Culture Media-Part 2-Semisolid Media- Dr Meera C R

 

Semisolid media have a consistency between Solid and liquid media. It has a soft custard or jelly like appearance.Its components are same as solid media. Only difference is in the concentration of agar used. In solid media 2% agar is used, ie: 2gm in 100 ml of the media. In semisolid media agar concentration used is 0.5% or less. They are used in demonstration of bacterial motility and to grow microaerophilic bacteria.

Bacterial motility is due to the presence of locomotory organelles like flagella. In motility test, bacteria is stabbed using a straight inoculation needle in to the semisolid media and incubated at 37⁰C overnight. After incubation tubes are observed for bacterial growth pattern. Non motile organisms will grow confined to the stab-line, having sharply defined margins and leave the surrounding medium clearly transparent. Motile organisms will move from the line of stab producing diffuse, misty growths that spread throughout the medium rendering it slightly opaque. 

Microaerophiles are bacteria that cannot tolerate atmospheric concentration of oxygen. However they need oxygen for their growth. These groups of bacteria can grow under reduced oxygen (5% to 10%) and increased carbon dioxide (8% to 10%) concentration. Higher amount of oxygen is inhibitory to their growth. Examples: Campylobacter jejuni, Helicobacter pylori, etc. Microaerophiles when inoculated to semisolid media, will move from the surface to the just lower layer where oxygen penetration is only in the required concentration.

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