Breathing, heartbeat and brain activity are controversial criteria for declaring a person dead. It is similarly complicated to judge whether a bacterium is dead or alive. This knowledge is important, for example when dealing with food.

 

The determination of the bacterial count, i.e. the number of microorganisms in a sample, also plays an important role in the assessment of soil and water quality and the purity and quality control of industrial products. But how can dead bacteria be distinguished from living bacteria? The oldest method is based on the fact that living bacteria divide and multiply. If bacteria multiply on nutrient-rich soils, this is a clear indication that they are alive. However, it becomes problematic when they are unable to multiply. Some bacteria fall into a kind of "deep sleep" when they are exposed to unfavourable living conditions such as a lack of nutrients or drought. In this state, the bacterial cell is referred to as a spore and can survive for up to several million years without dividing. Only when the cell recognises certain signals from outside does it "awaken". It would therefore be incorrect to characterise bacteria that do not reproduce as dead.

 

Another method of distinguishing between living and dead bacteria has to do with the cell membrane. It serves as a barrier and separates the cell contents from the outside world. Bacteria with holes in the membrane die because they lose their contents. In the laboratory, damaged and intact cells can be distinguished using fluorescent dyes (e.g. red propidium iodide and green fluorescent SYTO 9) and a microscope. Intact and living cells fluoresce green; damaged and therefore dead cells fluoresce red. 

Other fluorescent dyes can be used to test metabolic activity. This method tests whether certain enzymes in the bacteria are functioning. As the enzymes are required for the digestion of nutrients and the formation of cellular substances, their activity is vital for the bacteria. The same applies to the amount of DNA in the bacteria: As the DNA contains the entire genetic information of the cell, it is fatal for the cell if the DNA is damaged or degraded. Therefore, determining the amount of DNA in the bacteria can also provide information about the viability of the cells. These methods can be used to determine the extent of the damage in the bacterial cell, although there is still no clear definition of when the cell is no longer able to recover and can therefore be declared dead.