AST Flashcards
ANTIBIOTICS
These chemical substances are produced by microorganisms with the capacity to inhibit (_____) or kill (______) other microorganisms.
bacteriostatic
bactericidal
T or F
Antibiotics
• They can also be synthesized via chemical procedures that are independent from microbial activity.
True
is an overall profile of antimicrobial susceptibility testing results of a specific microorganism to a battery of antimicrobial drugs.
antibiogram
CLASSIFICATION OF ANTIBACTERIAL DRUGS
Natural
Semi-synthetic
Synthetic
These drugs are produced by bacteria or fungi
NATURAL DRUGS
Natural drugs ex
Amphotericin B
Erythromycin
Kanamycin
These are modified natural drugs with added chemical groups
SEMI-SYNTHETIC DRUGS
Semi-synthetic ex
Ampicillin
Carbapenicillin
Methicillin
These are chemically produced drugs
SYNTHETIC DRUGS
Synthetic ex
Sulfonamides
Trimethoprim
Ciprofloxacin
Isoniazid
CLASSIFICATION OF ANTIMICROBIAL AGENTS
Bacteriostatic
Bactericidal
Ex:
Chloramphenicol
Dapsone
Sulfonamides
Erythromycin
Tetracycline
BACTERIOSTATIC AGENTS
• These are antimicrobial agents that inhibit bacterial growth; but generally, they do not kill the microorganisms.
BACTERIOSTATIC AGENTS
• Used as a treatment for life threatening conditions
Ex: Aminoglycosides (Gentamicin, Amikacin, and Streptomycin)
BACTERICIDAL AGENTS
These are antimicrobial agents that usually kill or destroy organisms
BACTERICIDAL AGENTS
Group A
Ampicillin, Gentamicin, Tobramycin, and Cefazolin
Group B
Amikacin, Cefuroxime, Ciprofloxacin, Impenem, and Piperacillin
Group C
Aztreonam, Chloramphenicol, and
Tetracycline
Lowest concentration of drug that inhibit bacterial growth
MINIMAL INHIBITORY CONCENTRATION (MIC)
• Ratio of the toxic dose to the therapeutic dose and as such, the higher the therapeutic index, the more effective the chemotherapeutic agent
THERAPEUTIC INDEX
TYPES OF ANTIMICROBIAL AGENTS ACCORDING TO MECHANISM OF ACTION
Cell wall inhibitors
Protein synthesis inhibitors
Nucleic acid synthesis inhibitors
Cell membrane inhibitors
These drug are effective against gram positive bacteria.
CELL WALL INHIBITORS
• They inhibit the activity of transpeptidase enzymes in which cell growth stops and the death of cells often follows.
CELL WALL INHIBITORS
• These are most selective antibiotics with a higher therapeutic index
CELL WALL INHIBITORS
Cell wall inhibitors
- inhibits synthesis of peptidoglycan precursors
- inhibits transpeptidation
- can be bactericidal or bacteriostatic
- inhibits translocation and elongation of peptidoglycan
Bacitracin
B-lactams
Isoniazid
Vancomycin
Effective against M. tuberculosis, B. fragilis, N. meningitidis, H. influenza, and S.pneumoniae.
PROTEIN SYNTHESIS INHIBITORS
• These drugs target aerobic and anaerobic gram (+) and gram (-) bacteria;
PROTEIN SYNTHESIS INHIBITORS
These antibiotics bind to 30S subunit that results in the misreading of mRNA
PROTEIN SYNTHESIS INHIBITORS
Translation
(50S subunit)
• Macrolides
• Streptogramins
Translation
(30S subunit)
• Aminoglycosides
• Tetracyclines
Protein synthesis inhibitors
NUCLEIC ACID SYNTHESIS INHIBITORS
• - inhibits RNA polymerase and synthesis
• - effective for enteric bacteria (E. coli)
• - disrupts DNA and effective against anaerobic bacteria
Rifampicin
Quinolones
Metronidazole
Nucleic acid synthesis inhibitors
Rifampicin
Quinolones
Metronidazole
CELL WALL INHIBITORS
Bacitracin
B lactams
Isoniazid
Vancomycin
Polymyxin B and E are effective against gram (-) bacteria, like PAE
CELL MEMBRANE INHIBITORS
Folic Acid Inhibitors (Sulfamethoxazole)
Cord factor inhibitor (Isoniazid)
Take note that Trimethroprim and Sulfamethoxazole are synthetic drugs, they do not occur in nature.
ESSENTIAL METABOLITE INHIBITORS
ACTIONS OF ANTIMICROBIAL DRUGS
Inhibits cell wall synthesis
Inhibits cell membrane function
Inhibits protein synthesis
Inhibits nucleic acid synthesis
Inhibitors of Other Metabolic Processes
PRINCIPLE
• It measures the ability of an antibiotic or other microbial agent to inhibit bacterial growth
ANTIMICROBIAL SUSCEPTIBILITY TESTING
PRIMARY GOAL
• To determine whether the bacterial isolate is capable of expressing resistance to the antimicrobial agents selected for treatment
ANTIMICROBIAL SUSCEPTIBILITY TESTING
PURPOSE
To guide the clinician in selecting appropriate antimicrobial agent
To gather epidemiological data on microbial resistance
ANTIMICROBIAL SUSCEPTIBILITY TESTING
STANDARDIZATION
• To optimize bacterial growth conditions
• To optimize conditions for maintaining antimicrobial integrity and activity
• To maintain reproducibility and consistency in the resistance profile
ANTIMICROBIAL SUSCEPTIBILITY TESTING
DISK DIFFUSION METHOD (KIRBY-BAUER TEST)
• Medium:
Mueller-Hinton Agar
• Standard:
0.5 McFarland/ Barium Sulfate Suspension
99.5ml of 1% sulfuric acid
0.5ml 1.175% barium chloride
Standard inoculum:
1.5 x 10^8 CFU/ml
pH:
7.2-7.4
The standard susceptibility medium for non - fastidious bacteria.
MUELLER HINTON AGAR
MUELLER HINTON AGAR
Components
beef infusion
agar
nucleic acid
vitamins
casein
• An MH broth with_____ is used to improve the detection of oxacillin - resistant staphylococci.
2% NaCl
• An MH broth with ________is utilized for testing susceptibility for streptococci, N. meningitidis, and other fastidious organisms.
5% lysed horse blood or sheep blood
MHA
• Depth:
• Condition:
• Temperature:
• Incubation Time:
• Antibiotic disc:
• Petroff-Hausser:
4mm
Aerobic, No CO2
35-37°C
16-18 hours
6mm
bacterial count
• A calibrated chamber with a grid etched on its surface.
• Designed for accurate measurement of cell concentration in a given volume of liquid.
Petroff-Hausser Counting Chamber
FACTORS AFFECTING DISK DIFFUSION
Amount of inoculum
Thickness of susceptibility plate
Growth rate of organism
pH of medium
Number of disk per plate
Concentration of divalent bonds
If the agar is too thick, the zone sizes will be_____;
if the agar is too thin, the zone sizes will be_____
smaller
larger
•A temperature of_____ is best for most bacteria within______ of incubation
Temperatures higher than 35 degrees may lead to false detection of______
Lower temperature may lead to_____ ZOl Incubation with increased 5% to 10% carbon dioxide is not recommended except for capnophilic bacteria.
35 degrees; 16 - 18 hours
MRSA
larger
• Incubation of plates in CO2 could result in_______ pH.
• Increased pH (alk) results to_____ activity of tetracycline drugs.
• Low pH (acidic)______ the activity of aminoglycosides and erythromycin.
decreased (ACIDIC)
decreased
decreases
A 150mm plate can have a maximum of____ disks.
Placement of more than that may result in….
12
overlapping of zones
PROCEDURE AST
I. Pure inocula are obtained by selecting_____ colonies of the same morphology
- Colonies are suspended into____ to achieve a turbid suspension.
- Bacterial suspension &_____ solution are compared by matching the turbidity if the tubes against a____ background.
- Turn the plate____ between each streaking (Overlapping
Streaking) - Within____. of inoculation, antimicrobial agents are applied onto MHA.
- Within_____. of disk placement, plates are inverted & incubated at
35C for 16-18hrs. - Diameter of each inhibition zone is measured using a caliper or ruler.
4-5
0.85% NSS
McFarland; dark
60 degrees
15mins
15mins
MEASUREMENTS
= Disk distance
= 12 discs
= 5-6 discs
15mm
150mm
100mm
MEASUREMENTS
15mm = Disk distance
150mm =
100mm =
12 discs
5-6 discs
FALSE RESISTANT
15 mins (delay of disc application)
Increase moisture
Thick medium
Increase organisms = Decrease Zone of
Inhibition
FALSE SENSITIVE
15 mins (delay of incubation)
Increase drying
Thin medium
Decrease organisms = Increase Zone of
Inhibition
•: microorganism should respond to therapy with that antimicrobial agent
•: microorganism falls into a range of susceptibility in which the MIC approaches or exceed the level of antimicrobial agent that can be achieved and for which clinical response is likely to be less than with a suspetible strain
: no zone or small zone of inhibition - antibiotic is not the appropriate choice for treatment
Susceptible
Intermediate
Resistant
It is fully automated equipment designed for the identification of bacteria and AST
Optical readings are made every 15 minutes to measure transmitted light through each well
Final reading 6-8 hours
Vitek 2
Inocula are manually introduced to the broth microdilution tray
Growth patterns are automatically read and interpreted after incubation.
Microscan Walkaway System by Beckman Coulter
It uses manual gravity based inoculation technique.
Growth patterns are automatically read.
It has a mechanism to confirm ESBL of gram negative bacteria.
Results after 8 to 12 hours.
Phoenix System by BD Microbiology Systems
infection is caused by a strain of Staphylococcus bacteria that’s become resistant to oxacillin and methicillin
MRSA
2 TYPES OF MRSA
• Health care-associated MRSA (HA-MRSA)
• Community - associated MRSA (CA-MRSA)
Are associated with invasive procedures or devices, such as surgeries, intravenous tubing or artificial joints
Health care-associated MRSA (HA-MRSA)
Often begins as a painful skin boil. Spread by skin- to-skin contact.
At risk populations include groups such as high school wrestlers, child care workers and people who live in crowded conditions.
Community - associated MRSA (CA-MRSA)
