SUMMARY: The development and mechanism of formation of Mycoplasma and bacterial L-form colonies were studied. The micro-organisms penetrate into the agar within a few hours after inoculation. It is suggested that penetration is caused by capillary forces which draw the minute plastic organisms into the dried agar gel, together with the water surrounding them. Penetration does not take place when the agar surface is very moist. The organisms appear to develop in the interstices of the fibrillar network of the agar gel and form a firm and elastic ball-like colony growing in all directions. When reaching the agar surface the growth spreads into the thin free water film which covers the agar, forming the peripheral zone typical of mature colonies. Factors which decrease the thickness of the free water film, like drying the medium or increasing the agar concentration, cause a decrease of the peripheral zone dimensions. Factors which retard growth, such as high concentrations of inorganic salts or hydrogen ions, inhibit the formation of the peripheral zone by limiting the initial ball-like growth inside the agar, rendering it incapable of reaching the agar surface. Typical colonies composed of both central and peripheral zones appear also on other fibrillar media such as the cellulose pellicle of and gelatin.


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