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inhibitory concentrations much lower than bactericidal concentrations



inhibitory concentrations similar to bactericidal concentrations
must be used when local or systemic host factors are impaired (i.e. chemo)



minimum inhibitory concentration



minimum bactericidal concentration


How are MIC and MBC determined? Methods of testing

culture and sensitivity
-disk (innoculum on agar plate) diffusion method; Disk - Inhibition Diameter: set as sensitive, intermediate, resistant
-broth dilution method (VITEK or MICROSCAN); Broth or agar with increasing antimicrobial
concentration --> Inoculated with organism --> Check for growth after 18-24 hours; if clear, bacterial inhibition --> Aliquots of clear tube can be placed in antimicrobial free solution or on agar; conc with no growth is bactericidal


Concentration-dependent killing

-rate and extent of killing increases with increasing drug concentration (aminoglycosides, quinolones)


time-dependent killing

bactericidal activity continues as long as concentration above the MIC/MBC (beta lactams)


Postantibiotic effect (PAE)

persistent suppression of bacterial growth after limited exposure to an antimicrobial agent
-compares the time it takes a bacterial culture to reach logarithmic growth after drug removal vs an untreated culture
-Reasons: recovery after reversible non-lethal damage to cell structures takes time, persistence of drug at binding site or within periplasm, need to synthesize new enzymes before new growth can occur
-may be longer in vivo due to postantibioitic leukocyte enhancement (PALE)


empiric antimicrobial therapy

-initial therapy before causative pathogen is known
-covers likely pathogens until infecting organism is identified
-obtain specimen for laboratory - gram stain and culture
-treat based on likely infecting pathogen; goal is to choose drug that is selectively active for most likely infecting microorganism and least likely to cause toxicity


definitive treatment

-once infecting microorganism identified
-based on in vitro testing for microbial identified
-use narrowest spectrum available



postantibiotic leukocyte enhancement --> causes the PAE to be longer in vivo


Spectrum of action: narrow, broad, extended

i. Narrow - only effective against one class of bacteria
ii. Broad - if they are effective against a range of bacteria
iii. Extended - if a narrow spectrum is modified chemically the new compound is effective against more bacteria than the parent compound



Appearance of a new infection during chemotherapy of the primary one. Can happen if you remove normal body flora (they normally produce antibacterial substances (bacteriocins) and they compete for essential nutrients). The more "broad" the effect of the antimicrobial, the greater the possibility that a single microorganism will become predominant, invade the host and produce infection
-ex C. diff


Pregnancy safety categorites

A - studies in pregnant women, no risk

B - animal studies no risk, but human not adequate or animal toxicity but human studies no risk
i. Beta lactams: penicillins, BCN + BLI, cephalosporins, astreonam, ertapenem, doripenem, meropenem
ii. Fosfomycin, clindamycin
iii. Macrolides: erythromycin, azithromycin
iv. Metronidazole
v. Nitrofurantoin

C - animal studies show toxicity, human studies inadequate but benefit of use may exceed risk
i. Beta lactams: impenem/cilastin
ii. Ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin
iii. Linezolid, tedizolid
iv. Macrolides: clarithromycin
v. Sulfonamides/trimethoprim
vi. Vancomycin

D - evidence of human risk, but benefits may outweigh
i. Aminoglycosides: Amikagcin, gentamicin, isepamicin, neitilmicin, streptomycin, tobramycin
ii. Tetracyclines

X - fetal abnormalities in human, risk > benefit



total antibiotic activity greater than expected sum of the two (4-fold reduction MIC)
i. Ex) block sequential metabolic steps (TMP/SMX "Bactrim"); inhibitiron of enzymatic inactivation (Beta lactamase); enhanced antimicrobial uptake (aminoglycoside-PCN)



antibiotic activity is less than activity of either drug alone
i. Ex) Bactericidal activity inhibited by static drug (Tetracyclin-PCN í PCN blocks cell wall growth but organism needs to be growing for this to work); Induction of enzyme inactivation (induce B lactamase production)



combined antibiotic activity the same as that of the more active drug (broad spectrum abx coverage í neutropenic need broad spectrum)


When to use combination therapy

i. Treatment of mixed infections
ii. Broaden empiric coverage
iii. Enhancement of antibacterial activity for specific infections (pseudomonas (?), Strep viridans endocarditis - synergy)
iv. Prevention of emergence of resistant microorganism (Tb)


Antimicrobial prophylaxis - examples of when its commonly used

a. To protect healthy persons from acquisition of specific microorganisms
b. Situations: surgical prophylaxis most common, prevention of bacterial endocarditis (pts with strucutural heart problems undergoing dental, surgical, or other invasive procedures), recurrent infections, influenza, recurrent genital herpes simplex


Mechanism of resistance: beta-lactams

A) Inactivation by beta-lactamase
1. Most common
2. Hundreds identified (may be specific to type of abtibiotic: penicillinases, cephalosporinases, metallo-beta-lactamase, carbapenemases)
3. Some bacteria produce only one form (staph, H. flu)
4. Some bacteria produce beta-lactamases that hydrolyze both penicillins and cephalosporins (pseudomonas, enterobacter)

B) Alteration in target PCN Binding Proteins (PBPs)- MRSA, strep pneumoniae, enterococci 

1. Reduced affinity - can overcome with very high concentrations
2. Resistance in one organism may result from replacement of its PBP with PBP from resistant organism

C) Permeability barrier preventing penetration of abx- Gram - organisms
1. Can down-regulate porins or not make 

2. Not so critical by itself; important with B lactamase 

D) Efflux pump- Gram - organisms 


Mechanism of resistance: vancomycin

i. Modification of the D-ala-D-ala site; terminal D replaced by D-lactate


mechanism of resistance: quinolones

i. Mutation of bacterial DNA gyrase (primarily Gram -) or topoisomerase IV (primarily Gram +) Decreased quinolone penetration into bacterial cells (active efflux pump)
ii. Can be inhibited experimentally by reserpine


mechanism of resistance: tetracyclines

i. Decreased intracellular concentration due to decrease influx or increased efflux by active transport; plasmid mediated; often multi-drug resistance
ii. Production of proteins that interfere with ribosomal binding
iii. Enzymatic inactivation


How has strep pneumonia and MRSA developed resistance to beta-lactams?

Alteration in their target PCN binding proteins
-Alteration in target Penicillin Binding Proteins (PBP’s)
-Reduced affinity – can overcome with very high
-Resistance in one organism may result from replacement of its PBP with PBP from resistant organism


what is meant by MLS type B resistance

a. Macrolide-lincomycin-streptogramin resistance (Gram +)
b. Production of methylase enzyme that modifies the ribosomal target í decreased drug binding
c. Cross resistance with clindamycin, lincomycin, and streptogramin


Desired trough levels for vancomycin

Desired levels: trough 15-20 mcg/ml


Desired trough levels for aminoglycosides

Gent, tobra, netilmicin: Peak 5-8 mcg/ml, trough ≤ 2mcg/ ml; Amikacin: peak 20-30 mcg/ml; trough 10 mcg/ml