Susceptibility Testing
المؤلف:
Cornelissen, C. N., Harvey, R. A., & Fisher, B. D
المصدر:
Lippincott Illustrated Reviews Microbiology
الجزء والصفحة:
3rd edition , p30-31
2025-06-08
582
After a pathogen is cultured, its sensitivity to specific antibiotics serves as a guide in choosing antimicrobial therapy. Some pathogens, such as Streptococcus pyogenes and N. meningitidis, usually have predictable sensitivity patterns to certain antibiotics. In contrast, most gram-negative bacilli, enterococci, and staphylococcal species show unpredictable sensitivity patterns to various antibiotics and require susceptibility testing to determine appropriate antimicrobial therapy.
A. Disk-diffusion method
The classic qualitative method to test susceptibility to antibiotics has been the Kirby-Bauer disk-diffusion method, in which disks with exact amounts of different antimicrobial agents are placed on culture dishes inoculated with the microorganism to be tested. The organism’s growth (resistance to the drug) or lack of growth (sensitivity to the drug) is then monitored (Figure 1). In addition, the size of the zone of growth inhibition is influenced by the concentration and rate of diffusion of the antibiotic on the disk. The disk diffusion method is useful when susceptibility to an unusual antibiotic, not available in automated systems, is to be determined.
Fig1. A. Outline of disk-diffusion method for determining the sensitivity of bacteria to antimicrobial agents. B. Photograph of culture plate with antibiotic-impregnated disks.
B. Minimal inhibitory concentration
Quantitative testing uses a dilution technique in which tubes containing serial dilutions of an antibiotic are inoculated with the organ ism whose sensitivity to that antibiotic is to be tested. The tubes are incubated and later observed to determine the minimal inhibitory concentration (MIC) of the antibiotic necessary to prevent bacterial growth (Figure 2). [Note: MICs are now automated and often done simultaneously with automated biochemical identifications.] To provide effective antimicrobial therapy, the clinically obtainable antibiotic concentration in body fluids should be greater than the MIC. Quantitative susceptibility testing may be necessary for patients who either fail to respond to antimicrobial therapy or who relapse during therapy. In some clinical cases, the minimal bactericidal concentration may need to be determined. This is the lowest concentration of antibiotic that kills 100 percent of the bacteria, rather than simply inhibiting growth.
Fig2. Determination of minimal inhibitory concentration (MIC) of an antibiotic.
C. Bacteriostatic versus bactericidal drugs
As noted above, antimicrobial drugs may be bacteriostatic or bactericidal. Bacteriostatic drugs arrest the growth and replication of bacteria at serum levels achievable in the patient, thereby limiting the spread of infection while the body’s immune system attacks, immobilizes, and eliminates the pathogens. If the drug is removed before the immune system has scavenged the organisms, enough viable organ isms may remain to begin a second cycle of infection. For example, Figure 3shows a laboratory experiment in which the growth of bacteria is arrested by the addition of a bacteriostatic agent. Note that viable organisms remain even in the presence of the bacteriostatic drug. By contrast, addition of a bactericidal agent kills bacteria, and the total number of viable organisms decreases. Although practical, this classification may be too simplistic because it is possible for an antibiotic to be bacteriostatic for one organism and bactericidal for another (for example, chloramphenicol is bacteriostatic against gram negative rods and bactericidal against pneumococci).
Fig3. Effects of bactericidal and bacteriostatic drugs on the growth of bacteria in vitro.
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