SCREENING METHODS: Checkerboard method

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Checkerboard method

In other words, the concentration of a given compound needed to inhibit growth is given a value of 1, and the amount of the compound needed to inhibit growth when combined with another antimicrobial agent at a given concentration is expressed as a fraction. Most researchers add FICs for individual agents in a combination to produce a FIC Index (Barry, 1976). Calculation of a combine FIC yields a single number which can be indicative of additive, synergistic, or antagonistic effects. Theoretically, a combined FIC near 1 indicates additivity, <1 indicates synergy, and >1 indicates antagonism. However, the degree to which a value must be greater than or less than 1 to suggest antagonism or synergism is not clearly established (Plate-5).

Later, Orhan et al., (2005) represented a combination of antibiotic and crude extracts. The antibiotic in combination with the crude drug was serially diluted along with antibiotic and crude drugs. An inoculum with specific standard are prepared with a specific strain of Mycobacterium. The method is same as broth microdilution but it will differentiate by treatment with combination of crude drug and a specific antibiotic. Synergy is more likely to be expressed when the ratio of the concentration of each antibiotic to the MIC of that antibiotic was same for all components of the mixture. The XFICs were calculated as follows:^FIC = FIC A + FIC B, where FIC A is the MIC of drug A in the combination/MIC of drug A alone, and FIC B is the MIC of drug B in the combination/MIC of drug B alone. The combination is considered synergistic when the ^FIC is <0.5and antagonistic when the ^FIC is >2 (Orhan et al., 2005; Panda et al., 2010a).
plate5Screening Methods in the Study-8
Plate 5: Graph showing time kill kinetics and combination studies
a. Isobolograms displaying the three types of results possible with combination of antimicrobials
b. Type of growth and inhibition curves possible when a microorganism is Exposed to an plant extract/antimicrobials over time (Source-Davidson and Parish, 1989)
c. Time kill kinetics against S. epidermidis (Panda et al., 2011)

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