Different bacteria have different temperature range and optimum temperature for growth. Variable temperature is a stress factor: bacteria react to changing temperature, and the reaction costs energy and slows down the growth. Bacteria may only prefer variable temperature relatively to other bacteria, but not absolutely. (Cultivate the bacteria at constant temperature around the optimum: for the happiness of the bacteria and for the reproducibility of the cultivating conditions.)

Soil is a huge source of a diverse microbial population. For studying the soil bacterial growth, factors like temperature, pH, O2/shaking and nutrition matters the most. Depending on the type of bacteria, the factor may change for optimum. The soil sample, once inoculated in constant nutrition (same media conditions of constituents and pH and O2/shaking) may give different bacterial enrichment depending on incubation temperature and length of incubation. Hence researchers usually keep the temperature constant at the average sampling site temperature and study the microbial growth at that condition. 37'C is usually used to mimic body temperature but for ambient temperature studies, the temperature is usually kept WRT the sampling site.

Each bacteria have three temperature ( minimum, optimum and maximum temperture) , and the best degree of bacterial growth is optimum temperture, and in different enviroment like soil, the temperature of bacterial growth were vary depending on the temperature of soil.

bacteriological incubator is a basic instrument in microbiological lab when you want best microbial growth.

Thank you all for your valuable suggestions. By this answer, I get a new dimensions of thinking regarding my question

Soil contains huge communities of bacteria that undergo many activities among each other such as antagonism,synergism and commensalism.Thereafter and throughout certain bacterial types dominate on othe depending on their optimum conditions of temp ,pH value and soil salinity.When you isolate certain bacterial types of soil , accidently the dominant species grow well and this depends on soil conditions and soil temp...etc.So,depending on soil type, phsychrotrophs,mesophiles and thermophiles cold grow depending on your incubation temperature and temp growth range(optimum,minimum and maximum).

Adding on to what other colleagues have said, It will be adviceable to know the environmental factor specifics of the isolated bacteria in order to incubate it at appropriate temperature. It should be noted that the tropical temperatures are not constant; so for purposes of accountability, it is better to control temperature of the incubated bacteria.

Bacteriological incubator is a basic instrument in microbiological lab especially in tropical countries like India or in Asian continents because here temperatures changes frequently and we must provide our cultured organism a constant temperature for their best growth , weather from environment and Clinical Sample You can also adjust incubator temperature as you require.

Some bacteria's are well growth on specific temperature. Temperature reduce the bacterial cell metabolism which leads affect the bacterial growth rate

Thank you all for your suggestions

We need to understand that the researcher is working on soil samples that in which he has no target isolate in mind. Usually, Preliminary isolation might require an environment that will mimic their natural environment. if the artificial media does not provide such environment, best growth cannot be achieved. One can achieve results by incubating at room temperature but also a better results will be achieved if the isolates are incubated constantly at 30 degrees in tropical countries in order to avoid temperature stress.

Regards to aĺl what my colleagues hav said, each microbes (bacterial) has optimal growth temperature including specific medium. In culturing any species of bacterium, u needs to know the growth medium, temperature, incubating period etc because each bacterium grown well within its nutrients rich limit.

Air temperature may vary but soil temperature varies little. 37C for soil is probably inappropriate and its use may favor recovery of microbes less representative of your sample. Suggest you determine temp range of soil and incubate appropriately.

Each type of bacteria want a specific temperature to growth when on tropical temperature there is no incubator for a specific bacterial growth and we give the room temperature than there will unwanted bacterial growth and different bacteria will be grow.Common temperature is 35c for bacterial growth but in tropical temperature temperature is 18c so specific bacterial growth will be disturb and unwanted bacteria will be grow or may be no growth occur.

Each type of bacteria has optimum temperature for growth, therefore, incubator is required to give the optimum temperature to the bacteria under study.

the best degree of bacterial growth is optimum temperture, and in different environment like soil, the temperature of bacterial growth were vary depending on the temperature of soil. bacteriological incubator is a basic instrument in microbiological lab when you want best microbial growth.

I suggest checking the temperature of the soil sampled and the temperature of the room you will be incubating the plates. If there is a huge difference between the two, then you might not be able to grow much of the microbial flora found on the sample. Also, the incubation temperature may also provide you clues on the identification, and subsequently, other culture conditions for your isolates since the optimal temperature varies per species. But if temperature is not critical to your study, the temperature for tropical countries (i.e. Southeast Asia) is generally within the range mesophiles are capable of growing.