A systematic review of microorganisms as indicators of recreational water quality in natural and drinking water systems

The purpose of this review was to highlight the most frequent biological indicators used to estimate the microbiological quality of drinking and recreational water. It was observed that the incorporation of other microbiological indicators should be considered to strengthen the decision-making process on water quality management and guarantee its safe consumption in recreational activities.


INTRODUCTION
Water is one of the most valuable natural resources, an essential element for life development and human activities (Romeu-Álvarez et al. ). It is intended for different uses: human consumption, industrial processes, recreational activities of primary and secondary contact, ecological conservation, among others (Baird & Cann ).
Due to alterations in natural and anthropogenic processes, it is necessary to guarantee its quality so that there is no risk for humans and the environment These analyses are hard to execute due to the great variety of cultivable pathogenic bacteria, isolation tests, the low concentration of aggressive species, the need for specialized laboratories, high economic costs, and time-consuming. To identify the presence of pathogens in a reliable way, water quality monitoring should be done by searching for fecal contamination indicators approved by international and national standards.
Microorganisms must meet the following requirements: (1) be a normal constituent of the intestinal microbiota of healthy individuals; (2) be present exclusively in the feces of homeothermic animals; (3) be present where pathogenic intestinal microorganisms are; (4) appear in high numbers; (5) facilitate isolation and identification; (6) be unable to reproduce outside the intestine of homeothermic animals.
Their survival time must be equal to or greater than a pathogenic bacterium, and their resistance to environmental factors must be equal to or greater than fecal pathogens (Fernández et al. ). The objective of this review is to highlight the most common microbiological indicators that are used to evaluate the microbiological quality of drinking and recreational water to identify new methods that optimize microbiological monitoring of water quality.

METHOD
The research was conducted in the Science Direct, Redalyc, Scielo, and in the Google Scholar search engine selecting peer-reviewed documents and published research articles in the last ten years, based on the following keywords: microbiological indicators, drinking water, and recreational water. Articles' selection criteria were as follows: (1) biological indicators to assess the microbiological quality of drinking and recreational water quality; (2) water bodies that were affected by outflow wastewater; (3) the impact of microbiological water quality on individuals' health; and (4) recreational water affected by bathers.
Recreational waters in artificial systems such as swimming pools were not considered as they have different environmental conditions, compared to those of interest in this work. For example, swimming pool water quality criteria are like those for the quality of drinking water, including microbiological parameters that differ from the water quality characteristics of natural bodies (Carrasquero The above is not feasible from a technical and economic point of view for natural water bodies such as seas and rivers. Articles that did not meet the above criteria were excluded from the analysis. The guidelines were also based on microbiological quality standards for drinking and recreational water of the following organizations: the World Health Organization (WHO), the United States Environmental Protection Agency (EPA), and the standards of water quality of the European Union (EU). These served as the basis for defining the microbiological water quality criteria in Latin America and the world.

RESULTS AND DISCUSSION
The most damaging effect of contaminated water has been the transmission of diseases by microorganisms that can inhabit humans (see Table 1) (Romero et al. ).

Recreational water quality criteria
Water quality standards have been used through microbiological parameters that measure the degree of contamination (Bonamano et al. ). Table 2 shows the microbiological quality criteria for primary contact with recreational water. Table 3 shows studies of microbiological indicators for recreational waters.
Microbiological water quality analysis must consider the sources of microbial contamination of coastal areas since it has been found that fecal indicators, coliphages, and    It must be ensured that the supplied drinking water does not pose any health risk to consumers (Bridgeman et al. ).
Tables 4 and 5 show the recommended microorganisms for microbiologically evaluating the quality of drinking water.

CONCLUSIONS
The application of the most used microbiological indicators to assess the microbiological quality of drinking and recreational water has been described, highlighting reliable techniques for analyzing pathogenic microorganisms, considering technical and economic aspects. It is necessary to include other microbiological indicators to strengthen water quality research and guarantee the safety of its consumption and use in different recreational activities. These could be some biological agents that are not usually included in international regulations, such as viruses and parasites.