Biofilms are notoriously difficult to treat and are up to 1000 times more resistant to antibiotics than their planktonic counterparts (Sharma et al., 2019). Multiple factors contribute to antibiotic resistance exhibited by bacterial biofilms. These factors include limited antibiotic and biocide penetration, efflux pumps, persister cells, reduced growth rate and horizontal gene transfer (Verderosa et al., 2019). A biofilm’s first line of defense against antibiotics is the EPS matrix, which is achieved by limiting the penetration of antibiotics into the biofilm. As mentioned above, eDNA can chelate numerous antibiotics, thus trapping the antibiotics and preventing them from moving through the EPS matrix. Numerous studies have shown that a steep gradient difference in antibiotic concentration is observed when the concentration of antibiotics at the base of the biofilm is compared to the outer regions of the biofilm (Anderl et al., 2000; Doroshenko et al., 2014; Justo and Bookstaver, 2014). Another advantage is high cell density that facilitates horizontal gene transfer and effectively increases the spread of plasmid-borne antibiotic resistance genes, as demonstrated in S. aureus (Savage et al., 2013). Most antibiotics target actively growing and dividing cells; thus, the slow metabolic rate of bacteria deep within the biofilm protects them against antibiotics that do manage to penetrate the EPS (Ashby et al., 1994). Persister cells employ the same method to ensure survival and act as disease reservoirs once the antibiotic pressure is removed (Keren et al., 2004; Lewis, 2007). Although efflux pumps are present in planktonic cells, Zhang and co-authors found that efflux pumps involved in antibiotic resistance are upregulated in biofilms (Zhang and Mah, 2008). The Verderosa et al. (2019) review provides a more comprehensive summary of how biofilms evade antibiotics.

Industrial water usage requires water management systems which are usually prone to biofilm development (Figure 1). Petroleum refineries, steel mills, power generation plants, and petrochemical plants often have onsite industrial water management facilities, and biofilm communities colonize the surfaces of most of these as well as associated equipment such as fill material, reservoirs, submerged sight glasses and sensors, heat exchangers and pipelines (Liu et al., 2011; Rao, 2012; Di Pippo et al., 2018). Furthermore, the worldwide shortage of fresh water means that industrial water is constantly recycled, increasing the nutrient load of industrial water systems, thus creating the ideal environment for biofilm formation (Coetser and Cloete, 2005; Flemming et al., 2007). In addition, bacteria can also use some antiscalants and corrosion inhibitors as nutrient sources depending on their composition (Kusnetsov et al., 1993). Mechanical cleaning of cooling towers effectively removes biofilms, but structural design can make this method impossible. Thus, alternative methods are needed to remove biofilms and prevent biofouling in cooling towers (Wang et al., 2023). Various oxidizing (mainly chlorine, calcium hypochlorite, sodium hypochlorite, ozone, hydrogen peroxide, bromine chloride) and/or non-oxidizing (principally heavy metal compounds, amines, aldehydes, thiocyanates, isothiazolone, and organo-bromine compounds) agents are used to prevent microbial growth and microbially induced corrosion in many industrial water system components (Cloete and Flemming, 2012). Microbially-induced corrosion is a significant problem as numerous metals, such as nickel and aluminium-based alloys, comprise the base structure of industrial water-cooling circuits and towers. Exposure of these substances through an electrochemical reaction between the interfaces can lead to corrosion (Figure 1) (Beech et al., 2000).

Biocide treatment regimens are designed to treat wastewater streams for safe discharge into receiving waterbodies (AquaTech, 2019), with chlorine and ozone being the two major biocides used for years. Unfortunately, this process can lead to contamination of drinking water reservoirs and is considered toxic to the environment (Williams and McGinley, 2010). In addition, due to the high tolerance of bacteria in biofilms to these toxic biocides, higher than normal concentrations are needed which increases the cost and the burden on the environment (Maaike et al., 2006). As the biofilm thickens, higher biocide concentrations are required to penetrate the deeper levels and if sub-inhibitory concentrations of biocide are used, the chance of resistance developing increases. To improve the bioactivity of chemical biocides, industries have used surfactants, e.g., cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) in combination with chemical biocides to control and eradicate biofilm growth (Cloete et al., 1998; Simões et al., 2006).

Biosurfactants can be effectively introduced to industrial water systems by replicating methods for the removal of biofilms using conventional chemical surfactants (Table 2); several of these have been assessed in this regard (Table 3). Introducing biosurfactant molecules in solid or liquid form to biofilm-polluted sites could lead to biofilm dissociation by disrupting the water, nutrient and gaseous exchange channels. Biosurfactants can have intrinsic antimicrobial properties and therefore act as biocides in their own right (Płaza and Achal, 2020) or as dispersants of naturally derived compounds (Figure 3). In other words, biosurfactants can either kill the monolayers of cells after disruption or disrupt the mature biofilm by dispersion leaving the core cells alive. Lipopeptide 6-2 is a clear example of a biosurfactant that can act as killing agent as well as a dispersant agent (Song et al., 2016).

Biosurfactants used as biocides generally target the bacterial outer and cytoplasmic membranes and are usually more active against planktonic cells. Due to their small size and chemical properties, biosurfactants can penetrate the biofilm, directly killing microorganisms (Fleming and Rumbaugh, 2017). Many chemical biocides exhibit potent bactericidal activity but are ineffective at killing bacteria within biofilms (Singh and Sharma, 2020). The ability of some biosurfactants to show both dispersing and bactericidal activity makes them potent biocides. Although literature on biosurfactants used as biocides in industrial water systems is scarce, examples of biosurfactants with both antibacterial and anti-biofilm activity exist and include biosurfactants isolated from Lactobacillus spp. (Sambanthamoorthy et al., 2014), rhamnolipids (Elshikh et al., 2017) and sophorolipids (Diaz De Rienzo et al., 2015). While all the biosurfactants mentioned above were tested under laboratory conditions, they all show promise for industrial use. For example, the biosurfactants produced by L. jensenii and L. rhamnosus were not only able to prevent planktonic and sessile growth but were also able to disrupt preformed biofilms indicating that these biosurfactants can be used in industrial settings (Sambanthamoorthy et al., 2014). In another example, the rhamnolipid produced by Burkholderia thailandensis E264 prevents biofilm formation between 50% and 90% when plastic surfaces were precoated with a mixture of long-chain rhamnolipids (Elshikh et al., 2017). The long-chain biosurfactants also disrupted preformed immature biofilms by between 50% and 80%. Additionally, the Pf495-biosurfactant produced by Pseudomonas fluorescens inhibits pathogenic Listeria monocytogenes LO28 from attaching to polytetrafluoroethylene and stainless-steel surfaces (Meylheuc et al., 2001). This is significant as both polytetrafluoroethylene and stainless steel constitute major components in treatment plants, water-cooling circuits/systems, and industrial pipes. As shown by Song and co-authors (2016), B. amyloliquefaciens lipopeptide 6-2 could actively disrupt and kill the monolayers of two biofilm-forming species, Pseudomonas aeruginosa PAO1 and Bacillus cereus 1A06374 (Song et al., 2016).

Biosurfactants can also be used in combination with antibiotics to improve effectiveness or through synergistic interactions. For example, preformed biofilms of Escherichia coli CFT073 could not be removed by Bacillus licheniformis V9T14 lipopeptides, but when used in combination with different antibiotics, a significant difference was observed with more than 90% (1 log₁₀) biofilm reduction (Rivardo et al., 2011). In a follow-up patent report, the same Bacillus licheniformis V9T14 lipopeptides were used with polycide (a biocide), which further prevented the development of biofilms on abiotic and biotic surfaces (Ceri et al., 2013). Though both antibiotics and biocides may kill the free-living planktonic cells, they are not fully effective in killing organisms within a biofilm (Ceri et al., 2013). As mentioned, biosurfactants assist by penetrating the EPS and killing the monolayer cells. Although the above authors did not discuss usage in industrial water systems, we propose a similar synergistic approach as an effective manner to treat and prevent biofilm formation.

In some cases, biosurfactants bind to the cell surface or its components and alter cell-surface hydrophobicity. This prevents microbial strains from attaching to hydrophobic surfaces (Neu, 1996; Kuiper et al., 2004). Biosurfactants such as surfactin (Mireles et al., 2001), rhamnolipid (Bharali et al., 2013) and a cyclic lipopeptide isolated from Bacillus amyloliquefaciens AR2 (Satpute et al., 2016) change the cell surface hydrophobicity of bacteria affecting the integrity of the cellular membrane and interfering with cell adhesion (Shakerifard et al., 2009). Rhamnolipids also act on the biofilm matrix by forming micelles within the biofilm (Paraszkiewicz et al., 2021). The micelles affect the cell surface charge and hydrophobicity within the biofilm, thereby disrupting biofilm adhesion and permeabilizing the biofilm surface (Sotirova et al., 2008; Sotirova et al., 2009). Treatment of biofilms with surfactin leads to disruption of the crystalline/semi-crystalline packing of lipid bilayers in the bacterial cell membrane (Brasseur et al., 2007). This results in dissipation of the proton motive force while interrupting the electron transport chain (Sheppard et al., 1991; Rautela et al., 2014).

Biofilm dispersal is achieved when surfactants alter the cell surface tension properties of microorganisms, thus preventing adhesion to other microbial cells as well as surfaces. Many commercialized biodispersants highlighted in literature are often surfactant-based (Guilhen et al., 2017). They function by breaking and suspending biofilms in bulk water, and the resulting clumps are then removed from the system (Cooper, 2021). When compared to their synthetic counterparts, research on biosurfactant molecules that disperse biofilms in industrial water systems is not extensive (Fleming and Rumbaugh, 2017); however, their potential as dispersing agents by mixing with other compounds or solvents was highlighted by Cao (2015). Biosurfactants as dispersing agents can prevent biofilm formation by prior coating of equipment and attachment surfaces, thus altering the surface hydrophobicity or by dispersing the biofilm through the formation of channels within the biofilm matrix (Friedlander et al., 2019).

Some biosurfactants, such as rhamnolipids cause dispersal by disrupting the biofilm matrix and solubilizing components of the matrix (Pamp and Tolken-Nielsen, 2007). The surfactin from Bacillus tequilensis SDS21 showed promising activity in hard water and was able to remove biofilms grown on glass, stainless steel and polystyrene, indicating that this biosurfactant might be able to disperse biofilms that form in water pipes (Singh and Sharma, 2020). Additionally, the surfactin was still viable when exposed to boiling temperatures (100°C) for 3 h and an extreme pH range (pH 5–12) (Singh and Sharma, 2020), indicating that it will still be viable when used in harsher conditions such as those found in industrial wastewater treatment centres. Another biosurfactant produced by the Bacillus strain AR2 was also able to maintain its anti-biofilm activity under extreme conditions. The biomolecule inhibited biofilm formation at a 46%–100% efficiency rate while also dispersing 25%–100% of Candida matured biofilms (Rautela et al., 2014). Di Pippo et al. (2017) analyzed the biodispersant properties of polyglucoside (a biobased surfactant) to detach mature biofilms (grown over 28 days) using lab-scale systems and artificial industrial cooling tower water. The study provided the first insights into the ability of biodispersants to eradicate unwanted biofilms in industrial cooling tower systems. Although the biocide hypochlorite showed a higher biofilm removal rate than polyglucoside (at 0.50 and 0.25 g/L), polyglucoside still removed biofilms better than the non-green formulations such as Tween^® 80 and Chimec 7464. Also, they found that green bio-dispersants could perform better at higher concentrations when applied to biofilms grown in batch conditions. This is in comparison to semicontinuous experiments that favoured lower concentrations, with 58%–69% biofilm dispersal values achieved (Di Pippo et al., 2017).

Quorum sensing plays an essential role in the expression of virulent genes associated with biofilm formation and is yet another target for disruption. Biosurfactants isolated from Cobetia sp. MM1IDA2H-1 can interfere with quorum sensing signals related to biofilm formation in the fish pathogen Aeromonas salmonicida (Ibacache-Quiroga et al., 2013). The proposed mechanism here is ‘signal hijacking’ as production of the purple pigment, violacein, by the test organism used in these studies (Chromobacterium violaceum) is under quorum sensing control. The loss of the purple phenotype is associated with the biosurfactant out-competing the native C. violaceum quorum sensing signalling molecules (acyl-homoserine lactone-like compounds; 3-hydroxy fatty acids (Ibacache-Quiroga et al., 2013). The binding of the mimic, as opposed to the true signal, does not lead to induction of the quorum sensing pathway and consequently the absence of the pigment.

Anti-biofilm peptides including glycopeptides, lipopeptides, and cyclic peptides possess broad-spectrum activities that target the “biofilm lifestyle” (Pletzer et al., 2016; Pletzer and Hancock, 2016) which include bacterial membranes, adhesion organelles and molecules, biofilm structural composition, and the matrix components (Pletzer and Hancock, 2016). These peptides have successfully been used to disrupt biofilms produced by ESKAPE and non-ESKAPE pathogens on surfaces (known nosocomial pathogens exhibiting multidrug resistance and virulence) (Rajput and Kumar, 2018). Peptide-based surfactants can also induce changes in gene expression leading to inhibition of biofilm formation and ultimately cell death (de la Fuente-Nunez et al., 2012). This is demonstrated by cationic peptide 1037 against Listeria monocytogenes, Pseudomonas aeruginosa, and Burkholderia cenocepacia. The peptide targets and suppresses the multiple genes responsible for biofilm formation (de la Fuente-Nunez et al., 2012). Later, de la Fuente-Nunez et al. (2014) described a potent anti-biofilm peptide 1018 that induces a cellular response by binding, blocking, and degrading (p)ppGpp [guanosine 3′,5′-bis(pyrophosphate)], also known as the magic spot, which serves as an important signal and secondary messenger molecule in biofilm development. Interfering with the (p)ppGpp ultimately leads to the dispersal of biofilms (de la Fuente-Nunez et al., 2014). When developing biosurfactant formulations for use in industrial water systems, the focus should be complete disruption or prevention of biofilm formation to increase plant efficiency and eliminate or decrease the need for biocide use in industrial systems.

The accumulation of unwanted organic materials on any surface is called biofouling (Coetser and Cloete, 2005). Biofouling in industrial water systems involves microorganisms (e.g., bacteria, fungi, yeasts, moulds, diatoms, or algae) and macroorganisms such as mussels, protozoans, and barnacles (Melo and Bott, 1997). Biosurfactants used in anti-fouling strategies target biofilms that create dead biomass. Industrial biofouling in water systems results in several problems that include decreased membrane flux, shutdown, damage, reduced heat-exchanger efficiency, blockages, and energy loss (Coetser and Cloete, 2005). Many extreme conditions occur in industrial systems, thus the applied biosurfactants must have properties such as hard water resistance, stability, and ability to withstand harsh conditions such as high -acidic/alkaline conditions, temperature, and salinity. Other properties such as solubilizing properties, good wetting, interfacial action, and the ability to disrupt hydrophobicity between bacteria and surfaces are important for the successful removal of biofilms in industrial water systems (Sulaimon and Adeyemi, 2018). As discussed earlier, microbial fouling of industrial water systems can have severe health and economic consequences. Although most biosurfactant research focuses on treating biofilms in healthcare setups, we propose that biosurfactants can also play a big role in treating industrial wastewater since numerous biosurfactants can remove mature biofilms and prevent biofilm formation.

One approach to combat fouling problems is to mix paints with biosurfactants before applying them on surfaces. This improves resistance to fouling by modifying surface properties such as hydrophobicity, surface tension and charge. However, this strategy is challenging to implement as the dispersal and leach rate of the compound must be controlled and a coating mechanism that enables the gradual release of the compound needs development (Yebra et al., 2004). Aleman-Vega et al. (2020) reported a 70% biofouling decrease by degreasing and covering acrylic plates with paint formulated with biosurfactants isolated from Bacillus niabensis (My-30), B. niabensis (S-69) or Ralstonia sp. (S-74). The Bacillus amyloliquefaciens anti-CA lipopeptide also showed anti-fouling potential as the biosurfactant killed the Balanus amphitrite larvae and inhibited the growth of protozoans (Song et al., 2016), which are two organisms that contribute to fouling in cooling towers (Critchley and Bentham, 2007; Mizhir, 2012).

In addition to anti-fouling activities, biosurfactants can act as anti-corrosion agents (Table 4). For example, the biosurfactant alkylpolyglucoside inhibits corrosion of 907 steel and the efficacy could be altered by changing the alkyl chain lengths (Du et al., 2004). Another anti-biocorrosion strategy is the combination of biosurfactants and polymeric substances such as epoxy resins, polyester, polyaspartate, alginate, and polyglutamate. In a study by Zin et al. (2018), a biosurfactant complex containing rhamnolipids and an alginate-based biopolymer produced extracellularly by Pseudomonas sp. PS-17 inhibited the corrosion of aluminium alloy D16T successfully. A two-to four-fold increase in the repassivation kinetics was observed for the treated surfaces, and the effects were attributed to the rhamnolipids in the complex (Zin et al., 2018). Since microbially induced corrosion is a consequence of biofilm formation, efforts should be tailored towards preventing microbial contamination of metallic components of industrial water systems (Grasland et al., 2003; Silva et al., 2018). Metal is prone to biocorrosion and constitutes major components of industrial water systems such as water-cooling circuits, towers, vacuum pumps, treatment plants, pipes, and sensors. Bacillus species are well known for producing biosurfactants capable of combating biofilm-induced corrosion (Purwasena et al., 2019). Certain Bacillus spp. produce various surfactants that inhibit the growth of bacteria that induce metal surface corrosion (Jayaraman et al., 1999; Santos et al., 2016). Finally, the Pseudomonas stutzeri F01 biosurfactant is an eco-friendly biocide tested in the oil and gas industry, where 30%–40% of the corrosion problems are attributed to microbial corrosion (Parthipan et al., 2018). The authors report these biosurfactant compounds as efficient microbial inhibitors because they possess anti-biofilm properties against corrosive bacterial strains even at low concentrations.