Final answer:
Bacteria that grow at the bottom of the test tube in the absence of oxygen are called obligate anaerobes. They use anaerobic respiration or fermentation as they cannot tolerate oxygen due to a lack of protective cellular mechanisms.
Step-by-step explanation:
The bacteria that grow at the bottom of the test tube and for which oxygen is lethal are known as obligate anaerobes. These organisms thrive in environments that are completely devoid of oxygen, employing anaerobic respiration or fermentation to generate energy. In contrast to aerobes, which require oxygen to live, obligate anaerobes lack the cellular mechanisms to defend against oxidative stress caused by the presence of oxygen free radicals, which makes oxygen toxic to them.
Obligate anaerobes can be commonly found in various environments like deep soil sediments, still waters, or in the human gut, which is naturally low in oxygen. The study of these bacteria requires special techniques, such as cultivation in an anaerobic jar or a thioglycolate tube culture, which is designed to limit oxygen exposure. These methods are essential for understanding the growth characteristics of various bacteria in relation to oxygen availability.
The test-tube culture in thioglycolate medium, a reducing agent that removes oxygen, allows for the observation of growth patterns indicative of an organism's oxygen requirements. Therefore, the observed growth at the bottom of the test tube consistently indicates that the bacteria in question are obligate anaerobes.