Cellular tropism also referred to as “cellular predation,” is a regular interspecific antagonistic that occurs in diverse living habitats. It also defines an act of a predatory organism to kill and devour a prey organism for its nutritional requirements (Pérez et al., 2016). Predation behavior expands from primitive prokaryotic microbes to highly evolved mammals in the animal kingdom (Sinclair et al., 2003; Ripple and Beschta, 2004). Among the microorganisms, a family member of the virus, bacteria have developed predatory behaviors that are well investigated under in vivo and ex vivo conditions (Curds, 1982; Gonzalez et al., 1990; Parry, 2004). Myxobacteria and Bdellovibrio are δ-Proteobacteria in the bacteria phyla with well-known bacteriophagic nature (Stolp and Starr, 1963; Casida, 1982; Berleman et al., 2006; Martins et al., 2022). Through predation attitude, Myxobacteria, and Bdellovibrio contribute to community structuring and carbon recycling in the soil food web systems and play an important ecosystem function are well-known keystone taxa (Bratanis et al., 2020; Cavallo et al., 2021; Mookherjee and Jurkevitch, 2022; Ogundero et al., 2022; Whitworth, 2022; Wu et al., 2022). Thus, bacteria species in the actinomycetes species are gram-positive and mostly studied for secondary metabolites production and were recently discovered to exert a facultative predatory role (Zeph and Casida, 1986; Hoshino et al., 2015; Ait Barka et al., 2016; Katz and Baltz, 2016; Baltz, 2019; Ouchari et al., 2019; Ibrahimi et al., 2020; Korichi et al., 2021). Studies deciphering or investigating the facultative predation role of Actinomycetota are gaining growing research attention (Kumbhar et al., 2014; Ibrahimi et al., 2019, 2020; Baig et al., 2021) owing to their lifestyles adjustment to adapt to complex limited resources (Ibrahimi et al., 2019, 2020) and importantly potential application to design bio-pesticides molecules/products alternatives to heavy toxic pesticides inorganic molecules (Palaniyandi et al., 2013). The approach is driven by increasing consumer demands for safe, healthy, and organically produced foods globally (Alvarez et al., 2017, Sathya et al., 2017; AbdElgawad et al., 2020). However, in the context of Actinomycetota species, the mechanisms that underline opportunistic predation behaviors are under-investigated (Ibrahimi et al., 2019, 2020).

Predation is a bacteria co-evolutional trait in the Actinomycetota, owing to adjustments in lifestyles among the species, possible geographical local adaptations, and habitat change (Jousset, 2012; Ibrahimi et al., 2020). For a long time, Actinomycetota were only viewed as competitive, rather than predatory organisms (Kumbhar and Watve, 2013; Kumbhar et al., 2014; Ibrahimi et al., 2020; Pérez et al., 2020). As a consequence, despite their widespread ecological importance in the environment, there are very few investigations on Actinomycetota predation (Bentley et al., 2002; Mawang et al., 2021; Boubekri et al., 2022; Santos-Aberturas and Vior, 2022). The first evidence of the Actinomycetota opportunistic predation behaviors is described in the Streptomyces and Agromyces genera (Waksman and Woodruff, 1941; Casida, 1980). The author examined the utilization of Micrococcus luteus as prey by Streptomyces species. As versatile-opportunistic Actinomycetota, Streptomyces is also a non-obligate epibiotic predator of various microorganisms, specifically, Staphylococcus aureus, Escherichia coli, Bacillus sp., Pseudomonas aeruginosa, and Klebsiella sp. (Kumbhar et al., 2014). In addition, It was reported in the literature that under in vivo conditions, Streptomyces isolates exhibited the predatory ability on various prey bacteria cells (Gram+, Gram−) and multidrug-resistant strains (Ibrahimi et al., 2020). The latest supportive evidence of good predatory behavior by Streptomyces against various types of prey is reported by Baig et al. (2021). However, studies examining the mechanisms that underline predator–prey relationships and the diversity or identity of small signal molecules are lacking (Ibrahimi et al., 2020).

Increasing plant diseases due to pathogenic microbes represent an important global constraint for agricultural production and economic losses (Collinge and Sarrocco, 2022). Current interventions are toward the use of synthetic heavy toxic molecules pesticides for crop protection that negatively poses acute risks to human health and the environment (Rani et al., 2021). It is imperative to find alternative solutions for a sustainable crop yield (Collinge and Sarrocco, 2022). The increasing knowledge and understanding of plant–microbe interactions, in particular the predatory capability of Actinomycetota to design as a biopesticide product to combat plant-associated pathogenic microbes (Shivlata and Satyanarayana, 2017). The objective of the present study was to:

Predation by bacteria has traditionally attracted lower attention than their multicellular development or their production of bioactive compounds (Furness et al., 2020). In recent years, many aspects of bacterial predation are starting to be explored through research (Table 1). Since the purpose of a predatory bacteria is to kill and digest its prey, it remains necessary to understand the hunting and killing behavior of the predator. Most authors propose to classify bacterial hunting strategies into four general categories: epibiotic, group attack, or wolfpack, periplasmic penetration, and endobiotic predation or direct invasion (Martin, 2002; Jurkevitch, 2007; Berleman and Kirby, 2009; Pasternak et al., 2014; Pérez et al., 2016). Epibiotic is a tactic that requires close cell-to-cell proximity (Shi et al., 1993). When the predation is extracellular, the predator attached to the prey from outside does not invade either the periplasm or the cytoplasm of the prey, degrades and assimilates host molecules through specialized structures, but without penetrating the prey (Martin, 2002; Pérez et al., 2016), consuming it from the exterior before dividing into the daughter cells via binary division (Koval et al., 2013; Figure 1). Ensifer adhaerens (Druga et al., 2011), Myxococcus xanthus (Nair, 2016; Thiery and Kaimer, 2020), and Streptomyces (Casida, 1988; Ibrahimi et al., 2020) are some examples of epibiotic strategy. The cell-to-cell contact between the epibiotic predator and its prey is crucial for the transfer of compounds between their cells (Castelle and Banfield, 2018; Yakimov et al., 2022). As a consequence of the limited literature on culture-based investigations, our current state of understanding of the behavior of epibiotic lifestyles is relatively poor (Bor et al., 2020; Batinovic et al., 2021; Yakimov et al., 2022). However, Cross et al. (2019) suggested that to clearly demonstrate cell-to-cell contact, conditions that could enhance predator abundance to the degree that allows detailed microscopic characterization must be evaluated.

The second strategy is wolfpack or group attack or group predation, predatory bacteria in this kind of predation work as a group (Marshall and Whitworth, 2019), they are assumed to hunt collectively to attack prey (Figure 1). They assemble and collectively secrete a diversity of diffusible compounds like hydrolytic enzymes and extracellular antibiotics that degrade and kill nearby bacteria. Wolfpack strategy lyse prey cells from the exterior through concerted action (Martin, 2002). Furthermore, the aim of the process of lysis is to produce small molecules that are easily assimilated by the predator (Xiao et al., 2011). Moreover, higher predatory cell densities suggest higher diffusible compounds (Keane and Berleman, 2016). The most important property of wolfpack is the lysed prey will be consumed by the predatory bacteria secreting and non-secreting (Mendes-Soares and Velicer, 2013). Lysobacter sp. (Hashizume et al., 2004; Li et al., 2008), Myxobacteria (McBride and Zusman, 1996; Thiery and Kaimer, 2020), and Herpetosiphon sp. (Nett et al., 2006) are examples of predatory bacteria using this strategy.

The third approach is when cells enter the prey periplasm (Figure 1). The predator invades and develops in the periplasm of gram-negative bacteria (Hobot et al., 1984; Ferguson, 1990; Ferguson et al., 1992). Predator produces hydrolytic enzymes that promote penetration and damage the prey cell wall (Lerner et al., 2012). In parallel, the infected prey is destroyed once respiration comes to a halt and the outer membrane is damaged (Thomashow and Rittenberg, 1978). The invading predator attaches to the prey’s cytoplasmic membrane and initiates growth using the cytoplasm of the prey as a nutrient supply. The predator grows like a polynucleotide filament, the length of which is dependent on the prey size (Kessel and Shilo, 1976). In the end, filaments septate into individual attack-phase cells that grow a flagellum, induce the formation of pores in the cell wall and burst into the external medium to engage in another cycle (Fenton et al., 2010). Therefore, the predator kills the prey by ingesting its cytoplasm. Bdellovibrionales (Jurkevitch and Davidov, 2006), Bdellovibrio bacteriovorus, Bacteriovorax marinus, Bacteriolyticum stolpii, and Peredibacter starrii, use periplasmic predation (Sockett and Lambert, 2004; Pasternak et al., 2014). However, the mechanism used by predators to release their intracellular contents to achieve their original cell cycle is poorly described (Laloux, 2020).

Moreover, some predatory bacteria can utilize more than one hunting strategy, like Bdellovibrio, which employs the periplasmic and epibiotic strategy; also, Myxobacteria can use both epibiotic and wolfpack strategies.

The last category of predation includes all predators that penetrate the host cytoplasm (Figure 1). This approach is also known as the invasion of cytoplasm or diacytotic strategy (Moulder, 1985). After the penetration, the predator grows and divides inside the cytoplasm. Daptobacter is the only bacteria that employ this strategy (Guerrero et al., 1986), but no other study has been reported about this group. Among predatory Actinomycetota, a few investigations have highlighted strategies used in predation because they have not received the level of attention of competitors (Ibrahimi, 2020). Pérez et al. (2016) suggest that bacteria secreting secondary metabolites, including Actinomycetota, can attack their prey in groups, which is supported by the fact that they are social prokaryotes that form and develop in multicellular structures. Correspondingly, Kumbhar and Watve (2013) reported that antibiotic producers such as Actinomycetota are unable to use an endobiotic strategy and direct contact with the prey is not required. Kumbhar et al. (2014) showed that Actinomycetota are non-obligate epibiotic predators of diverse prey such as S. aureus, E. coli, Bacillus spp., Pseudomonas aeruginosa, and Klebsiella spp. Recently, a study by Zeng et al. (2021) demonstrated that Streptomyces globisporus, a predatory Actinomycetota, preyed on Microcystis through an epibiotic mode of predation. Overall, among the phylum Actinomycetota, only two genera, Agromyces and Streptomyces, are known to have an epibiotic predatory behavior against other bacterial species (Casida, 1983; Arcamone et al., 2000; Ibrahimi et al., 2020). All this information allows us to believe that the predatory Actinomycetota can hunt their prey through wolfpack and epibiotic strategies. Therefore, our understanding of Actinomycetota predation is still very fragmentary, including gaps in their mechanisms of predation. Understanding these mechanisms by Actinomycetota species is required for better knowledge and understanding of their effect on prey structure to develop a new strategy to control plant disease and multidrug-resistant pathogens.

Predatory bacteria represent a diversified collection of prokaryotic organisms that have the ability to consume other bacteria (Jurkevitch, 2007). While some of these bacteria act as solitary hunters, others are known to hunt in groups in a wide mixture before they attack their prey (Martin, 2002). This predatory strategy generally implicates the production of lytic enzymes and small bioactive compounds as predatory weapons (Rosenberg and Varon, 1984; Reichenbach and Höfle, 1993; Berleman and Kirby, 2009), while genome sequencing programs of these microorganisms have revealed the presence of very broad and varied secondary metabolites (Kiss et al., 2011). Therefore, extraction and purification of antimicrobial molecules from predatory bacteria have yielded the discovery of numerous novel molecules, as illustrated by jahnellamides (Plaza et al., 2013), salimyxins (Felder et al., 2013), cystomanamides (Etzbach et al., 2014), and precorallopyronin (Schäberle et al., 2015). It is confirmed by Xiao et al. (2011) that this variety of secondary metabolites is supposed to be involved in the death of the prey. It has been found that a defect in the production of these substances significantly affects predatory activity. In this section, all research relative to the results of chemical studies of compounds produced by predatory bacteria will be reviewed and critically analyzed in Table 2.

Traditionally, Actinomycetota species are renowned for their excellent potential to produce secondary metabolites and antibiotics compounds (Hoshino et al., 2015; Ait Barka et al., 2016; Ouchari et al., 2019; Ibrahimi et al., 2020; Korichi et al., 2021). These molecules are produced to antagonize the growth of surrounding microbes (Ortiz-Ortiz et al., 2013). To date, up to 33 new secondary metabolites have been successfully isolated from 12 Actinomycetota through the co-culture (Hoshino et al., 2019). With their novel roles as predatory microbes, the diversity and identity of molecules produced during predation strategies should be considered an untapped source of biomolecules. A further co-culturing of predator and prey monitored for 15 days induced an increase in the total amount of methylated fatty acids biomarker of the predatory Actinomycetota responsible for the predation process (Ibrahimi et al., 2019). Therefore, secondary metabolites engaged in the facultative predation and antagonistic mechanism are likely to differ, from the Actinomycetota strain, the origin of habitat, or the ecology of isolated strains (McBride and Zusman, 1996; Jurkevitch, 2007; Octaviana, 2021).

Predatory Actinomycetota are mostly isolated from marine and soil environments (Kumbhar et al., 2014; Ibrahimi et al., 2020; Baig et al., 2021). They exhibit a wide range of predatory activities against diverse bacteria (Kumbhar et al., 2014; Ibrahimi et al., 2020; Baig et al., 2021). This is explained by the bioactive compound that they produce, which possess a range of antimicrobial activities. These molecules are used as a weapon by predatory Actinomycetota to kill their prey (Kumbhar and Watve, 2013). Recently, Baig et al. (2021) demonstrated a strong correlation between predation and enzyme inhibition, particularly trypsin and chymotrypsin inhibition, in which predatory Actinomycetota were found to release more enzymes in the presence of prey. On this basis, it has become apparent that Actinomycetota can exhibit the great potential to produce antibiotics and enzymes during predation. Also, it should be noted that, to date, no compounds produced by predatory Actinomycetota during predation behavior have been elucidated. Therefore, it will be interesting to conduct further studies to isolate new predatory Actinomycetota as well as the extraction and identification of their molecules involved in predation lifestyle.

Nowadays, one of the most alarming world wild problems is the increase of plant diseases caused by pathogenic bacteria, which causes great economic, environmental, and human health damage. Therefore, finding alternative and sustainable solutions to confront these pathogens represent one of the biggest challenges. In this review, we suggest the use of predatory Actinomycetota as an effective biocontrol agent. Predation is an important cause of mortality and determines the structure and activity of microbial communities in both terrestrial and aquatic ecosystems, with a complex process involving several components such as prey finding, recognition, consumption, and digestion. The literature has concentrated largely on the presence of predatory Actinomycetota and their effective activity against a panel of pathogenic microorganisms but has not examined the mechanisms of the prey lysis. Indeed, a good knowledge of the predation mechanism used by predatory Actinomycetota is essential to improve the efficiency of the lysis of the prey cells and predation performance. The application of predatory Actinomycetota in large-scale systems and field experiments must be examined to determine if there will be ecological consequences. Furthermore, it is suggested that research evaluating predators’ efficiency should include molecular viability assays, such as EMAqPCR and EMA-Illumina, to determine the efficacy of treatment. Finally, during this review, we noticed that antibiotics produced during the process of predation by predatory Actinomycetota are demonstrated but have not yet been identified. Consequently, it seems advisable to discover such compounds. In the future, actinobacterial predation could be a new approach to control plant pathogen, cyanobacterial blooms; however, intensive research efforts are required to pursue this aim.

MI wrote the manuscript. YO and MJ refining and critical reading of the manuscript. LL, SL, and MH revised the manuscript. All authors contributed to the manuscript and approved the submitted version.