AST Flashcards
what is the difference between a bactericidal and bacteriostatic?
bactericidal: kills bacteria
bacteriostatic: inhibits bacteria
what is it called when an organism becomes resistant to several drugs at once when a single change occurs in the bacterial cell?
cross resistance
what is constitutive resistance expression?
organism is constantly expressing the resistance mechanism
what is inducible resistance expression?
resistance is expressed when exposed to the appropriate antimicrobial agent
what is constitutive-inducible resistance expression?
organism is constantly expressing resistance at low level. and resistance at high level when exposed to appropriate antimicrobial agent
what is homo and heterogenous resistance expression?
homo: entire bacterial population expresses resistance
hetero: some bacterial cells in a population express resistance, but others do not
what are the four classes of beta-lactam antimicrobial agents?
penicillins, cephems, carbapenems, and monobactams
what is the mechanism of beta-lactams? Why are they considered bactericidal?
they inhibit cell wall synthesis by binding to penicillin binding proteins
bactericidal due to simulating autolytic enzymes that lyse bacterial cells
most __ produce penicillinase and are resistance to natural penicillins
Staph spp.
extended-spectrum penicillins are developed to treat ____, and these include what three kinds of penicillins?
treat GNB
Aminopenicillins, carboxypenicillins, and ureidopenicillins
what is one example for 1st and 2nd generation cephams?
1st gen: cefazolin
2nd gen: cephalosporins
Aminoglycosides are inhibitors of ______ ______. What are two side affects of these kinds of antimicrobials? What bacteria are they used for?
inhibit protein synthesis
side affects: nephrotoxicity and ototoxicity
used for enterics and P. aeruginosa
Vancomycin and Teicoplanin are both examples of _________. They interfere with the formation of ________ cross-links by binding to cell wall components, which means they are active against gram ____ organisms
glycopeptides
they interfere with formation of peptidoglycan cross-links
work against gram pos organisms
what are VRE and VISA/VRSA?
VRE: vanco resistance enterococci (resistance genes VanA and VanB)
Vanco-intermediate-resistance Staph aureus
Quinolones work by inhibiting ____ synthesis by binding to ___ _____. You do not give this drug to which population?
DNA
DNA gyrase
do not give to pregnant women and patients younger than 18 years old
sulfanomides and trimethoprims (SXT) are used in combination against agents that do what?
bacteria that inhibit folic acid synthesis
What spp. of bacteria have intrinsic resistance to SXT drugs and why?
Enterococci due to their ability to use exogenous folates, thymine, and thymidine
Rifampin is used to inhibit _____ synthesis and how? Used prophylactically to treat people with _______. What color body fluids can be a side affect of taking this drug?
inhibits RNA synthesis by binding to RNA polymerase
treat people with meningitis
red-orange fluids
metronidazole becomes a bactericidal when _________ (and enzyme in anaerobes) reduces the ____ group on a parent molecule that disrupts bacterial ___
when nitroreductase reduced the nitro group on a parent molecule. Disrupts bacterial DNA
metronidazole is active against ____ and ________ parasites
anaerobes and protozoan parasites
(doesn’t work on aerobes due to these not having nitroreductase)
Nitrofurantoin is used to treat ____
UTIs
bacitracin acts on cytoplasmic membrane and is active against what kind of bacteria?
GPC
polymyxins are used topically usually because they are ____- and ______. What bacteria are they active against?
neurotoxic and nephrotoxic
active against certain GNB
Isoniazid (INH) treat what kind of organisms and how?
inhibit the synthesis of mycolic acid (treats mycoplasmas) and interact with catalase or peroxidase to form free radicals, which are toxic
susceptibility testing standards are described by the ___?
CLSI (clinical and laboratory standards institute)
what four things can affect test results when using a Mueller-Hinton agar/broth?
cation concentration
pH
thymidine
blood
what is the standard turbidity for McFarland standards?
“standardized suspension”: 0.5 (has approx 1.5 x 10^8 CFU per mL)
Broth dilution tests:
macrodilution is performed in tubes that contain ___ of broth with diff concentrations of drug
microdilution is performed in microtiter trays where each well contains ___ to __ of broth
macro: 1 mL
micro: 0.05 to 0.1 mL
size of zone (when it comes of disk diffusion tests) depends on what three things?
- ability of test drug to diffuse through the agar
- susceptibility of the organism to the drug
- agar depth
positive beta-lactamase tests indicate that an isolate produces beta-lactamase, but it cannot do what?
it can’t be used to predict the organism’s susceptibility or resistance to a particular drug. But neg doesn’t mean that it’s susceptible either
Beta-lactamase is produced constantly by what five organisms?
- H. influenzae
- N. gonorrhoeae
- M. catarrhalis
- enterococci
- Bacteroides spp.
Beta lactamase is inducible by what one spp.?
Staph spp.
Nitrocefin is a _________ cephalosporin that is ___ when beta lactam ring is intact and turns ___ when the ring is hydrolyzed by beta lactamase
it’s a chromogenic cephalosporin that is yellow when ring is intact and red when the ring is broken
acidimetric beta lactamase testing uses ____ ___ to detect pH ____ when penicillin is hydrolyzed to penicilloic acid
uses phenol red
yellow = positive
red = negative
iodometric beta lactamase testing uses _____-_____ complexes (blue) to detect penicilloic acid formation
starch-iodine
(iodine reduced by penicilloic acid cannot form a color and the suspension becomes colorless, aka positive)
beta lactamase tests should NOT be performed on what two groups of bacteria?
Enterobacterales and nonfermenters
Oxacillin is used to screen for _____, and vancomycin is used to screen for ____
MRSA
VRSA
uncomplicated enterococci infections may be treated with a single agent: _________. What kind of agars can be used to screen for specific species of Enterococcus?
ampicillin
Chromogenic agars
Strep pneumo may now have ___ resistance
penicillin
what is the minimum bactericidal concentration?
measures the ability of a normally bactericidal agent to kill a test organism
- uses log-phase MIC growth method
what are schlichter tests?
measures the antibacterial activity of a patient’s serum against the patient’s infecting organism
what is a bacteriostatic/serum inhibitory test (SIT)?
is the highest serum dilution with no growth
what is serum bactericidal titer?
the highest dilution that kills 99.9% of inoculum
