Figure 1 shows that some databases had more articles related to the topic than others because some databases specialize in biomedical literature such as the PubMed platform [33,34]. A disadvantage of this database is that articles on biosensors mostly consider medical applications, which limits finding articles on biosensors with applicability in other fields, unlike the Web of Science and Scopus [35] databases. On the other hand, the ScienceDirect database provided the highest number of publications (n = 1129) found between 2012–September 2022, followed, in order, by SpringerLink (n =816), Scopus (n = 108), and PubMed (n = 41). The greater number of publications is possibly because non-specialized databases cover more multidisciplinary literature, with Scopus being one of the three most important sources in the last 15 years [36]. However, when the inclusion and exclusion criteria were applied, the number of publications was reduced (n = 45), and the number of publications related to the topic varied for each database. In another sense, the databases that had the most articles found were ScienceDirect and SpringerLink, which was possibly because the search engine of these databases has shown better precision compared to other databases (PubMed and Google Scholar) [37,38].

Figure 2 shows a comparison between the graph obtained with the number of publications selected and related to the use of microbial biosensors in wastewater monitoring and the graph obtained in Scopus when using the search formula: “Biosensor” AND “Bacteria” AND “wastewater”. Figure 2 was obtained after applying the inclusion and exclusion criteria, which show the same tendency to increase over the last 10 years (from 2012 to September 2022). This comparison was possible thanks to the fact that the Scopus database allows for bibliographic analysis. The similarity in the increase of research related to the topic also shows the importance that microbial biosensors have gained in the last decade in relation to their application in the monitoring of different pollutants present in different types of wastewater.

The reduction in the total number of publications after applying the exclusion and inclusion criteria was due to the fact that during the analysis, duplicate publications were detected between the different databases. In addition, publications that were not specifically related to wastewater monitoring using microbial biosensors were excluded. The duplicity of publications was due to the fact that the databases share the following characteristics as search subcategories other bibliography search engines such as Medline, PubMed [38], and SpringerLink.

Figure 2 shows that, although in the last decade there has been an increasing trend in research, this has not been significant, as can be compared with the total number of articles related to the use of microbial biosensors from 1981 to 2017, where a total of 2323 publications were registered in the databases [39]. However, there was no exact data on the number of publications about wastewater monitoring using these microorganism-based devices. The monitoring of this type of water possibly began due to the concerns about transferring pathogens and contaminants that put human health at risk [23,28]. However, the use of microbial biosensors in wastewater monitoring continues to attract the attention of the scientific community due to the need for new monitoring alternatives capable of detecting minimum concentrations of pollutants in real-time and thus being able to guarantee public and environmental health [4,40]. Another problem that may have delayed research in this area of biomonitoring was the COVID-19 pandemic [41,42]. According to Riccaboni and Verginer (2022), subsidies from other areas unrelated to COVID-19 were displaced during the pandemic [43], while Gao et al. (2021) reported that the average number of hours dedicated to research decreased in the first year of the pandemic [44].

Another important aspect that stands out is the recent use of genetically manipulated microorganisms [62,69,70,71,73,75,81,85,86,88], which improves the biosensor’s specification and sensitivity [95]. This is possible thanks to recombinant DNA technology that allows genes encoding transcriptional regulators to be integrated into biorecognition genes. Some of the most widely used genes are lux/luc, lacZ, and gfp, which code for the enzyme firefly/bacterial luciferase, β-galactosidase, and green fluorescent protein, respectively [78,96]. On the other hand, the microorganisms most used in biosensors for the monitoring of environmental contaminants are bacteria because they are easy to reproduce in cheap media, resistant to stress, detect specific signals, and also provide online analysis, in vivo, and dose-response. Of these bacteria, the E. coli species is the most widely used due to its easy handling [78]. In this way, this species can be used with the respective genes for the detection of environmental contaminants such as heavy metals and various inorganic contaminants [61,62,63,66,69,74,75,76,78,81,88] present in wastewater, as described and demonstrated in the analyzed articles. However, other bacterial species can also be used as components of biosensors with good detection limits, such as species of Pseudomonas, Bacillus, Burkholderia, Vibrio, etc. [97].

It can also be seen that emerging technologies such as microbial fuel cells (MFCs) have been used as electrochemical microbial sensors, they have been very useful in monitoring BOD [46,47,48,49,50,51,52,53,54,55,56,57] and other pollutants such as Zn²⁺ [63] and linear alkyl benzene sulfonate [65]. The latter is one of the most dangerous contaminants in wastewater and comes from the detergent industry, which is extremely important to detect early [65]. This utility as a microbial biosensor is possible due to its operation. Chu et al. (2021) [98] explained how these electrochemical devices function as biosensors, where the anode biofilms fulfill the role of component detection, monitoring toxic compounds by monitoring the extracellular transfer of electrons (ETE) by electroactive microorganisms and the anode. However, the cathode can also function as a biosensor, which is based on the electrochemical reduction of the analytes to be detected or through the inhibition of the oxygen reduction reaction. Likewise, the same author emphasized that through this type of biosensor, an early warning of the toxic compounds present in a body of water can be generated. Another novelty of these devices was presented by Emaminejad et al. (2022), who evaluated for the first time in the long term, the quantification of the sensitivity to variations in the organic load in a channel of primary effluents for 247 days, yielding encouraging results. However, it is also necessary to appreciate the environmental factors such as pH, the concentration of volatile fatty acids, and temperature that influenced the accuracy of the electrochemical biosensor [99].

The versatility of MFCs provides potential to monitor heavy metals in wastewater and, at the same time, generate bioelectricity, as shown in Zhang et al. (2022) [64] and Do et al. (2022) [100]. Likewise, Hui et al. (2022) highlighted the advantages of these electrochemical biosensors to detect toxic compounds in polluted water bodies since they are easy to operate, provide fast results in real-time [101], and can be built on small scales, which adds to their portability, making them very useful tools for in-situ tests. On the other hand, Tucci (2020), in his academic work, showed that electrochemical biosensors are useful for the detection of pollutants related to agriculture, such as herbicides, since for their detection there are classical analytical techniques (HPLC, GC-MS, etc.,), which are expensive and take a long time to issue results [102]. Although the low output potential and the scaling of MFCs indeed represent a challenge for electricity generation, that is different from its potential as a biosensor, of which it is a very practical monitoring system [103]. On the other hand, the sensitivity and specificity of these biosensors are lower than that of a subcomponent-based electrochemical biosensor (for example, an electrochemical enzyme biosensor) and an electrochemical sensor equipped with a chemically modified electrode, respectively. However, enzyme purification techniques make these other biosensors expensive and laborious, representing an economic disadvantage for researchers and companies [98].