General Microbiology

Inoculation and Isolation Technique

Written by MicroDok


Inoculation is a microbiology technique which is used to introduce or place specimens and microbial cultures on or into a culture medium. The organism to be transferred into or onto the surface of the culture media (inclusive of solid and liquid or broth media) is known as inoculum. Inoculation is performed in the microbiology laboratory with an important piece of apparatus known as the inoculating loop (Figure 1). Sometimes or in some quarters, inoculating loop can also be referred to as wire loop. Inoculation technique allows microbiologists to obtain pure cultures of microorganisms through a process known as streaking. Streaking technique is explained in the subsequent section.

Figure 1: Inoculating loop. Microorganisms or specimens are picked up with the loop (arrow) and transferred onto sterile culture media or other reagents. A loopful or speck of the microbial culture or specimen is obtained with the loop or tip of the wire which is adequate for any given experiment and not a mass of the sample.


Isolation technique is a microbiology procedure which is used to obtain pure cultures of microorganisms especially from a mixed culture or specimen. It includes culturing microorganisms or specimens on selective media which allow the growth of particular organisms. After culturing, bacterial growth is often expressed on solid culture media as colonies which may contain contaminants and representative colonies of the organism of interest (Figure 2). But through subsequent subculturing onto fresh culture media, pure cultures of the organism of interest can easily be obtained via isolation as one kind of bacterium known as pure cultures. Pure cultures in microbiology are cultures or microbial growth in which all the microorganisms on solid culture media are of the same strain or species. Pure cultures are also known as axenic cultures; and they are mostly used for further studies in the laboratory such as in performing antimicrobial susceptibility testing (AST) and other biochemical studies. They usually appear as discrete colonies on culture media plate; and isolated colonies of bacteria are often the best for further microbiological tests in the laboratory especially for performing antimicrobial susceptibility studies and other molecular studies (e.g. amplification of bacterial gene by PCR).

Figure 2: Illustration of isolated or discrete colonies (arrows) of bacteria on growth media plate. Discrete colonies of bacteria are pure cultures of microorganisms that are of the same species or strain. Pure cultures are different from mixed cultures which are usually made up of diverse microbial community. Isolated colonies (i.e. pure culture) appear on culture media plate after incubation.


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