1. Introduction
Water is one of the essential resources for sustaining life. However, anthropogenic activities have caused dangerous pollution problems, especially in freshwater environments [1] . One of the main factors that cause deterioration in these ecosystems is the release of effluents without treatment, mainly by agricultural, industrial, and livestock activities, which can cause damage to the health of people and the organisms that live there [2] .
Water can carry harmful bacteria to health since these microorganisms can cause waterborne diseases capable of leading to death, known to adapt to most different environments in isolation or groups [3] . These microorganisms, in turn, play a fundamental role in providing resistance and resilience against changes/pollution, which can measure the environmental stressors present by the practice of either-or inadequate management of these ecosystems [2] .
According to the United Nations (UN), most diseases and deaths in developing countries are caused by microorganisms waterborne diseases, mainly due to the lack of sanitation, which can increase expenses with hospital admissions [4] . Therefore, these bacteria represent a risk to human health, causing approximately 870,000 deaths per year [5] [6] .
For this reason, the microorganisms investigation in freshwater aquatic environments is essential to assess changes in these ecosystems. Thus, the main objective of this article was to carry out a systematic analysis through the literature on the main microorganisms found in impacted aquatic environments and to relate the data with the risks to public health.
2. Methodology
Protocol
This investigation was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and was organized into the planning, conducting, and reporting data phases.
Eligibility criteria
There used the PECO strategy to conduct this investigation: Population—Microorganisms, Exposure—Freshwater aquatic environments, Comparison—not applicable, and Outcomes—Main microorganisms found in freshwater environments.
Studies that considered the main microorganisms isolated in freshwater environments without year and-or language restrictions were eligible. The exclusion competed with editorial files, typical discussion documents, comments, letters, incomplete or insufficient data studies (studies that did not contain information on microorganism identification methodology), duplicates and titles that do not corroborate with the theme.
Information and Research Sources
The searches were carried out in the electronic databases PubMed, Medline, and LILACS. There selected the definition of descriptors and synonyms from the Medical Subject Headings (MeSH) and Decs (Descriptors of Health Sciences) and were also used keywords to assist in the controlled search strategy of scientific studies. Therefore, the resulting terms for the search strategy were: “Bacteria” and “Ecosystem” and “Environment” and “Aquatic” and “Impacted”.
Selection of studies
The selection of studies had the participation of two independent reviewers, which became reliable, separately and blindly, since the following variables were considered: Reasons for inclusion and exclusion of studies. The 1st stage consisted of studies analysis by title, so duplicates were eliminated. The 2nd stage consisted of the eligibility criteria discussion with the relationship of the PECO strategy since those studies that were not related to the proposed strategy were eliminated. The 3rd and last stage included the studies elimination after reading the abstract and total studies, which did not provide sufficient information and data to conduct this systematic review.
Data collection process
Soon after the selection of studies, there extracted information through a form created by the authors to record the selected studies’ data, which there used a defined protocol. The included items consisted of the First author, year of publication, place of study, genus and species of the isolated microorganism, and type of environment in which the author(s) carried out the study; thus, they were tabulated in an Excel spreadsheet. Any necessary calculations for the data were performed by two researchers, and if there were discrepancies, they would be resolved through group discussion.
Risk of Bias
The risk of bias was assessed using the Joanna Briggs Institute (JBI) Critical Assessment Checklist for Qualitative Research [7] . Risk classification consisted of High (greater than 49% of studies scoring “yes”), Moderate (studies achieving 50% to 69% of “yes” scores), and Low (studies reaching higher than 70% of “yes” scores); thus, there excluded those with high publication bias. Because of this, two reviewers independently evaluated each study. Then, the information was verified so that in the event of ambiguity, a third reviewer was involved in the assessment of bias in the studies.
3. Results
According to the searches carried out in the electronic databases, there obtained 1058 studies in total, of which 173 were eliminated by duplication, 716 by title, and 103 because they did not meet the eligibility criteria, accounting for a total of 31 studies included in this systematic review. Figure 1 corresponds to the flowchart showing the respective quantitative and qualitative data for the exclusion and inclusion articles.
Seeing the eligible studies, there performed the first analysis to identify the microorganism predominance in aquatic environments distributed on different continents. As a first result, there observed that the North American and Asian continents had the highest number of published studies on this topic, of which China and the United States were the most contemplated publications in the area. About the Latin American continent, Brazil stands out as the country with the highest number of publications. The data synthesis related to publications on
the subject by continent and their respective countries can be seen in Table 1, Figure 2.
Regarding the main genera and species of microorganisms found in these environments, the qualitative distribution can be seen in Table 2. There observed a total of 99 different bacterial genera and 150 species.
The bacterial genera that were found with the highest incidence were: Pseudomonas (n = 15), Aeromonas (n = 11), Escherichia (n = 9), and the most abundant species were: Pseudomonas—P. putida, P. aeruginosa, P. taiwanensis, P. geniculata, P. otitidis, Aeromonas—A. veronii, A. caviae, A. hydrophila; Bacillus—B. licheniformis; Enterobacter—E. clocae, E. asburiae; Escherichia—E. coli, and Klebsiella—K. pneumoniae.
Regarding the assessment of the risk of JBI bias, there observed that most of the responses to the critical evaluation questionnaire of the 31 studies consisted of “Yes” answers (>70%), indicating that the respective studies had a low risk of bias. Therefore, they have a high methodological quality.
4. Discussions
Aquatic environments are formed by marine and/or freshwater ecosystems.
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Table 1. Quantitative synthesis of publications by continent and countries.
Source: Authors, 2023.
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Table 2. Qualitative synthesis of the main genera and species of microorganisms found in impacted freshwater environments.
Source: Author, 2023.
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Source: Autores, 2023.
Figure 2. Distribution and predominance of microorganisms across continents.
However, freshwater is the primary environment in which there is a growing anthropogenic intervention, such as domestic, hospital, industrial, agricultural, and livestock effluents [2] ; since rivers, streams, and water residues are among the leading sources that have suffered from the contamination of these emerging [39] [40] .
Therefore, the continuous release of these organic compounds of anthropogenic origin has caused environmental degradation, in addition to increasingly resulting in the appearance of microorganisms [41] . These have as main characteristics high growth rates in addition to a higher metabolism, which also have particularities of being influenced by physicochemical changes and the introduction of organic, inorganic, and polluting compounds in the environment in which they inhabit [42] .
The present research carried out a systematic review in which the Asian continent presented greater microbial diversity when compared to other continents, with China being the country with the highest bacterial representation. Considering the population and development characteristics that have been taking place over time in their areas, especially urban areas [1] [43] , it is likely that these events have contributed to water pollution in freshwater environments and, consequently, greater bacterial diversity.
A point that deserves attention is that when identified in aquatic environments, these microorganisms can be used as a bioindicator of water contamination, reflecting the quality of the water, whether for bathing or drinking. An example of a bacterial species used to assess water quality is E. coli, one of the leading indicators of fecal contamination in freshwater aquatic environments, which has a strong relationship with risks of gastrointestinal diseases in humans [44] .
Pseudomonas aeruginosa is a bacterium that is frequently isolated in lakes and rivers, in addition to wastewater, due to the high load of nutrients found in these environments. Recent studies give a fundamental role of water in the colonization of this microorganism in humans since these bacteria have multiple virulence factors that promote their high pathogenicity [45] .
Klebsiella pneumoniae is a microorganism that can inhabit aquatic environments, soils, plants, and sewage. This bacterium can be isolated from the oropharynx, the gastrointestinal tract of humans and mammals, and the feces of healthy individuals, besides having the ability to colonize humans asymptomatically. The most common leading infections are pneumonia, urinary tract infections, wounds, bacteremia, and meningitis [46] .
Regarding the species of the genus Aeromonas, they are found in different habitats, particularly in aquatic environments, from which it is widespread. In freshwater environments, they are mainly present in wastewater, of which some species present themselves as relevant zoonotic pathogens. Among these, A. veronii, A. caviae, and A. hydrophila have been linked to diseases in humans, especially with symptoms of diarrhea, septicemia, and soft tissue infections of wounds after exposure to water, considered an emerging threat in different environments, mainly to human health [47] .
Therefore, the contamination of pathogenic microorganisms in aquatic environments, mainly freshwater, has caused an extensive public health problem worldwide, especially concerning waterborne diseases, since many of these microorganisms are enteric and can lead to death [44] [48] .
5. Conclusions
The findings and evidence showed that the genera Pseudomonas, Aeromonas, and Escherichia and their respective species were the main microorganisms isolated, that is, they proved to be the most predominant, which indicates that these pathogens can present themselves as leading indicators of quality in freshwater environments that are being impacted.
These data support the relevance of the continuous analysis of microorganisms in freshwater aquatic environments, mainly to assess the impact on the local fauna, the dynamics of water quality, and the survival of the beings that are there, helping to monitor this ecosystem and indicating the possible risks to the population that depends directly or indirectly on these places. The data presented reinforce that these environments not only harbor but spread pathogens, representing a risk to public health when not monitored or preserved.