alteration of target site: PBPs, cell wall precursors, ribosomes, DNA gyrase/topoisomerase
3. Altered permeability of bacterial cell: efflux pumps, porin changes

Question 15

What are the major mechanisms of resistance for specific bacterial pathogens to the HECK-YES Ma’aM organisms?

Answer 15

These are all cephalosporinases (AmpC)

Enterobacter cloacae
Citrobacter freundii
Klebsiella aerogenes
Serratia marcescens
Morganella species

1. Induction of β-lactamase production. At first, the gene for β-lactamase production is repressed, then it’s induced, then the gene is derepressed, then there is increased β-lactamase production
   - Cefoxitin is a strong, labile inducer of β-lactamases

Due to this, these are not inhibited by the other β-lactamase inhibitors. But they ARE inhibited by the newer β-lactamase inhibitors.

Question 16

How can resistance occur to third generation cephalosporins by selection of stably derepressed mutants and what is the clinical significance of this phenomenon? What do we treat them with then?

Answer 16

Selection - Some organisms have the AmpC gene derepressed all the time. This is really rare, but it happens. Using a 3rd gen cephalosporin may kill 99.9% of the organisms, but then that 0.1% will replicate. This is associated with increased mortality.

Treatment:
- Cefepime (4th gen cephalosporin)
- Carbapenem (meropenem, imipenem, ertapenem)
- Non-β-lactam antibiotics

Question 17

What is the clinical significance of extended-spectrum β-lactamases? What are risk factors for infections caused by these β-lactamases? What is the appropriate treatment of infections caused by organisms producing these enzymes?

Answer 17

ESBL - the enzyme has changed so that it has expanded the drugs that it can hydrolyze.
- Ex. CTX-M-15

Risk factors: prolonged hospital/ICU/neonatal ICU stay, residency in long-term care facility, exposure to 3rd gen cephalosporin, exposure to ciprofloxacin, etc.

Treatment: carbapenems (meropenem, imipenem, dorpenem, ertapenem
- careful if they are on valproic acid, since it increased the metabolism of valproic acid

Question 18

What are the examples of narrow-spectrum β-lactamases, ESBL, serine carbapenemases, metallo-β-lactamases, and cephalosporinases?

Answer 18

Narrow-spectrum: staphylococcal penicillinase

ESBL: CTX-M-15

Serine carbapenemases: KPC-1, KPC-2, KPC-3

Metallo-β-lactamases: VIM-1, IMP-1, NDM-1

Cephalosporinases: AmpC

Question 19

What is the mechanism of vancomycin resistance in enterococci? What is the importance of the vanA phenotype?

Answer 19

Vancomycin inhibits cell wall synthesis by binding to the D-alanine-D-alanine terminus of the pentapeptide. If the D-alanine-D-alanine terminus is changed, vanc will have significantly less binding affinity to the sequence.
- Treatment is daptomycin or linezolid

We can test for VanA and VanB. These cause a high level of vancomycin resistance AND they are on a plasmid (highly transferable).

Question 20

What is the preferred treatment options for organisms with serine carbapenemases (KPC), metallo-β-lactamases (NDM, VIM, IMP), or OXA-type in CRAB?

Answer 20

Serine carbapenemases (KPC)
- ceftaidime/avibactam
- meropenem/vaborbactam
- imipenem/cilastatin/relebactam
- Cefiderocol (Fetroja)

Metallo-β-lactamases (NDM, VIM, IMP)
- aztreonam + ceftazidime/avibactam
-Cefiderocol

OXA-type in carbapenem-resistant Acinetobacter baumannii (CRAB)
- High dose sulbactam
- Sulbactam/durlobactam
- Cefiderocol
- Minocycline

Question 21

What are the 3 mechanisms of resistance for aminoglycoside-modifying enzymes?

Answer 21

1. Acetylation
2. Nucleotidylation
3. Phosphorylation

These modify aminoglycoside structure by transferring a chemical group to a specific side chain. This impairs uptake and/or binding to the bacterial ribosome.

Question 22

Which drugs are the biggest sustrates of the efflux pumps?

Answer 22

Ciprofloxacin
Levofloxacin

(quinolones)

Question 23

What are the usual methods used in antimicrobial susceptibility testing and how do we use information from these to make clinical decisions?

Answer 23

Broth dilution - Gold standard where there are two-fold dilutions of drug concentration in liquid media (no protein) [ex. 1mcg/mL, 2, 4, 6, 16mcg/mL]. Then, add the organism and incubate for ~24 hours. From this we can determin the MIC.

Agar dilution - two-fold dilutions of antibiotic incorporated into molten agar that’s poured into petri dishes and allowed to solidify. Then we let the bacteria inoculate over time. The MIC would be the agar plate with the lowest concentration of drug to prevent growth.

Disc diffusion - A fixed amount of antibiotic is impregnated into a disc. We put the organism on the surface of the agar, then incubate it. We can observe the zone of inhibition from this. The only results are S, I or R (not MIC).

Etest - (Epsilometer test) These are rectangular strips that have a concentration gradient of the antibiotic. We take the organism and streak it across the plate, incubate it, then see where the growth stops. The MIC is the conc on the strip where the bacterial growth intersects that strip.

We use these to determine the MIC, which we can use to decide treatment

Question 24

What is MIC and MBC?

Answer 24

MIC - Minimum inhibitory concentration. lowest drug concentration that prevents visible growth (unaided eye)

MBC - Minimum bactericidal concentration. Lowest drug concentration resulting in ≥99.9% decrease in initial bacterial inoculum. We don’t do this clinically.

Question 25

What do these terms mean: susceptible, susceptible-dose dependent, intermediate, resistant

How does an organism get categorized as susceptible, intermediate, or resistant based on interpretive guidelines?

Answer 25

Susceptible (S) - These organisms are expected to be inhibted by the concentration of the antibiotic at normal dosing regimen (clinical efficacy)

Susceptible-Dose Dependent (S-DD) - Susceptibility is dependent on the dosing regimen used (higher doses, more frequent dosing, extended or continuous infusions)

Intermediate (I) - Response rates may be lower, treatment may be successful when max doses are used or if the drug is concentrated at the site of infection.

Resistant (R) - Organisms not inhibited by the usual concentrations. Clinical efficacy hasn’t been reliable.

Nonsusceptible (NS) - Used for isolates where only a susceptible breakpoint is designated. If the MIC is above (or if zone diameter is below) the susceptible breakpoint, the isolate is categorized as NS.

Question 26

How are interpretive criteria (breakpoints) established?

Answer 26

Established by the Clinical & Laboratory Standards Institute (CLSI)

Breakpoint - the MIC or zone diameter used to categorize an organism into 1 of 5 criteria (susceptible, suceptible-dose dependent, intermediate, resitant, or nonsusceptible)

Established by:
- Clinical pharmacology of the drug
- Clinical and bacteriologic outcomes from human studies
- Distribution of MICs w/in a bacterial population
- Phenotypic detection of bacterial isolates with certain resistance mechanisms

Question 27

How can changes in the interpretive criteria (breakpoints) result in changes of reported susceptibility and resistance rates?

Answer 27

When breakpoints change, susceptibilities will likely change. This has nothing to do with resistance!! We just changed the definition of S, I, and R.

Question 28

What is the inoculum effect and tolerance. What are the potential impacts of these issues on clinical outcome?

Answer 28

Inoculum effect - When you test a higher inoculum (10^7 CFU instead of 10^5) and thus get an increase in MIC. This makes sense because you would need more drug to kill more organisms.

Tolerance - When MBC ≥ 32xMIC. Clinical significance is not well known, but it’s been associated with poor outcome. We don’t really look at this because we don’t routinely determine MBCs.

Question 29

How do you calculate MIC50, MIC90, geometric mean MIC, and cumulative susceptibility to describe the susceptibility of a population of microorganisms to antimicrobial agents?

Answer 29

MIC50 - the antibiotic concentration that inhibits 50% of the bacteria tested

MIC90 - antibiotic concentration that inhibits 90% of the bacteria tested

Geometric mean MIC - the antilog of the mean of the log MICs (this is a better measure of central tendency.

Question 30

What is synergy, antagonism, and additivity/indifference? What are the two major methods used to test for this effect?

Answer 30

Synergy - the activity of an antimicrobial combination is greater than that expected from the additive activity of the individual agents. (1 + 1 = 10)

Antagonism - activity of an antimicrobial combination is less than that expected from the additive activity of the individual agents ( 5 + 5 = 2)

Additivity/indifference - neither synergy nor antagonism (1 + 1 = 2)

Methods:
1. Checkerboard: measures inhibitory activity of the combination
2. Time-Kill Curves: measures rate and extent of bactericidal activity

Question 31

What is the methodology for determination of the postantibiotic effect (PAE)? What is the importance of PAE in the pharmacotherapy of certain infectious diseases?

Answer 31

Postantibiotic effect (PAE) - persistent effect (inhibition or killing) of the antibiotic on bacterial growth after brief exposure of the organism to the drug.

Methods:
- Expose bacteria to a fixed antibiotic concentration. Then we remove the drug (dilute it). The growth of exposed bacteria is compared to growth control. The difference in time required for exposed and control cultures to increase shows the PAE.

Question 32

What is the role of rapid diagnostic testing in antimicrobial stewardship?

Answer 32

It speeds up the process for the clinical team so we can treat the patient with the correct drug sooner (narrowest spectrum). For this reason, rapid diagnostics can help prevent resistance.

Question 33

What is the recent progress in rapid diagnostic testing used to identify infectious pathogens and specific resistance mechanisms?

Answer 33

We use the PCR nasal swab to test MRSA a lot. How we can quickly determine if pts have MRSA or not, which reduces vanc usage.

We have a Multiplex PCR that can detect multiple bacterial, fungal, and viral pathogens (Biofire, Verigene).

Question 34

What are the potential limitations for widespread clinical use of rapid diagnostic tests?

Answer 34

• Most assays detect select pre-specified pathogens and/or resistance mechanisms (there are many ESBLs not detected, maybe target mutations won’t be picked up)
• Many tests still rely on positive blood cultures
• Discrepant results with standard susceptibility testing
• Cost
• Fear of litigation

Question 35

What are the pharmacodynamic parameters that correlate with the clinical outcomes for β-lactams, aminoglycosides, and fluoroquinolones for animal, in vitro, and human studies?

Answer 35

β-lactams: Clinical/microbiologic outcome relates to the time the the free drug concentration remains above the MIC (fT>MIC). These are time-dependent bactericidal drugs. Generally, these do not have a post antibiotic effect in gram-nagatives (except carbapenems).
- ex. penicillins, cephalosporins, carbapenems, monobactam

aminoglycosides: concentration dependent. Outcome relate to the peak/MIC ratio (want 8-10:1), but some say AUC/MIC is better. These have a PAE for gram pos. and gram neg. bacteria. We also need to factor in adaptive resistance.

fluoroquinolones: there outcome correlates with the AUC/MIC ratio (AUC/MIC > 125 is ideal). These are rapidly bactericidal and they have a PAE for both gram pos. and neg. bacteria.
- ex. ciprofloxacin, levofloxacin, and moxifloxacin

Question 36

What are the pharmacodynamic parameters that predict outcome for macrolides, clindamycin, vancomycin, and daptomycin?

Answer 36

macrolides - time dependent. Look at AUC/MIC

clindamycin - time dependent. Look at AUC/MIC

vancomycin - time dependent bactericidal activity. The target is an AUC/MIC of 400-600 (ideal ~515). Elevated AUC (>600) is a risk factor for nephrotoxicity.

daptomycin - concentration dependent. Look at AUC/MIC and Peak/MIC.

Question 37

How would we make a therapeutic plan for a patient that takes into account the pharmacodynamics of β-lactams and fluoroquinolones?

Answer 37

β-lactams - maximize time above the MIC. This means either give a bigger dose or shorten the dosing interval. Also if the patient is overweight, they will need a higher dose to achieve the same drug concentration.

fluoroquinolones - optimize the free AUC/MIC ratio.