FUNDAMENTALS OF MICROBIOLOGY - V. M. Samygin - 2015

CHAPTER 3. DISTRIBUTION OF MICROORGANISMS IN NATURE

Microorganisms are extremely widespread in nature, inhabiting soil, Water, air, plants, as well as the bodies of animals and humans.

3.1. Soil Microflora

Soil serves as the primary habitat for microorganisms. Their Abundance depends on both the natural CHARACTERISTICS OF THE soil and its chemical composition. Because soil contains nutrients and moisture, the microbial population per 1 gram of soil reaches colossal figures: ranging from 200 million Bacteria in clay soils to 5 billion in chernozem. A single gram of arable soil can contain up to 10 billion bacteria. The surface layer of soil is unfavorable for microbial life due to the destructive effects of direct sunlight and desiccation. The plow layer at a depth of 5-15 cm is the richest in microorganisms.

The soil microflora includes nitrifying, nitrogen-fixing, and denitrifying bacteria, Cellulose-degrading microbes, sulfur bacteria, pigment-producing microbes, Fungi, and Protozoa. Nitrogen-fixing bacteria are of particular importance, as they can assimilate atmospheric nitrogen and bind it into chemical compounds utilized by other living organisms. Pathogenic microorganisms can also enter the soil. Spore-forming microbes—such as the causative agents of tetanus, gas gangrene, botulism, and anthrax—persist in the soil for the longest periods.

Many microbes thrive in the most hostile environments. For instance, nitrifying bacteria live on bare rocks, obtaining mineral nutrients directly from the stone they inhabit or from wind-borne dust. They acquire life-sustaining nitrogen in the form of ammonia and its compounds, which are formed in the atmosphere during electrical discharges and deposited on the rock via dew or raindrops.

3.2. Aquatic Microflora

Water is the most ancient habitat for microbes; they are found in A wide variety of water bodies—standing and flowing, shallow and deep, thermal and icy, saline and fresh, pristine and polluted—in lakes, swamps, seas, and oceans.

The composition of microflora in freshwater bodies and rivers is largely determined by the microbial communities of adjacent soils. The concentration of microbes in 1 ml of river water can reach 5 million organisms. Their numbers are highest near the shores, decreasing with distance from the bank and increasing depth. Investigations of the Pacific Ocean revealed bacteria at a depth of 10,462 m in one of the deepest trenches of the World Ocean.

Microorganisms also inhabit hot springs (thermophilic microorganisms). In moderately acidic waters, bacteria survive at temperatures exceeding 90 °С, while in alkaline waters they can withstand even 100 degrees. In Antarctica, researchers have isolated many species of psychrophilic (cold-tolerant) bacteria; up to 100 bacteria have been found per gram of ice, and certain species were isolated from the lower strata of three-meter-thick ice sheets. Highly saline water bodies (such as the Dead Sea) harbor halophilic bacteria adapted to life in hypertonic environments. A peculiar record belongs to microorganisms (bacteria and Yeasts) isolated from the small Antarctic Don Juan Pond, whose waters feature an extremely high salt concentration and low Temperature. The water here does not freeze even at -24 °C due to a salt content 13 times greater than that of seawater. Bacteria can even multiply in distilled water.

Depending on the degree of pollution, pathogenic bacteria may be present in water bodies and remain viable for a certain period. For example, *Salmonella* can persist in tap water, river water, and well water from 2 days to 3 months, while *Vibrio cholerae* survives in river and sea water for up to several months.

3.3. Airborne Microflora

Air is an unfavorable medium for the reproduction of microorganisms. The lack of nutrients, solar radiation, and desiccation cause microbes in the air to die rapidly. The composition of airborne microflora depends on soil and water microflora, as well as seasonal and meteorological conditions. The higher the concentration of dust, smoke, and soot in the air, the greater the microbial load. Each particle of dust or smoke is capable of adsorbing numerous microorganisms onto its surface. The air of forests, particularly coniferous ones, exerts a lethal effect on microbes owing to the bactericidal properties of phytoncides—volatile plant substances.

In summer, the air is contaminated with microorganisms at twice The rate of winter. Precipitation cleanses the air of dust and reduces its microbial content.

The species composition of airborne microorganisms is diverse, most frequently represented by Spore Forms, sarcinae, yeasts, pigment-producing forms, staphylococci, and Molds.

Airborne transmission is possible for the causative agents of Influenza, measles, scarlet fever, diphtheria, pertussis, staphylococcal, streptococcal, and meningococcal infections, tonsillitis, tuberculosis, acute respiratory infections (ARIs), and other diseases.

3.4. Food Microflora

The presence of Proteins, CARBOHYDRATES, Vitamins, and other nutrients in foods promotes not only the survival but also the proliferation of microorganisms. Food microflora is categorized into specific and nonspecific. Specific microflora is utilized in the preparation of all fermented dairy products, bread, beer, wine, pickled vegetables, etc. For instance, *Lactococcus lactis* is used to produce kefir, kumis, sour milk, cottage cheese, sour cream, and butter. To achieve a creamy consistency, *Streptococcus cremoris* is added. Lactic acid cocci and rods hydrolyze lactose to form lactic acid, which is essential for the METABOLIC ACTIVITY OF microscopic fungi that drive Alcoholic Fermentation. Concurrently, the product becomes saturated with carbon dioxide and acquires a distinct flavor. In other words, specific microflora imparts desirable taste qualities and a specific consistency to food products.

However, foods may also contain microorganisms or their spores introduced from the environment—this constitutes nonspecific microflora. It comprises saprophytic, opportunistic, and pathogenic microorganisms. The proliferation of certain microbes renders foods inedible; globally, 25% of produced food fails to reach the consumer due to spoilage, predominantly caused by microbes. Food products can be contaminated with salmonellae, staphylococci, *Escherichia coli*, the botulinum toxin producer, and other bacteria.

Milk and most dairy products provide a favorable medium for various microorganisms. Microbes enter milk during the milking process. Raw milk may contain micrococci, streptococci, as well as *Klebsiella*, *Yersinia*, *Proteus*, and *E. coli*. Contaminated milk may harbor bacteria, yeasts, and microscopic fungi. The agents responsible for souring fresh milk are lactic acid bacteria, which ferment sugars into lactic acid. Following these lactic acid bacteria, other microbes begin to multiply in the milk, such as microscopic fungi that utilize lactic acid in their METABOLISM.

Sour cream, kefir, cottage cheese, and sour milk exhibit greater storage stability than milk, acting as hostile environments for The Development of many pathogenic bacteria. This is attributed to the elevated acidity of these products and the antibiotic properties of certain starter cultures. The manufacture of Fermented milk products involves starters containing pure cultures of lactic acid streptococci, Bulgarian and acidophilus bacilli, or mixtures thereof. Kefir production relies on kefir grains—a symbiotic association of yeasts and other microorganisms. Cheeses are produced by curdling milk with lactic acid bacteria, after which the ripening process proceeds with the participation of microbes responsible for lactic acid and Propionic Acid Fermentation. The production of certain soft cheeses employs mold cultures of the genus *Penicillium*.

Meat is a highly favorable medium for numerous microorganisms. Contamination of meat can occur ante-mortem As a result of animal or poultry disease, as well as during slaughter, carcass dressing, improper storage, and transportation. The surface of meat typically harbors putrefactive, lactic acid, butyric acid bacteria, and micrococci. *E. coli* frequently proliferates on raw meat. Under high atmospheric humidity, microscopic fungi develop on meat, forming colored spots. Furthermore, meat from diseased animals may harbor pathogens hazardous to humans.

Fish is contaminated with a wide variety of microbes originating from water, scales, the intestinal tract, the hands of Processing personnel, and various equipment (knives, tables, cutting boards, decks, etc.). The most hazardous microorganisms include the botulism bacterium—which releases exotoxin in canned goods—and enteric salmonellae.

Fruits and vegetables become contaminated by microbes residing in the soil and on the hands of personnel involved in harvesting, packaging, transportation, and retail distribution. Improperly canned vegetables (such as tomatoes), mushrooms, and other preserves occasionally cause botulism.

The microflora of eggs can be of lifetime (endogenous) origin, but is more frequently exogenous, resulting from external shell contamination. Various microorganisms, including pathogens such as salmonellae and fungi, quite often penetrate into eggs, while staphylococci often contaminate egg powder. The eggs of waterfowl frequently harbor salmonellae, which are causative agents of food poisoning.

Of all agents causing food poisoning in humans, 70% are pathogenic bacteria. Particular hazards are posed by salmonellae, staphylococci, and streptococci, which multiply and accumulate in food products without altering their organoleptic properties (color, taste, and odor).