What are the common methods used to isolate microbes from mixed cultures?

Microbes are found mixed with many other forms and types of life in nature. In order to identify and study a specific microbe, it first needs to be isolated from the mixed culture. 

These are the 6 most common methods used to isolate microbes from mixed cultures: 

• Streak Plate Method: This method is widely used to isolate pure cultures of bacteria. A small amount of mixed culture, which is placed on the tip of an inoculation loop, is streaked across the surface of the agar medium. The successive streaks reduce the density of the inoculum sufficiently so that the micro-organisms are distinctly isolated from one another. This method is based on the principle that streaking establishes a dilution gradient across the surface of a Petri plate, facilitated by the deposition of bacterial cells onto the agar surface.

• Pour Plate Method: In the pour plate method, the mixed culture is diluted directly by placing it in tubes containing melted agar medium. The melted agar medium and bacteria are mixed well and plated. The pour plate method is based on the principle that diluting the inoculum in liquefied agar medium allows the bacterial cells to get more thoroughly dispersed within the medium.  

• Spread Plate Method: In the spread plate isolation method, the mixed culture is diluted in a series of tubes containing sterile liquid, usually water or physiological saline. The medium is incubated by placing a drop of diluted liquid from each tube in the center of an agar plate and spreading the drop evenly over the surface to encourage colonies to develop on the agar plates. As colonies develop on the agar plates, certain plates display well-isolated colonies due to the individual microorganisms becoming separated through dispersion across the diluted liquid on the plate's medium. The isolated colonies are then selected and transferred onto fresh medium to ensure their purity. This technique exclusively yields surface colonies unlike the pour plate methods. The advantage of this method is that microorganisms are not required to withstand the temperature of the liquefied agar medium.

• Serial Dilution Method: Serial dilution is a commonly-used isolation method for obtaining pure cultures of those microorganisms that grow exclusively in liquid media and have yet to be successfully cultivated on solid media. The serial dilution method is based on the principle that the pure form of the dominant microorganism within a mixed culture can be obtained by subjecting the culture to a series of dilutions. The goal is to inoculate a series of tubes with a microbial suspension so dilute that some tubes exhibit the growth of only one single microbe. If a tube shows any microbial growth, there’s a high likelihood that this growth has resulted from a single microorganism in the medium, essentially representing its pure culture.

• Single Cell Isolation Methods: Single cell isolation methods aim to pick out an individual target cell from a mixed culture and allow the selected cell to grow. There are two types of single cell isolation methods: Capillary pipette method and Micromanipulator method. In both methods a drop that contains only one microorganism is isolated from the mixed culture and used to establish a pure culture of the required microorganism. 

• Enrichment Culture Method: In this isolation technique, a special cultural environment is created by adding a specific nutrient in the medium and by changing the physical conditions of the incubation. This new environment allows the desired microbes to grow and thrive while deterring the growth of undesirable microorganisms. The enrichment culture method is typically used for isolating microbes that are present in smaller numbers or that have slower growth rates as compared to other species present in the mixed culture.