Distribution of microorganisms in the soil
The number of microbes in the soil reaches incredible values—up to 10 billion cells per gram. However, along the vertical profile, they are distributed extremely unevenly. On the soil surface itself, microorganisms are relatively few. This is due to the detrimental impact of aggressive environmental factors, primarily direct ultraviolet rays and constant drying.
The maximum concentration of microbial biomass is observed in the surface layer at a depth of up to 10 centimeters. As depth increases further, the number of bacteria and fungi begins to drop sharply. The lower limit of habitat is located at a depth of 3–4 meters—there microorganisms are almost completely absent.
The qualitative and quantitative composition of the flora depends on many factors: the type and current state of the soil, the nature of the vegetation growing on it, and key abiotic indicators. Optimal conditions for the vast majority of soil inhabitants are a neutral pH level, high relative humidity, and a temperature range of 25 to 45 °C.
Physiological groups of soil bacteria
Depending on the biochemical functions performed, bacteria are divided into several main groups that ensure the cycle of substances in nature.
- Nitrogen-fixing bacteria. Their main task is the assimilation of molecular nitrogen directly from the atmosphere. Typical representatives include microorganisms of the genera Azotobacter, Azomonas, and Mycobacterium. Cyanobacteria (blue-green algae), which are actively used in modern agricultural technology to increase land fertility (for example, in flooded rice fields), also belong here.
- Ammonifying (putrefactive) bacteria. This group of microorganisms is responsible for the mineralization of complex organic substances. Among them are spore-forming forms, such as non-pathogenic soil bacilli (Bacillus megaterium, Bacillus subtilis), and non-spore-forming species—various representatives of pseudomonads and microbes of the genus Proteus.
Pathogenic bacteria and sanitary-epidemiological significance
From the standpoint of medicine and epidemiology, soil is considered a potential source of infectious diseases. All pathogenic flora can be divided into two broad categories.
- Resident flora (spore-forming pathogens). This group includes true inhabitants of the soil, for example, the causative agents of anthrax (Bacillus anthracis), as well as bacteria of the genus Clostridium (causative agents of botulism, tetanus, and gas gangrene). Due to their ability to form highly resistant spores, they can remain viable in adverse conditions for decades. Moreover, some of them, in particular Clostridium botulinum, are able to actively multiply directly in the soil.
- Transient flora (enteric bacteria). Representatives of the family Enterobacteriaceae (such as Escherichia coli, Shigella, Salmonella) enter the soil transiently along with human or animal feces. In the soil, they generally do not multiply due to the lack of a suitable nutrient medium and gradually die off.
Normally, in clean soil, enteric bacteria are extremely rare. That is why the detection of coliform bacteria (the Escherichia coli group) serves as a reliable indicator of fecal contamination. This fact is a crucial sanitary indicator criterion that directly indicates the sanitary distress of the territory and the high risk of transmission of intestinal infections.
Soil fungi and protozoa
In addition to bacterial flora, the microbiocenosis includes numerous eukaryotic organisms that play an equally important role.
- Soil fungi. They play a key ecological role in soil formation processes and the complex transformation of nitrogen compounds. Fungi possess high biochemical activity: they are able to synthesize various antibiotics and toxins. The medical significance of these microorganisms lies in the fact that toxin-producing species can get onto agricultural food products, causing severe diseases in humans—mycotoxicosis.
- Protozoa. These microorganisms are also an integral part of the ecosystem. Their numbers vary greatly and can range from 500 to 500,000 individuals per 1 gram of soil. Protozoa feed on organic residues and bacteria, thereby actively influencing changes in the composition of organic matter in the soil.
Lichens, which are a unique symbiotic association of fungi and cyanobacteria, also frequently inhabit the soil.
Briefly: answers to topical questions
Which microorganisms are used as sanitary indicators to assess fecal contamination of soil?
To assess fecal contamination of soil, intestinal group bacteria, clostridia, and thermophiles are used as sanitary-indicator microorganisms.
- Coliform bacteria (Escherichia, Citrobacter, Enterobacter, Klebsiella) are the main indicator of fecal contamination.
- Enterococci are an auxiliary indicator of recent fecal contamination.
- Clostridia (Clostridium perfringens) are an indicator of long-standing contamination (determined by the perfringens titer).
- Thermophiles are an indicator of contamination with manure and compost.
Which bacteria belong to the group of nitrogen-fixing soil microorganisms?
The group of nitrogen-fixing soil microorganisms includes bacteria capable of assimilating molecular nitrogen from the atmosphere.
- Free-living bacteria (Azotobacter, Azomonas, Mycobacterium) assimilate nitrogen independently.
- Nodule bacteria are plant symbionts.
- Clostridia and Beijerinckia also participate in nitrogen fixation.
- Cyanobacteria are used in agricultural technology to increase fertility.
What diseases (mycotoxicoses) are caused by toxin-producing soil fungi in humans?
When ingested through food, toxin-producing soil fungi cause food poisoning—alimentary mycotoxicoses in humans.
- Ergotism (Claviceps purpurea) occurs when consuming cereals contaminated with ergot.
- Alimentary-toxic aleukia (Fusarium sporotrichiella) is a severe disease with impaired hematopoiesis caused by overwintered cereals.
- Drunken bread syndrome (Fusarium graminearum, Aspergillus flavus) causes symptoms similar to alcohol intoxication.
- Aflatoxicosis is food poisoning caused by toxins of pathogenic fungi.
Which pathogenic bacteria are capable of actively multiplying directly in the soil (causative agents of sapronoses)?
Some causative agents of sapronotic and saprozoonotic infections are capable of multiplying and accumulating directly in the soil.
- Causative agent of botulism (Clostridium botulinum) is capable of multiplying directly in the soil.
- Causative agents of yersiniosis—soil and soil waters serve as the primary reservoir where not only the preservation but also the active accumulation of the pathogen occurs.
What is the role of actinomycetes in the soil microbiocenosis and what is their medical significance?
Actinomycetes play a key ecological and synthetic role in the soil microbiocenosis and also have immense medical significance.
- Role in the soil—inhabiting the rhizosphere, they convert soil substrates into compounds accessible to plants, enter into symbiosis with them, and exhibit antagonism toward phytopathogens.
- Medical significance—actinomycetes (primarily the genus Streptomyces) synthesize antibiotics as a tool for competition with other bacteria, serving as the source of about 80% of all natural antibiotics.
To memorize dangerous pathogens found in soil, use the "Spore Disease" rule: Anthrax, Tetanus, Gas gangrene, Botulism — all of them form protective spores.
Class and exam questions
Why are there almost no bacteria on the very surface of the soil?
The surface is directly exposed to ultraviolet rays and constant drying, which is lethal to most microbial cells.
What does the presence of Escherichia coli in the soil mean?
The detection of Escherichia coli serves as a sanitary indicator of fecal contamination and indicates the risk of infection spread.
Can pathogenic bacteria multiply directly in the soil?
Yes, some resident species, such as the botulism agent (Clostridium botulinum), are capable of active reproduction in soil under suitable conditions.
At what depth are most microorganisms located?
The maximum biomass concentration is recorded in the surface layer at a depth of up to 10 centimeters.
What else to study on this topic
- Features of the sporulation process in bacteria of the genera Bacillus and Clostridium.
- Mechanisms of molecular nitrogen fixation by the genus Azotobacter.
- Mycotoxicoses: the effect of specific fungal toxins on the human body.
- Sanitary and microbiological indicators for assessing soil quality.
- Symbiotic interactions of cyanobacteria and fungi in lichens.
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