Tiêu đề 16. MICROBIOLOGICAL CULTURE SENSITIVITY TESTS.pdf ✅

PHARMD GURU Page 1 SENSITIVITY TESTING: It is the degree of activity of the selected antimicrobial agent against the infecting bacterial strains. INTRODUCTION: • Usually almost all the bacteria in infectious disease are drug resistant. • Hence sensitivity test is performed to select the correct antimicrobial drug of choice. • It may also help to identify the pathogen. LIMITATIONS: • It helps us to measure only the antimicrobial activity against bacteria under laboratory conditions and not in the patients. • The patient’s clinical condition, type and site of infection, drug hypersensitivity ADME, characters of the patients are not taken in to consideration in sensitivity testing techniques. Sensitivity testing can be performed by: 1) Diffusion technique and 2) Dilution technique. 1) DIFFUSION TECHNIQUE: 1) AGAR DIFFUSION SENSITIVITY TEST: a) A disc of blotting paper is impregnated with a known volume and appropriate concentration of antimicrobial placed on a plate of sensitivity agar inoculated with test organism. b) The antimicrobial diffuses from the disc in to the medium. After 24 hours, the culture is examined for areas of growth around the disc. c) Growths of the sensitive strains are inhibited for a distance while for resistant strains it grows up to the edge of the disc. MICROBIOLOGICAL CULTURE SENSITIVITY TESTS
PHARMD GURU Page 2 d) The zone of inhibition caused by the antimicrobial is compared with the control. e) The volume, moisture content, PH, constituent of agar medium, concentration, storage and application of dose influence the diffusion technique. f) Agar diffusion sensitivity tests are carried out either by Kirby-Bawer (KB) method, ICS method or by Stocks method. g) Modified KB method is recommended by the National Committee for clinical Laboratory Standards (NCCLS) and the WHO. 2) DILUTION SENSITIVITY TESTS: Dilution sensitivity tests usually measures the minimum inhibitory concentration (MIC) or minimum bactericidal concentration (MBC) required to kill the bacteria. a) Here dilutions of antimicrobials are added to the broth or agar. b) A standardized inoculum of test organism is added. c) After overnight the lowest antimicrobial required to prevent visible growth is taken in to consideration. Dilution technique needs:  Careful standardization  Broth and agar medium  Antimicrobial solution  Incubation time and  Dilution time.
PHARMD GURU Page 3 General requirements for sensitivity testing: 1. SENSITIVITY TESTING AGAR:  Suitable media include Mueller Hinton agar, Iso sensitest agar and Gibco sensitivity testing agar no.2.  Meat infusion 2.0 g/l  Casein hydrolysate 17.5 g/l  Starch 1.5 g/l  Agar-agar 13.0 g/l 2. ANTIMICROBIAL DISC:  This disc should be refrigerated at a temperature instructed by the manufacturer.  This should not be used after expiry date.  The working stock disc should be warmed to room temperature, avoid keeping in direct sunlight. ANTIMICROBIAL RESISTANCE:  Antimicrobial resistance can arise in bacteria in several ways.  Microbes acquire resistance after a change in their DNA. Such changes may occur by : a) Genetic mutation i.e. by alteration in the structure of their own DNA. b) Genetic exchange i.e. by acquisition of extra- chromosomal DNA from other bacteria. DRUG RESISTANCE: It refers to unresponsiveness of a micro-organism to an antimicrobial agent. They are of 3 types: 1) Natural resistance 2) Acquired resistance
PHARMD GURU Page 4 3) Cross resistance. NATURAL RESISTANCE:  Some microbes have always been resistant to certain AMA.  They lack the metabolic process or the target site which is affected by the particular drug. Ex: a) gram negative bacilli are normally unaffected by Pencillin G b) M.tuberculosis is insensitive to tetracyclines. ACQUIRED RESISTANCE: It is the development of resistance by an organism (which was sensitive before) due to the use of an AMA over a period of time. CROSS RESISTANCE: 1) Cross-resistance is the tolerance to a usually toxic substance as a result of exposure to a similarly acting substance. 2) It is a phenomenon affecting e.g. pesticides and antibiotics as an example.