Introductory Microbiology

Maintenance media

• Satisfactory maintenance of the viability and physiological characteristics of a culture over time may require a medium different from that which is optimum for growth. Prolific, rapid growth may also be associated with rapid death of the cells at the end of the growth phase. For example, glucose in a medium frequently enhances growth, but acid harmful to the cells is likely to be produced. Therefore, omission of the glucose is preferable in a maintenance medium.

Based on consistency, media are classified into two types.
Solid media
In addition to liquid media, solid and semi-solid media are useful for isolating bacteria or for determining the characteristics of colonies. The solidifying agent is usually agar, which at concentrations of 1.5 to 2.0 percent forms firm, transparent gels that are not degraded by most bacteria. Silica gel is sometimes used as an inorganic solidifying agent for autotrophic bacteria. e.g., Nutrient agar
Semi-solid media
Semi-solid media, prepared with agar at concentrations of 0.50 percent or less have a soft, custard like consistency and are useful for the cultivation of microaerophillic bacteria or for determination of bacterial motility. e.g.,Nitrogen free malic acid BTB medium

Based on nature and chemical composition, the media are classified into two types.
i) Synthetic or chemically defined medium: Those media are prepared by mixing all the pure chemicals of known composition, for eg. Czapek dox medium.
ii) Semi-synthetic or un-defined medium: Such are those media, where exact chemical composition is unknown, e.g. potato dextrose agar.