Physical Requirements for Growth of microorganisms

When we talk about microbial growth, we are really referring to the number of cells, not the size of the cells. Microbes that are "growing" are increasing in number, accumulating into colonies (groups of cells large enough to be seen without a microscope) of hundreds of thousands of cells or populations of billions of cells. Although individual cells approximately double in size during their lifetime, this change is not very significant compared with the size increases observed during the lifetime of plants and animals.

Microbial populations can become incredibly large in a very short time. By understanding the conditions necessary for microbial growth, we can determine how to control the growth of microbes that cause diseases and food spoilage.

The requirements for microbial growth can be divided into two main categories: physical and chemical. Physical aspects include temperature, pH, and osmotic pressure.

Most microorganisms grow well at the temperatures that humans favor. However, certain bacteria are capable of growing at extremes of temperature that would certainly hinder the survival of almost all eukaryotic organisms. Microorganisms are classified into three primary groups on the basis of their preferred range of temperature: psychrophiles (cold- loving microbes), mesophiles (moderate-temperature- loving microbes), and thermophiles (heat-loving microbes).

Most bacteria grow best in a narrow pH range near neutrality, between pH 6.5 and 7.5. Very few bacteria grow at an acidic pH below about pH 4. This is why a number of foods, such as sauerkraut, pickles, and many cheeses, are preserved from spoilage by acids produced by bacterial fermentation.

Some organisms, called extreme halophiles, have adapted so well to high salt concentrations that they actually require them for growth. They may be termed obligate halophiles. Organisms from such saline waters as the Dead Sea often require nearly 30% salt, and the inoculating loop (a device for handling bacteria in the laboratory) used to transfer them must first be dipped into a saturated salt solution. More common are facultative halophiles, which do not require high salt concentrations but are able to grow at salt concentrations up to 2%, a concentration that inhibits the growth of many other organisms. A few species of facultative halophiles can tolerate even 15% salt.

Most microorganisms, however, must be grown in a medium that is nearly all water. For example, the concentration of agar (a complex polysaccharide isolated from marine algae) used to solidify microbial growth media is usually about 1.5%. If markedly higher concentrations are used, the increased osmotic pressure can inhibit the growth of some bacteria.

VI Remember the following word and word-combinations:

Number of cells, lifetime, to determine, condition, disease, food spoilage, survival, osmotic pressure, sauerkraut, pickles, saline water, device for handling bacteria, to saturate, salt concentrations, to inhibit the growth, to tolerate.

VII Insert the missing words:

1. Physical aspects include…

2. Most microorganisms grow well at the temperatures that…

3. Most bacteria grow best in a narrow pH range near neutrality…

VIII Find the sentences with the following word-combinations:

Число клеток, причина болезней и порча продуктов, бактериальное брожение, концентрация агара, Мертвое море.

IX Answer the questions:

1. How is divided the requirements for microbial growth?

2. What do include physical aspects for microbial growth?

3. What temperature the most optimal for microbial growth?

4. What range of pH the most optimal for microbial growth?

5. What organisms are called halophiles?