Destruction of microbes. Part 2


A logarithmic scale is used to evaluate or compare the effectiveness of microbial killing methods.

Destruction logarithm

If you place bacteria in a warm, humid environment and provide them with good nutrition, they will multiply rapidly. On a clean and dry surface, they die, so cleanliness alone is enough to control microorganisms. Viruses can only reproduce in living cells, this happens only when they manage to penetrate the human body. Shaking the hand of an infected person or touching a surface they have previously come into contact with allows several viruses to get into your eyes or mouth, putting you at risk of infection.

In the external environment, especially on a contaminated surface, viruses are able to remain viable for several days, but in cleanliness and dryness they are quickly destroyed. In most cases, fungi are multicellular organisms, although unicellular organisms are also found among them, they need a moist environment to thrive, and they pose less of a threat to humans than bacteria and viruses.

The microbes that surround us number in the millions and billions. Their destruction with the help of chemical agents is very effective, and they die in such quantities that a logarithmic scale is used to describe this process. Thus, the decimal logarithm of destruction can be equal to four or even six. In the first case, this means that 99.99% of the bacteria population died, in the second — 99.9999%. But, of course, the remaining bacteria will multiply rapidly and, given the availability of nutrients, will be able to produce millions of offspring per day.

The phrase «logarithm of destruction» sounds rather strange if you do not know that the logarithm means the order by which the number of bacteria decreased as a result of disinfection. Suppose we have a million bacteria (this number can be written as 106), after the death of 90% there will be 100,000, or 105 bacteria, and the decimal logarithm of destruction will be equal to one, that is, the algebraic difference of 6 and 5, in other words, we can say that the number of bacteria decreased by a factor of ten. (You will achieve the first log kill if you wash your hands with soap.) Killing 90% of the remaining 100,000 will reduce the number of bacteria to 10,000, or, in other words, to 104, microorganisms, in which case the decimal log kill in this case will be 2, you can also to say that we have killed 99% of the original million bacteria. (Through handwashing can achieve this level of pathogen elimination as well.) By killing 90% of the remaining bacteria, we will reduce their number to 1000 (103), and the elimination rate will be 99.9%, the logarithm of elimination will be equal to 3. With after killing 90% of the pathogens, only 100 (102) bacteria will remain alive, and this will mean that we have destroyed 99.99% of the microorganisms, that is, the logarithm of destruction has become 4. After killing another 90% of the bacteria, we will have only ten cells, and the log kill will be 5. The next 90% reduction in the number of microorganisms will leave us with only one bacterium, and the decimal log kill will be 6. In the end, it turns out that out of the original million, 999999 bacteria died and the percentage of destruction is 99.9999%. If we started our calculations with a billion bacteria, the log kill could be 7 and 8, but for most disinfectants the log kill is six.

In order for a healthy person to develop an infectious disease, infection with several thousand viruses and more than 100,000 bacteria is necessary. In susceptible populations such as children, the sick and the elderly, as little as a thousand microbes can develop disease. Once in favorable conditions, microorganisms begin to multiply at a tremendous rate: one bacterium can give rise to a million offspring in less than 24 hours.

Is it possible to be too clean?

There is no doubt that modern hygiene practices save the lives of many people, nothing confirms this better than the significant reduction in child mortality in recent decades. But can hygiene be excessive? Throughout the evolution of the Homo sapiens species, our ancestors have always relied on their immune system as a reliable defense against germs. Newborn children with a deficiency of the immune system did not withstand natural selection, they died soon after the cessation of breastfeeding, when they were deprived of mother’s milk, rich in protective substances and antibodies.

Some scientists believe that we are overprotecting young children from exposure to environmental substances, which is why their immune system is not competent enough. This is the reason for the development of hyperreactivity to harmless substances, such as proteins of pets and plants, house dust particles. Because of this, the incidence of bronchial asthma and allergies increases. Contact with such materials is necessary for the formation and maintenance of pools of specialized immune cells, and there is strong evidence for this theory. However, the reason for the increase in the frequency of allergic diseases, most likely, lies in the change in the composition of the food of modern man.

When the immune system is functioning adequately, certain types of white blood cells release cytokines that instruct infected cells to kill bacteria or viruses. Another task of the cells of the immune system is to protect against parasites living in our intestines. These cells release antibodies and activate mast cells to expel intruders. Mast cells release large amounts of histamine, which causes hydration of the intestinal mucosa and inhibition of microbial growth. Such a response can also be triggered by a false identification of a pathogen, which is why we become allergic to agents such as animal dander, plant pollen or peanuts.

In ordinary life, we must keep the house clean, but the need for disinfection, as a rule, does not arise. However, there are times when we feel vulnerable to infection, and then we need extra protection, something stronger. The table below lists the main disinfectants used in the household and describes their effects on four types of pathogens.

Table. Disinfectants and their effect on four types of microbes

Dezinfectant Action of bacteria Action on viruses Action on mushrooms Action on disputes Mechanism of action
Hydrochloride Effective Effective Effective Effective Oxidizes organic molecules
Chlorophenols Effective Effective in large cases Effective Effective Denature proteins
Quaternary ammonium salts Act on some bacteria Rarely effective Effective Resistant Destroy the membrane
Hydrogen peroxide (hydrogen peroxide) Effective Effective in large cases Rarely effective May be effective Affects DNA

Source: based on the book by John Emsley «On the benefits and harms of the products we love to buy»

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