tetrakis(hydroxymethyl)phosphonium sulfate (THPS) can stimulate biofilm formation by Pseudomonas aeruginosa, which, in turn, accelerates corrosion of materials like carbon steel. This article examines how even low levels of biocides can have a paradoxical effect, increasing microbial growth and corrosion. It underscores the importance of precise biocide dosing and better management to prevent unintended material degradation in industrial settings.
In industrial water systems, biocides play a critical role in controlling microbial growth and mitigating microbiologically influenced corrosion (MIC), which can significantly damage metal infrastructure. However, recent studies have revealed a surprising twist: sub-minimum inhibitory concentrations (sub-MICs) of biocides like tetrakis(hydroxymethyl)phosphonium sulfate (THPS) may actually promote the formation of biofilms by certain bacteria, such as Pseudomonas aeruginosa. These biofilms not only protect the bacteria from biocides but also accelerate the corrosion of materials like carbon steel. This paradoxical effect raises concerns about biocide dosing and the management of microbial growth in industrial environments. This article explores the impact of sub-MIC levels of THPS on biofilm formation and corrosion, highlighting the need for precise biocide application.
Biocides like THPS are non-oxidizing agents commonly used in industrial settings for their ability to kill a wide range of microorganisms, including bacteria, fungi, and algae. These biocides work by disrupting the integrity of microbial cell walls, leading to cell death. They are typically applied in concentrations above the minimum inhibitory concentration (MIC), the lowest concentration needed to halt bacterial growth. However, emerging research suggests that when biocides are applied at sub-MIC levels, they may have unintended consequences, including the stimulation of biofilm formation. Biofilms are clusters of microorganisms that attach to surfaces and are encased in a protective matrix of extracellular polymeric substances. These biofilms can be extremely damaging to industrial materials, accelerating corrosion by creating an environment that fosters microbial activity and shields bacteria from biocides.
In the study discussed here, sub-MIC levels of THPS (specifically 160 µg/ml) were introduced to Pseudomonas aeruginosa cultures to observe their effects on biofilm formation and corrosion of carbon steel. The results were striking: even at concentrations too low to inhibit bacterial growth, THPS promoted an increase in both biofilm thickness and the corrosion rate of the metal. Biofilm thickness increased from 82 µm to 97 µm, and the corrosion rate jumped from 10 mils per year to 18.7 mpy. These findings were confirmed through a variety of electrochemical techniques, such as electrochemical impedance spectroscopy, Tafel polarization, and linear polarization resistance, which all demonstrated an acceleration of corrosion in the presence of sub-MIC THPS.
Further analysis revealed that the biofilms formed under the influence of sub-MIC THPS exhibited enhanced characteristics that contributed to their ability to accelerate corrosion. The biofilms contained higher levels of EPS, which made the matrix more robust and better able to adhere to the surface of the carbon steel. This increased structural integrity made the biofilms more resistant to biocide treatment, essentially shielding the bacteria from the biocide’s intended effects. Additionally, the biofilms demonstrated an extended dispersal phase, during which microbial cells detached and spread, potentially leading to the formation of new biofilms in other areas of the system. This ability to disperse and colonize new surfaces could amplify the corrosion problem across a larger area of the system, leading to widespread material degradation.
Confocal laser scanning microscopy provided further insight into the biofilm structure and dynamics. Images revealed that the inner layers of the biofilm contained a higher density of viable sessile cells, bacteria that adhere to the surface and are less exposed to biocides due to their location within the biofilm. The presence of more viable cells deep within the biofilm suggests that sub-MIC concentrations of THPS did not effectively inhibit bacterial growth but instead promoted the persistence and growth of the biofilm. This phenomenon is similar to what has been observed with other antimicrobial agents, such as antibiotics, where sub-MIC levels can trigger biofilm formation and increase microbial resistance to treatment.
The issue of sub-MIC concentrations promoting biofilm growth and corrosion is particularly relevant in industrial settings, where biocides may be unevenly distributed due to poor mixing, incorrect dosing, or fluctuations in biocide concentration over time. Microorganisms in certain areas of the system may therefore be exposed to sub-MIC levels of biocides, which could foster the growth of biofilms in these pockets. Over time, these biofilms can contribute to the degradation of infrastructure, such as pipelines, cooling towers, and storage tanks, leading to higher maintenance costs, equipment failures, and safety risks.
This study underscores the need for careful monitoring and control of biocide concentrations in industrial systems. While biocides are an essential tool for managing microbial growth, sub-MIC levels may inadvertently promote the very problems they are meant to solve. The findings suggest that industries must take a more nuanced approach to biocide application, ensuring that concentrations are maintained at effective levels throughout the system. Additionally, further research is needed to explore the full scope of how sub-MIC biocide concentrations affect biofilm formation and corrosion in different microbial species and industrial materials.
The phenomenon of sub-MIC levels enhancing biofilm growth is not unique to THPS or Pseudomonas aeruginosa. Similar effects have been observed with other antibiotics and biocides, indicating that sub-MIC concentrations can trigger biofilm formation in a wide range of bacteria. This highlights the need for a broader understanding of how biocides interact with microbial communities and the biofilm formation process. With more precise dosing and better monitoring, industries can minimize the risks associated with microbiologically influenced corrosion and improve the efficiency of biocide treatments.
In conclusion, this research emphasizes the importance of accurate biocide dosing and monitoring in industrial environments. Sub-MIC levels of biocides like THPS can inadvertently stimulate biofilm formation and accelerate corrosion, which can have serious consequences for the integrity of industrial materials. Understanding and controlling the complex interactions between biocides, microbes, and metal surfaces is essential for ensuring the long-term durability of industrial infrastructure and minimizing the economic and safety risks associated with corrosion.
