AST Flashcards
Antibiotic produced by Bacillus subtilis
Bacitracin
Antibiotic produced by Bacillus polymyxa
Polymyxin
Microorganism that produces Cephalosporin
Cephalosporium
Microorganism that produces Penicillin
Penicillium notatum
Microorganism that produces Erythromycin
Streptomyces erythraeus
Microorganism that produces Neomycin
Streptomyces fradiae
Antibiotic produced by Micronospora purpurea
Gentamicin
Microorganism that produces Amphotericin B
Streptomyces nodusus
Antibiotic produced by Streptomyces noursei
Nystatin
Microorganism that produces Chloramphenicol
Streptomyces venezuelae
Classification of antibacterial drugs
Natural, Semi-synthetic, Synthetic
Drugs produced by bacteria or fungi
Natural Drugs
Examples of natural drugs
(AKE)
Amphotericin B
Kanamycin
Erythromycin
Define semi-synthetic drugs
Modified natural drugs with added chemical groups
Examples of semi-synthetic drugs
(CAM)
Carbapenicillin
Ampicillin
Methicillin
Chemically produced drugs
Synthetic Drugs
Examples of synthetic drugs
(SITC)
Sulfonamides
Isoniazid
Trimethoprim
Ciprofloxacin
1st Gen Drugs
(RIPES)
Rifampicin
Isoniazid
Pyrazinamide
Ethambutol
Streptomycin
2nd Gen Drugs
(CCOKE)
Capreomycin
Ciprofloxacin
Ofloxacin
Kanamycin
Ethionamide
Antimicrobial agents that inhibit bacterial growth; Stops multiplication via binary fission
Bacteriostatic Agents
Examples of bacteriostatic agents
(DECTS)
Dapsone
Erythromycin
Chloramphenicol
Tetracycline
Sulfonamides
Antimicrobial agents that kill microorganisms; Treatment for life-threatening conditions; Disrupts bacterial structure
Bactericidal Agents
Examples of bactericidal agents
Aminoglycosides (GAS)
*Gentamicin
*Amikacin
*Streptomycin
Group A Primary Test (Drugs)
(CATG-CATriona Gray)
Cefazolin
Ampicillin
Tobramycin
Gentamicin
Group B Primary Test Selectivity (Drugs)
(PICCA)
Piperacillin
Imipenem
Cefuroxime
Ciprofloxacin
Amikacin
Group C Supplemental Report Collectively
(CAT)
Chloramphenicol
Aztreonam
Tetracycline
Define MIC
(Minimal Inhibitory Concentraton)
Lowest concentration of drug that inhibits bacterial growth
Increasing dilution –> smaller activity of drugs against organisms
Define MBC
(Minimal Bactericidal Concentration)
Lowest concentration of drug that kills bacteria
Ratio of the toxic dose to the therapeutic dose; Ratio of how much dosage is safe and lethal
Therapeutic Index
Higher TI –> more effective chemotherapeutic agent
What are the mechanisms of action of antibacterial agents?
Cell Wall Inhibitors
Protein Synthesis Inhibitors
Nucleic Acid Inhibitors
Cell Membrane Inhibitors
Essential Metabolite Inhibitors
Most selective antibiotics with a higher therapeutic index
Cell Wall Inhibitors
Inhibit the activity of transpeptidase enzymes in which cell growth stops and death of cells often follows
Cell Wall Inhibitors
These drugs are effective against Gram (+) bacteria
Cell Wall Inhibitors
Give the examples of cell wall inhibitors and their action
*Bacitracin (inhibits synthesis of peptidoglycan precursors)
*B-lactams (inhibits transpeptidase)
*Isoniazid (can be bactericidal or bacteriostatic)
*Vancomycin (inhibits translocation and elongation of peptidoglycan)
These antibiotics bind to 30S subunit (smaller) that results in the misreading of mRNA.
Protein Synthesis Inhibitors
These drugs target aerobic and anaerobic, gram (+) and gram (-) bacteria
Protein Synthesis Inhibitors
Protein synthesis inhibitors are effective against what examples of microorganisms?
(FIMPT)
Bacteroides fragilis
Haemophilus influenzae
Neisseria meningitidis
Streptococcus pneumoniae
Mycobacterium tuberculosis
Examples of protein synthesis inhibitors
Aminoglycosides, Tetracycline
Examples of nucleic acid inhibitors and their action
*Rifampicin (inhibits RNA polymerase and synthesis)
*Quinolones (effective against enteric bacteria like E. coli)
*Metronidazole (treatment for amoeba, disrupts DNA, effective for anaerobic bacteria)
What drugs are effective against Gram (-) bacteria
Cell Membrane Inhibitors
Examples of cell membrane inhibitors
Polymyxin B and E
First mutant bacteria
Pseudomonas aeruginosa
This targets the permeability of cell walls
Cell Membrane Inhibitors
Examples of essential metabolite inhibitors
Folic Acid (Sulfamethoxazole)
Cord Factor (Isoniazid)
*For TB; Virulence Factor
Principle of AST
Measures the ability of an antibiotic or other microbial agent to inhibit bacterial growth
What is an antibiogram
Profile of the susceptibilities of specific pathogenic bacteria against antimicrobial agents
Primary goal of AST
To determine whether the bacterial isolate is capable of expressing resistance to the antimicrobial agents selected for treatment
Purpose of AST
*To guide the clinician in selecting appropriate antimicrobial agent
*To gather epidemiologic data on microbial resistance
Purpose of standardization
*To optimize bacterial growth conditions
*To optimize conditions for antimicrobial integrity and activity
*To maintain reproducibility and consistency of the resistance profile
Most common method in Antimicrobial Susceptibility Testing
Disk Diffusion Method (Kirby-Bauer Test)
Medium used in Kirby-Bauer Test
Mueller-Hinton Agar
Standard used in DDM and its components
0.5 Mcfarland/Barium Sulfate Suspension
*99.5 mL of 1% Sulfuric Acid
*0.5 mL or 1.175% Barium Chloride
Standard inoculum in DDM
1.5 x 10^8 organisms/mL
pH of DDM
7.2-7.4 (slightly alkaline)
Depth of plate in DDM
4 mm
Condition of DDM
Aerobic (NO CO2)
Temperature of DDM
35-37*C
Incubation time in DDM
16-18 hours
Diameter of antibiotic disk
6 mm
Bacterial count in DDM
Petroff-Hausser
If sample is too turbid then Mcfarland, what do you do?
Dilute and add NSS
If sample is too diluted than Mcfarland, what do you do?
Add colony
The standard susceptibility medium for non-fastidious bacteria
Mueller-Hinton Agar
Composition of MHA
(BANCV)
Beef infusion
Agar
Nucleic Acid
Casein
Vitamins
Used to improve the detection of oxacillin resistant staphylococc
MH broth with 2% NaCl
Utilized for testing susceptibility for streptococci, N. meningitidis, and other fastidious organisms
MH broth with 5% lysed horse or sheep blood
Where is sheep’s blood collected?
Jugular vein
Preferable when preparing culture media for BAB due to the large RBC compared to humans
Sheep’s blood
Alternative method of testing for MRSA aside from adding 2% NaCl
Plate containing 6 ug/ml of oxacilin in Mueller-Hingon Agar supplemented with 4% NaCl
Factors affecting disk diffusion
(ATGpNC)
*Amount of inoculum
*Thickness of susceptibility plate
*Growth rate of microbe
*pH of medium
*Number of disks per plate
*Concentration of divalent bonds
If the agar is too thick –>
Smaller ZOI
If the agar is too thin –>
Larger ZOI
If the temperature is higher than 35*C –>
False detection of MRSA (due to mutated mecA gene)
If the temperature is lower than 35*C –>
Larger ZOI
Incubation of plates in CO2 leads to –>
Decreased pH
Increased pH –>
Decreased activity of tetracycline drugs
Decreased pH –>
Decreased activity of aminoglycosides and erythromycin
Placement of more than 12 may result in –>
Overlapping of zones
Can affect the testing of aminoglycosides and
tetracycline against PAE.
Concentration of divalent bonds (Ca ang Mg)
Procedure for AST
- Pure inoculum are obtained by selecting 4-5 colonies of the same morphology.
- Colonies are being suspended into 0.05% NSS to achieve a turbid suspension.
- Bacterial suspension & Mcfarland solution are compared by matching the turbidity of the tubes against a dark background.
- Perform Lawn Streaking. Turn the plate 60 degrees between each streaking
- Within 15 minutes of inoculation antimicrobial agents are applied into your MHA.
- Within 15 minutes. of disk placement, plates are inverted & incubated at 35C for 16-18hrs.
- Diameter of each inhibition zone is measured using a caliper or ruler.
If there is no ZOI –>
Measure the size of the antibiotic disk in mm. Do NOT report as zero
Distance between each disk
15 mm
How many discs fit in 150 mm?
12
How many discs fit in 100 mm?
5-6
Causes of false resistant AST
*15 mins delay (disc application)
*increased moisture
*too thick medium
*increased organisms –> decrease ZOI
Causes of false sensitive
*15 mins delay (incubation)
*increased drying
*too thin
*decreased organisms –> larger ZOI
Define susceptible
Microorganisms should respond to therapy with that antimicrobial agent; Effective against the microorganism
Define intermediate
*MIC approaches or exceeds the level of antimicrobial agent that can be achieved
*Clinical response is likely to be less than with a susceptible strain
*Can be both effective or resistant against the organism
Define resistant
No or small ZOI. Antibiotic is not the appropriate choice for treatment
In case the organism is susceptible to 2 antibiotics, what should you do?
Determine the drug that works best for the px
It uses manual gravity based inoculation technique; Has mechanism to read ESBL; Results after 8-12 Hours
Phoenix System
Inocula are manually introduced to the broth microdilution tray
Microscan Walkaway System
Fully automated equipment that is designed for the identification of bacteria and AST; Optical readings every 15 minutes; Final reading is 6-8 hours
Vitek System
2 types of MRSA and define
*Healthcare Associated MRSA (due to invasive procedures or devices such as surgeries, artificial joints, intravenous tubing)
*Community Associated MRSA (begins as a painful skin boil, spread by skin-to-skin contact, at risk populations are crowded areas)
The only difference between CA and HA MRSA
location where it is acquired
Tests for MRSA that are recommended by CLSI
(BCLPCN)
*Broth microdilution testing
*Cefoxitin disk screen test
*Latex agglutination for PBP2a (Penicillin‐binding protein 2a)
*Plate containing 6 ug/ml of oxacilin in Mueller-Hingon Agar supplemented with 4% NaCl
*Chromogenic agars
*Nucleic acid amplification test (PCR) –> detects mecA gene
