Principles of Antimicrobials

What is an antibiotic?

substances produced by microorganisms that inhibit or kill other microorganisms

What is a antimicrobial agent?

include synthetic, semisynthetic and natural substances that kil or inhibit the growth of a microorganism

What is bactericidal?

the organism is killed; mainly attacks the cell wall

What is bacteriostatic?

the organisms are prevented from growing; mainly attacks protein synthesis

Why are bactericidal antibiotics better for an immunocompromised patient?

bacteriostatic antibiotics needs an intact immune response

What is the ratio of minimal bactericidal concentration to minimal inhibitory concentration (MBC to MIC) in bacteriostatic agents?

large

What is the ratio of minimal bacteristatic concentration to minimal inhibitory concentration (MBC to MIC) in bactericidal agents?

small

Between bacteriostatic and bactericidal, which is more prone to causing superinfection? Why?

bactericidal due to altering normal flora

What are the commonly infecting organisms in the genitourinary tract?

gram-negative aerobes

What are the commonly infecting organisms in abdominal infections?

gram-negative aerobes and then anaerobes

What are the antibiotics choices for gram positive aerobes?

B-lactam, cephalosporins, trimethoprim-sulfa, quinolone

What are the antibiotics choices for gram negative aerobes?

quinolone, 3rd generation cephalosporins, trimethoprim-sulfonamides, aminoglycosides

How long does it take to get incubation identification and sensitivity report on a standard aerobic culture?

preliminary report in 24 hrs and ID/sensitivity in 2-3 days

How long does it take to get incubation identification and sensitivity report on a standard anaerobic culture?

incubation/report in ~7 days

What is a semi-quantitative (disk diffusion)?

antibiotic sensitivity is determined by measuring the radius of the zone of inhibition and comparing to a laboratory standard for organism

What is the quantitative method (dilution/agar)?

minimal ratios

What is minimal inhibitory concentration (MIC)?

the lowest concentration of antibiotic that prevents visible growth after 18-24 hours incubation

What is minimal bactericidal concentration?

the lowest concentration that kills bacteria

What does a MIC greater than (>) local drug concentration tell you?

no effect: resistant

What does a MIC equal to (=) local drug concentration tell you?

doubtful: intermediate

What does a MIC less than (<) local drug concentration tell you?

successful therapy: susceptible

What is the epsilometer test?

microbial agent is microencapsulated on the back of a plastic strip and placed in agar plate to diffuse out; usually used for anaerobic bacteria

What is a breakpoint?

MIC selected to predict clinical outcome for a specific pathogen, in a specific disease, in a specific species, given a specific regimen (dose, route, duration, frequency); cut off used to interpret the MIC

What should you keep in mind when medicating a newborn with an antimicrobial?

sulfonamides cause CNS disorder

What should you keep in mind when medicating a young animal with an antimicrobial?

tetracyclines cause retardation of bone growth and discoloration of teeth

In regards to toxicity, what classification of antimicrobials is known to have allergic reactions?

beta-lactam

What toxicity can be seen with procaine?

CNS side effects

What animals are sensitive to procaine-penicillin?

guinea pigs, birds, snakes, turtles

In regards to toxicity, what classification of antimicrobials is known to have ototoxicity and nephrotoxicity?

aminoglycosides

In regards to toxicity, what classification of antimicrobials is known to cause teeth mottling/discoloration and impaired bone development in fetus and young animals? Along with hepatotoxicity and nephrotoxicity.

tetracycline

What is the toxicity associated with doxycycline in cats?

esophageal strictures

What is the toxicity associated with doxycycline when given IV to horses?

cardiac toxicity

What causes retinal degeneration and permanent blindness in cats?

enrofloxacin

What is a post-antibiotic effect?

persistent suppression of bacterial growth that continues after an antibiotic has been completely removed or its concentration in the body drops below the minimum inhibitory concentration

What is an concentration-dependent antibiotic?

high plasma concentration (Cmax) relative to the MIC is the major determinant of clinical efficacy, area under the curve determines efficacy; long dosing intervals

What is a time-dependent antibiotic?

time that the antimicrobial concentration exceeds the MIC determines efficacy, objective is to keep the average plasma drug concentration above the MIC of pathogen, lack post-antibiotic effect

What is a drug combination that show synergism?

b-lactams and aminoglycosides

What is a drug combination that prevents rapid development of resistance?

erythromycin plus rifampin against R. equi infections

What is an example of synergistic activity by inhibition of sequential pathways toward the same target?

potentiated sulfonamides

What is a super infection?

appearance of bacteriological and clinical evidence of a new infection during the therapy of a primary one