This method is commonly used to obtain pure cultures of those microorganisms that have not yet been successfully cultivated on solid media and grow only in a liquid media. A microorganisms that predominates in a mixed culture can be isolated in pure form by a series of dilutions. the inoculum is subjected to serial dilution in a sterile liquid medium, and a large number of tubes of sterile liquid medium are inoculated with aliquotes of each successive dilution. the aim of this dilution is to inoculate a series of tubes with a microbial suspension, so dilute that there some tubes showing growth of only one individual microbe.
for convenience, suppose we have a culture containing 10 ml of liquid medium containing 1000 microorganisms i,e 100/ml of the liquid medium if we take out 1 ml of this medium and mix it with 9 ml of fresh sterile liquid medium, we would then have 100 microorganisms in 10 ml or 10 microorganisms/ml. if we add 1 ml of this suspension microbes to another 9 ml of fresh sterile liquid medium, each ml would now contain a single microorganisms. if this tube shows any microbial growth, there is a very high probability that this growth has resulted from the introduction of a single microorganism in the medium and represents the pure cultures of that microorganisms.
Hence from this method we can cultivate a pure single strain of microbes.
