Biofilms as Hidden Harborage

Biofilms are among the most persistent challenges faced by operators of water systems, food plants, and animal facilities. They are surface-associated microbial communities embedded within a self-produced EPS matrix composed of polysaccharides, proteins, lipids, and extracellular DNA. This matrix confers structural stability, nutrient retention, and chemical resistance, enabling microbial survival under hostile conditions.

While biofilms are sometimes beneficial-for example, in fixed-film bioreactors used for wastewater treatment-their uncontrolled growth creates significant operational burdens. Biofilm formation on clarifier walls, trickling filters, drain lines, and cooling tower surfaces can reduce hydraulic capacity, impede heat transfer, generate odors, and harbor pathogens. Less appreciated, but equally critical, is the role biofilms play in supporting pest populations, particularly flies whose larvae thrive in moist, organic-rich environments.

Biofilm development is a multistage process involving initial attachment, irreversible adhesion, maturation, and dispersion. During initial attachment, planktonic bacteria encounter a surface and adhere via weak forces or pili-mediated contact. If conditions are favorable, the cells transition to irreversible adhesion, secreting EPS that glues them to the surface and traps additional cells.

The EPS matrix is a hydrated gel that can retain water at up to 95% of its mass. It is composed of polysaccharides, proteins, extracellular DNA (eDNA), and lipids. This complex matrix not only supports microbial growth but also creates a diffusion barrier, slowing penetration of biocides and allowing time for neutralization reactions. Mature biofilms often contain multi-species consortia, including aerobic, facultative, and obligate anaerobes living in stratified layers, allowing biofilms to persist under fluctuating conditions.

Biofilms function as ideal microhabitats for the breeding and development of several invertebrate pests, most notably filter flies (Psychodidae). These insects are notorious nuisances in wastewater plants, food processing facilities, and livestock barns, where their presence can lead to customer complaints, regulatory citations, and secondary contamination.

The EPS matrix retains moisture, preventing desiccation even under intermittent drying conditions, which is critical for the survival of fly eggs and larvae. Trapped organic matter serves as a continuous nutrient source. Psychodid larvae actively graze on biofilm microbial biomass, creating feeding channels and aerating the matrix. Biofilm’s diffusion barrier limits exposure of larvae to chemical interventions, with chlorine consumed at the outer layers before reaching interior zones, allowing survival despite disinfection efforts.

Biofilms frequently harbor opportunistic pathogens such as E. coli, Salmonella, and Pseudomonas aeruginosa. Adult flies emerging from these sites can mechanically transfer pathogens to food-contact surfaces, equipment, and human-occupied spaces, creating a public health hazard and regulatory liability.

Biofilm presence fundamentally undermines sanitation programs. Chlorine demand can increase up to 100× in biofilm-rich systems as oxidizing agents are rapidly consumed by EPS components.

Microorganisms deep in the matrix remain viable, leading to regrowth within hours to days. Fogging and surface sprays suppress adult flies temporarily but leave larval habitats intact, causing rapid population rebounds.

The economic burden is considerable: facilities face repeated cleaning cycles, increased labor, chemical costs, downtime, and risk of fines. The persistence of biofilm is a key reason why pest and odor issues become chronic rather than episodic.

Several strategies are currently used to control biofilms and associated pests, each with strengths and weaknesses:

Method Strengths Limitations

Mechanical Cleaning Immediate biomass removal Labor-intensive, regrowth within days

Chemical Oxidizers Widely available, familiar to operators High demand in biofilm, risk of corrosion

Fogging/Insecticides Fast knockdown of adults No effect on larvae, frequent reapplication needed

Enzymatic Cleaners Target EPS specifically Costly, sensitive to environment

Biosurfactants Eco-friendly, potential residual activity Limited field data

Literature consensus emphasizes that biofilm must be addressed at the matrix level to achieve sustainable control. Future solutions must penetrate EPS, slow recolonization, and provide residual protection to reduce both chemical use and labor demand. Biosurfactants-amphiphilic molecules with hydrophilic and hydrophobic domains-offer a promising approach, enabling displacement of biofilms and allowing water to penetrate. Early research shows they can disrupt biofilm integrity while being environmentally safe.

Biofilms are dynamic ecosystems that enable pest persistence, increase chemical demand, and threaten public health. Conventional methods are insufficient to break this cycle, making biofilm-targeted strategies essential. Integrated approaches combining biofilm removal with residual surface activity represent the future of pest management and sanitation science, offering a pathway toward cleaner, safer, and more sustainable facilities.