MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 8. MICROORGANISMS AND THE ENVIRONMENT

Aquatic Microorganisms

Microorganisms are found in A wide variety of Water bodies: saline and fresh, transparent and turbid, shallow and deep, standing and flowing, thermal and ice-bound. Water dissolves various substances essential for Microbial Nutrition. Within aquatic environments, microorganisms live, multiply, assimilate nutrients, and excrete metabolic products.

The quantitative composition of aquatic microorganisms is influenced by the water's biological and physicochemical composition (the content of organic and inorganic substances, Temperature, pH values, radiation, dissolved CO₂ and O₂ saturation, etc.), reservoir depth, current velocity, flora and fauna, wastewater influx, and other factors. On the water surface, microorganisms are adversely affected by solar radiation, whereas at depth, they encounter high hydrostatic pressure, low temperatures, and oxygen depletion.

While certain microorganisms are permanent inhabitants of water bodies, many others enter from the soil, especially after precipitation. For instance, while a typical lake water sample may contain about 10 microbial Cells per 1 mL, this number can surge to 1,200 following rain. These largely comprise Representatives of the genera Achromobacter, Azotobacter, Flavobacterium, Nitrobacter, Micrococcus, Proteus, Pseudomonas, Spirillum, and others. The influx of high loads of organic matter leads to the appearance of Clostridium, Spirochaeta, and Vibrio species. In sulfur-rich water bodies, photosynthetic Bacteria actively develop.

The microbial population in water is also affected by proximity to human settlements, such as towns and villages. Upstream from populated areas, the microbial count in water is significantly lower than adjacent to or downstream from them.

Microbial Abundance fluctuates with depth as well. For example, Professor M. Fedorov, having investigated lake microflora, reported the following data: 73 microbial cells per 1 mL of water at the surface; 142 cells at a depth of 5 m; 197 at 10 m; 147 at 20 m; 50 at 40 m; and merely 6 cells at 54.5 m.

Microorganisms washed into water from soil can survive for varying periods depending on the species and the CHARACTERISTICS OF THE water. Aquatic environments may harbor Vibrio cholerae, Salmonella typhi, Shigella dysenteriae, Francisella tularensis, Bacillus anthracis, and numerous other pathogens hazardous to humans and animals.

It is estimated that approximately 25% of infectious diseases are waterborne. For instance, Salmonella can persist in clean well water from 2 days to 3 months, Shigella for 5–9 days, and Leptospira for 7–150 days. Tubercle bacilli remain viable in river water for up to 5 months. Typhoid bacilli can even overwinter in ice without losing their pathogenic properties.

Water quality is primarily determined by the total microbial count it contains. Drinking water is deemed safe for consumption if 1 mL contains no more than 100 microbial cells. The degree of biological water pollution is assessed using the coli-titer and coli-index. The coli-titer is the smallest volume of water from which a single Escherichia coli Cell is isolated. The coli-index represents the number of E. coli cells detected in 1 L of water. According to current standards for tap water, the coli-titer must not be less than 300, and the coli-index must not exceed 3.

Microbial life is even more intense in river and lake bottom sediments (silt). Indeed, 1 g of wet silt contains between 200 and 400 million microbial cells. The surface layer of silt is rich in iron and sulfur bacteria. Because numerous living organisms inhabit and subsequently die in water, and the decomposition of organic matter generates hydrogen sulfide (H₂S), significant amounts of this gas—toxic to both plants and animals—accumulate in silt. Sulfur bacteria, however, are capable of oxidizing hydrogen sulfide into sulfuric acid. Without this process, hydrogen sulfide would diffuse into the water Column and destroy living organisms.

Microorganisms also inhabit deep layers of silt. These are anaerobes that derive the energy necessary for their vital activity through Fermentation.

The species composition of silt is quite diverse. One gram of silt contains from 100,000 to 1 million cells of sulfate-reducing bacteria, 10,000 to 100,000 ammonifiers, about a thousand nitrifiers (which oxidize ammonia to nitric acid and promote saltpeter formation), 10,000 to 100,000 denitrifiers, and roughly 100 anaerobic and aerobic Cellulose-degrading bacteria.

Marine and oceanic waters are also rich in microorganisms. They have adapted not only to highly saline environments but also to extreme hydrostatic pressure. For instance, microorganisms have been discovered in one of the deepest trenches of the World Ocean (10,462 m). At such depths, these microorganisms thrived at a temperature of 2.5 ^oC and a pressure of about 1,000 atm. Marine and oceanic microflora play a direct role in the biogeochemical cycles of carbon, iron, nitrogen, sulfur, phosphorus, manganese, and potassium. Marine bacteria decompose complex Organic compounds and convert them into nutrients accessible to aquatic flora and fauna. Chitin-degrading bacteria also play a vital role, as chitin forms the rigid exoskeleton of marine animals.

The exceptional ability of microorganisms to adapt to environmental conditions is also evident at high temperatures. Thermophilic bacteria have been discovered in thermal geysers at temperatures exceeding 90 ^oC. Interestingly, these microorganisms reproduce exclusively at high temperatures.

Sanitary and microbiological water testing is conducted for routine monitoring and epidemiological purposes. The primary targets of such investigations are:

- centralized drinking water supplies;

- decentralized drinking water supplies;

- surface and groundwater sources;

- wastewater;

- coastal marine waters;

- swimming pool water.

The principal officially regulated indicators of the sanitary and microbiological quality of water are:

1. Total bacterial count (TBC) – the number of mesophilic bacteria per 1 mL (cm³) of water.

2. Coliform bacteria index is the number of coliforms per 1 L of water. If it is necessary to investigate waterborne infectious disease outbreaks, a more detailed sanitary and microbiological water analysis is performed, determining the presence of enterococci, salmonella, *Vibrio cholerae*, and enteroviruses.

Air is an unfavorable physical environment for the growth of microorganisms. The lack of nutrients, sunlight, and other factors lead to the rapid death of Microorganisms in the air. However, certain species can persist in it for quite a long time.

The composition of air microflora is determined by its contamination with mineral and organic particles, temperature, terrain characteristics, soil and water microflora, as well as the season and meteorological conditions. The air in large cities is more polluted with microorganisms than in rural areas. The air in forests, mountains, and over the water surface of seas and large lakes contains even fewer microbial cells.

In summer, the air is contaminated with microorganisms twice as much as in winter. If we take the total number of microorganisms in winter as 1, in spring it will be 1.7, in summer - 2, and in autumn - 1.2. Thanks to precipitation (rain, snow), the air is cleansed of dust particles and the microorganisms settled on them.

The species composition of air microorganisms is quite diverse. They enter the air from soil, plants, and living organisms. Spore-forming forms, pigment-producing bacteria (staphylococci, micrococci), Molds, Yeasts, and actinomycetes are the most common in the air. The number of microorganisms in indoor air depends on the frequency of ventilation, cleaning Methods, lighting, and other factors.

The number of microorganisms in the air ranges from a few individuals to many tens of thousands per 1 m³. For instance, Arctic air contains 2-3 microbial cells per 20 m³, whereas industrial cities have over 3,000 microbial cells per 1 m³. Spore-forming bacteria and fungal conidia are detected at altitudes of over 20 km. One gram of dust can contain up to 106 microbial cells. The spread of microorganisms in the air is associated with The formation of an aerosol—a colloidal system consisting of liquid droplets (or small solid particles) suspended in a gaseous medium. Microorganisms present in the air can exist in three states of bacterial aerosol: droplet, droplet-Nucleus, and dust. Aerosol stability depends on particle size, surface energy, magnitude of electrical charge, and Properties of the dispersion medium. Particles larger than 10 µm settle rapidly because their gravitational force exceeds air resistance.

Indoor air becomes contaminated with microflora from the Upper Respiratory Tract and integument of humans and animals. In rooms housing sick animals or humans, pathogenic microorganisms may be detected in the air: cocci, *Mycobacterium tuberculosis*, causative agents of diphtheria, anthrax, pertussis, pneumonic plague, tularemia, etc. The causative agents of these infections can be transmitted via droplets of mucus or sputum during sneezing, coughing, or talking.

Sneezing and coughing propel up to 60,000 droplets of various sizes into the air over a distance of up to 2-3 m. Large droplets (100-2,000 µm in size) settle quickly, while small bacterial aerosol droplets (1.0-10 µm) can remain suspended for a long time (from several hours to several days). Even short-term presence of pathogenic microorganisms in the air can cause the spread of infectious diseases.

The following methods are used to study air microflora:

1. Natural sedimentation (Koch's plate method) involving passive deposition of microbial cells onto The surface of a solid nutrient medium for a specific period (usually 5-10 min).

2. Forced sedimentation using special devices—impactors such as the Krotov apparatus (microbial cells settle onto the surface of a solid nutrient medium) and impingers such as the Dyakonov apparatus (air is blown through so that microorganisms enter a liquid nutrient medium).

3. Filtration method—air is blown through water or membrane filters, followed by inoculation into a nutrient medium.

The first two methods are more reliable as they provide a quantitative assessment of microbial air pollution.

The criteria for evaluating the sanitary and microbiological condition of indoor air are:

- total microbial count (TMC) - the number of bacteria per 1 m³ of air that grew after inoculation onto the surface of nutrient Agar (cultures are incubated for 24 h at a temperature of 37 ^oC, followed by another 24 h at 20 ^oC);

- index of sanitary-indicator bacteria - the number of opportunistic pathogens of the respiratory tract per 1 m³ of air, including hemolytic streptococci, *Staphylococcus aureus*, Gram-negative bacteria, Yeast-like Fungi, and molds.