transformation is a form of DNA transfer that appears to be an adaptation for repairing DNA damages. Batch culture is the most common laboratory growth method in which bacterial growth is studied, but it is only one of many. It is ideally spatially unstructured and temporally structured. The bacterial culture is incubated in a closed vessel with a single batch of medium. In some experimental regimes, some of the bacterial culture is periodically removed and added to fresh sterile medium. In the extreme case, this leads to the continual renewal of the nutrients. This is a chemostat, also known as continuous culture. It is ideally spatially unstructured and temporally unstructured, in a steady state defined by the rates of nutrient supply and bacterial growth. In comparison to batch culture, bacteria are maintained in exponential growth phase, and the growth rate of the bacteria is known. Related devices include turbidostats and auxostats. When Escherichia coli is growing very slowly with a doubling time of 16 hours in a chemostat most cells have a single chromosome. Bacterial growth can be suppressed with bacteriostats, without necessarily killing the bacteria. Certain toxins can be used to suppress bacterial growth or kill bacteria. Antibiotics (or, more properly, antibacterial drugs) are drugs used to kill bacteria; they can have side effects or even cause adverse reactions in people, however they are not classified as toxins. In a synecological, true-to-nature situation in which more than one bacterial species is present, the growth of microbes is more dynamic and continual. Liquid is not the only laboratory environment for bacterial growth. Spatially structured environments such as biofilms or agar surfaces present additional complex growth models. Environmental conditions Environmental factors influence rate of bacterial growth such as acidity (pH), temperature, water activity, macro and micro nutrients, oxygen levels, and toxins. Conditions tend to be relatively consistent between bacteria with the exception of extremophiles. Bacterium have optimal growth conditions under which they thrive, but once outside of those conditions the stress can result in either reduced or stalled growth, dormancy (such as formation spores), or death. Maintaining sub-optimal growth conditions is a key principle to food preservation. Temperature Low temperatures tend to reduce growth rates which has led to refrigeration being instrumental in food preservation. Depending on temperature, bacteria can be classified as: Psychrophiles Psychrophiles are extremophilic cold-loving bacteria or archaea with an optimal temperature for growth at about 15 °C