Pseudomonas fluorescens, Micrococcus roseus, etc., are among the specific aquatic aerobic microorganisms. Anaerobic bacteria are very rarely found in water.

The microflora of rivers depends on the degree of pollution and the quality of purification of sewage waters flowing into river beds.

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Micro-organisms are widespread in the waters of the seas and oceans and have been found at different depths (3700-9000 m).

The degree of contamination of the water with organisms is expressed as saprobity, which designates the total of all living matter in water containing accumulations of animal and plant remains. Water is subdivided into three zones. The polysaprobic zone is strongly polluted water, poor in oxygen, and rich in organic compounds. The number of bacteria in one ml reaches 1000000 and more. Colibacilli and anaerobic bacteria predominate, which bring about the processes of putrefaction and fermentation. In the mesosaprobic zone (zone of moderate pollution), the mineralisation of organic substances with intense oxidation and marked nitrification takes place. The number of bacteria in one ml of water amounts to hundreds of thousands, and there is a marked decrease in the number of colibacilli. The oligosaprobic zone is characteristic of pure water. The number of microbes is low, and in one ml, there are a few tens or hundreds; this zone is devoid of the colibacillus.

Depending on the degree of pollution, pathogenic bacteria can survive in reservoirs and, for a certain time, can remain viable. Thus, for example, in tap water, river, or well water, salmonellae of enteric fever can live from two days to three months, shigella — from 5-9 days, and leptospira — from seven to 150 days. The cholera, vibrio El Tor, lives in water for many months and is the causative agent of tularaemia, lasting from a few days to three months.

Tap water is considered clean if it contains a total amount of 100 microbes per ml, doubtful if there are 100-150 microbes, and polluted if 500 or more are present. In well water and in open reservoirs, the amount of microbes in one ml should not exceed 1000. Besides, the quality of the water is determined by the presence of E. coli and its variants.

The degree of faecal pollution of water is estimated by the coli titre or coli-index. The coli titre is the smallest amount of water in milliliters in which one E. coli bacillus is found. The coli-index is the number of individuals of E. coli found in 1 litre of water. Tap water is considered good if the coli titre is within the limits of 300-500. Water is considered to be good quality if the coli-index is 2-3.

Due to the fact that Str. faecalis (enterococci) are constant inhabitants only of the intestine in man and warm-blooded animals and are highly resistant to temperature variations and other environmental factors, they are taken into account with the coli titre and coli-index for the determination of the degree of faecal pollution of water, sewage waters, soil, and other objects. At present, new standards of enterococcus indices are being worked out.

Water is an important factor in the transmission of a number of infectious diseases (enteric fever, paratyphoid, cholera, dysentery, leptospirosis, etc.).

Due to the enormous sanitary-epidemiological role of water in relation to the intestinal group of diseases, it became necessary to work out rapid indicator methods for revealing colibacillus and pathogenic bacteria in water.

These include the methods of luminescent microscopy for the investigation of water for the presence of pathogenic microbes and the determination of the increase of the titre of the phage. Upon the addition of specific phages to liquids containing a homologous microbe in 6-10 hours a considerable increase in the amount of phage particles can be observed.

For a more complete and profound study of the microflora of the soil and water, capillary microscopy is used. The principle is that very thin capillary tubes are placed in the soil or water reservoirs, after which their contents are exposed to microscopic investigations. This method reveals those species of micro-organisms that do not grow in ordinary nutrient media and which, for many years, were unknown to microbiologists.

The author is an associate professor (retd.) and former head of the department of botany at Ananda Mohan College.