Bacterial Cultures – Optimal Conditions


Basic methods of laboratory diagnosis in microbiology presume the isolation of the microbial culture for its further identification (i.e., the determination of microbial genus and species).

To aim this, in laboratory conditions the bacteria are cultured in various nutrient media at a constant optimal temperature.

The optimal temperature range is of great value for the successful propagation of bacterial cells.

Depending on temperature, suitable for their growth and reproduction, all microbial agents are divided into several groups: – psychrophilic microorganisms (Gk. psychros – cold, philein – love) that have the permissible temperature range from -10-0oC up to 25-30oC with optimum about 10-20oC;

mesophilic microorganisms (Gk. mesos – intermediate) that live in the range from 10-25oC up to 40-45oC with optimum about 20-40oC;

thermophilic microorganisms (Gk. thermos – warm) that prefer the temperature range from 25-45oC up to 70-80oC with optimum with 50-60oC.

These grades indicate that the bacteria cover a broad temperature scale for their growth – at least, from +10 to +80°C.

Most of pathogenic representatives pertain to mesophilic bacteria. The are cultured within the moderate temperature range of 20 to 45°C with optimal temperature near 37oC.

Likewise, the optimal concentration of hydrogen ions or pH of the medium is of great significance for microbial propagation as well. Overall, the saprophytic microbes can live within the long range of pH values – from extremely acidic (pH~0.6) up to highly alkaline (as pH about 11). Pathogenic bacteria are characterized in most cases by relatively narrow range of optimal pH – within 6.0-8.0.

Similarly, the total concentration of ions or ionic strength (e.g., concentration of Na+ or Cl-) as well as osmotic pressure play a substantial role in normal growth and function of bacterial cells. In general, the bacteria demonstrate the high limits of salt tolerance. Nonetheless, for optimal growth of the most of bacteria, the medium should be closer to isotonic (~0.15 M NaCl). By contrast, some groups of bacteria termed as halophiles prefer the increased concentration of salts – in the range from 0.3 M of sodium chloride to more than 5 M that is equivalent to 30% of NaCl. As an example, the causative agent of cholera, Vibrio cholerae, is the halophylic bacterium.

The microbial agents that can live and propagate far beyond the natural limits for habitation of conventional microorganisms are called extremophiles – for their life within the extremal conditions. For instance, hyperthermophils can thrive at temperature 100-120oC, deep psychrophiles – at -15oC, alkaliphiles – at pH>9.0, already mentioned halophiles – at 20-30% NaCl concentration, xerophiles – under extremely dry desert conditions. The existence of such bacteria clearly expands the borders for life; on the other hand, they are used as valuable sources of biological products with outstanding characteristics (e.g., microbial strains for wastes degradation, thermostable enzymes, etc.).