It's not just a simple case of heat stroke. To understand what heat does to a bacterium, we need to know about its structure. A bacterium is a single-celled organism. Think of it like a studio apartment, one room containing all the things a person needs to live: food, water, air. The walls of the apartment enclose the electrical wiring and gas pipes that deliver energy, along with the sewage pipes that get rid of waste products. In contrast to the size of this single-celled organism, even an animal as small as a mouse would be like a huge city with thousands of buildings and extensive infrastructure to keep it "alive."
In more scientific terms, a bacterium is made up of the cell envelope, the cytoplasm and, often, the flagella. Besides holding in the cytoplasm, the cell envelope is where energy-generating functions like photosynthesis and respiration happen. The cytoplasm refers to everything inside the cell envelope, a mixture of water, ribosomes, chromosomes, nutrients and enzymes -- all the things that keep the bacterium alive and kicking. Enzymes are especially important because they cause the chemical reactions that make up the cell's metabolism. The flagella are tiny appendages on the outside of the bacterium that help it move around, attach to surfaces or fend off enemies.
Now that we've set the scene and introduced the characters, here comes the dramatic climax. When the temperature gets hot enough, the enzymes in the bacterium are denatured, meaning they change shape. This change renders them useless, and they're no longer able to do their work. The cell simply ceases to function.
Heat can also damage the bacterium's cell envelope. Proteins and fatty acids making up the envelope lose their shape, weakening it. At the same time, fluid inside the cell expands as the temperature rises, increasing the internal pressure. The expanding fluid pushes against the weakened wall and causes it to burst, spilling out the guts of the bacterium.
Thermoduric bacteria are more heat-resistant and harder to kill. In terms of our apartment analogy, thermoduric bacteria have reinforced walls, double-paned windows, insulated pipes and an emergency supply of water and food. These heat-defying bacteria have to be kept under control by refrigeration, which keeps them from multiplying. [source: Todar]