AST Flashcards by Kyra Catedral

ANTIBIOTICS
These chemical substances are produced by microorganisms with the capacity to inhibit (_____) or kill (______) other microorganisms.

bacteriostatic

bactericidal

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T or F

Antibiotics

• They can also be synthesized via chemical procedures that are independent from microbial activity.

True

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is an overall profile of antimicrobial susceptibility testing results of a specific microorganism to a battery of antimicrobial drugs.

antibiogram

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CLASSIFICATION OF ANTIBACTERIAL DRUGS

Natural
Semi-synthetic
Synthetic

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These drugs are produced by bacteria or fungi

NATURAL DRUGS

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Natural drugs ex

Amphotericin B
Erythromycin
Kanamycin

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These are modified natural drugs with added chemical groups

SEMI-SYNTHETIC DRUGS

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Semi-synthetic ex

Ampicillin

Carbapenicillin

Methicillin

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These are chemically produced drugs

SYNTHETIC DRUGS

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Synthetic ex

Sulfonamides
Trimethoprim
Ciprofloxacin
Isoniazid

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CLASSIFICATION OF ANTIMICROBIAL AGENTS

Bacteriostatic

Bactericidal

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Ex:
Chloramphenicol
Dapsone
Sulfonamides
Erythromycin
Tetracycline

BACTERIOSTATIC AGENTS

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• These are antimicrobial agents that inhibit bacterial growth; but generally, they do not kill the microorganisms.

BACTERIOSTATIC AGENTS

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• Used as a treatment for life threatening conditions

Ex: Aminoglycosides (Gentamicin, Amikacin, and Streptomycin)

BACTERICIDAL AGENTS

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These are antimicrobial agents that usually kill or destroy organisms

BACTERICIDAL AGENTS

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Group A

Ampicillin, Gentamicin, Tobramycin, and Cefazolin

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Group B

Amikacin, Cefuroxime, Ciprofloxacin, Impenem, and Piperacillin

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Group C

Aztreonam, Chloramphenicol, and
Tetracycline

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Lowest concentration of drug that inhibit bacterial growth

MINIMAL INHIBITORY CONCENTRATION (MIC)

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• 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

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TYPES OF ANTIMICROBIAL AGENTS ACCORDING TO MECHANISM OF ACTION

Cell wall inhibitors
Protein synthesis inhibitors
Nucleic acid synthesis inhibitors
Cell membrane inhibitors

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These drug are effective against gram positive bacteria.

CELL WALL INHIBITORS

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• They inhibit the activity of transpeptidase enzymes in which cell growth stops and the death of cells often follows.

CELL WALL INHIBITORS

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• These are most selective antibiotics with a higher therapeutic index

CELL WALL INHIBITORS

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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 2. Colonies are suspended into____ to achieve a turbid suspension. 3. Bacterial suspension &_____ solution are compared by matching the turbidity if the tubes against a____ background. 4. Turn the plate____ between each streaking (Overlapping Streaking) 5. Within____. of inoculation, antimicrobial agents are applied onto MHA. 6. Within_____. of disk placement, plates are inverted & incubated at 35C for 16-18hrs. 7. 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)