As early as 1676, Antonie van Leeuwenhoek observed microbial aggregates in dental plaque, yet the biofilm theory had not been established.
In the 1930s, Henrici and Zobell discovered that aquatic microorganisms could adhere to solid surfaces and form microcolonies, laying the foundation for early phenomenological research.
The term “biofilm” first appeared in wastewater engineering literature in 1975. In 1978, J. William Costerton systematically proposed the modern biofilm concept. He stated that after microorganisms attach to interfaces, they secrete extracellular polymeric substances (EPS) to encase microbial cells and form communities with three-dimensional architecture, water channels and distinctive physiological traits. This lifestyle differs substantially from planktonic bacteria and confers enhanced resistance to environmental stresses. Since then, biofilm theory has been widely applied in the research of microbiologically influenced corrosion, marine biofouling, environmental bioremediation and other fields.
Classically defined, a biofilm is a community of aggregated microbial cells embedded within a self-produced extracellular polymeric matrix and adherent to biotic or abiotic interfaces. It contains internal water channels, nutrient gradients and quorum-sensing systems, and represents the predominant lifestyle of microorganisms in nature.
“Biofilm” is the standard academic nomenclature, especially in the fields of microbiologically influenced corrosion and environmental microbiology. A biofilm refers to a three-dimensionally structured microbial community formed when microorganisms attach to solid surfaces (or liquid–liquid and gas–liquid interfaces) and secrete EPS to encapsulate themselves. Rather than a simple pileup of bacteria, it is an organized collective lifestyle, constituting two distinct bacterial phenotypic states together with free-living planktonic cells (Flemming, 2024).
A corrosive biofilm develops when corrosive microorganisms adhere to metal surfaces and secrete EPS. Its microenvironment (oxygen gradients, acidic microzones, metabolic products and electron transfer) can initiate or accelerate metal corrosion. The biofilm serves as a carrier, and microbial metabolism within the biofilm drives corrosion. The term corrosive biofilm (short for corrosion-type biofilm) is commonly adopted in academic papers.
The core characteristics of biofilms are summarized as follows:
1. Heterogeneity of interfacial microenvironments. Even in bulk aerobic liquids, aerobic microbes within biofilms consume oxygen and create anaerobic microzones at metal interfaces. Microbes also produce organic acids metabolically to form localized acidic microregions that disrupt metal passive films. Gradients of oxygen, pH, and redox potential across the biofilm are critical for corrosion initiation.
2. Quorum sensing. Microorganisms secrete signaling molecules to regulate adhesion, EPS synthesis and metabolic pathways, and coordinately express corrosion-related functional genes.
3. Electron transfer pathways. Two major corrosion routes are recognized: indirect mechanisms, where microbes produce corrosive metabolites such as sulfides and organic acids to corrode metals via chemical reactions; direct electron transfer, in which electroactive microbes form electrical connections with metals and directly withdraw electrons from elemental iron, leading to electro-microbiological corrosion.
4. Stress resistance. Microbes embedded in biofilms exhibit far higher tolerance to biocides and environmental stress than planktonic cells, so conventional cleaning and dosing strategies hardly achieve complete removal.
Biofilms consume oxygen at interfaces and generate anaerobic niches, facilitating the proliferation of anaerobic corrosive bacteria such as sulfate-reducing bacteria. These microbes metabolically generate sulfides and organic acids, induce local acidification and dissolve passive films. Electroactive microorganisms within biofilms mediate direct metal electron transfer and accelerate anodic metal dissolution. EPS and precipitated corrosion minerals inside biofilms lead to localized occlusion, forming occluded cells and aggravating localized pitting corrosion.
MIC induced by corrosive biofilms can cause localized pitting, crevice corrosion in carbon steel, stainless steel and pipeline steel, resulting in failure of pipelines, marine structures and oilfield equipment. Corrosion attacks tend to be localized and may cause perforation within a short period, bringing enormous economic losses.
Metal corrosion can be mediated by diverse microorganisms. Core corrosive bacteria are classified into aerobic and anaerobic groups according to their oxygen requirements for metabolism. Typical functional taxa include slime-forming bacteria, sulfate-reducing bacteria, iron-oxidizing bacteria and iron-reducing bacteria.
Microbiologically mediated corrosion mostly manifests as localized destructive damage, including metal pitting, aggravated erosion, dealloying, stress corrosion cracking, intensified galvanic corrosion and hydrogen embrittlement. Bacterial EPS promotes stable cell attachment on material surfaces. Its organic components such as polysaccharides, proteins and fatty acids modulate surface wettability, charge properties and surface free energy, thereby markedly altering the electrochemical behavior at the metal–biofilm interface and participating in and promoting the progression of microbiologically influenced corrosion.
Materials exposed to humid atmospheres, soils, aqueous media and other natural environments generally suffer from microbiological corrosion to varying degrees. This phenomenon is particularly prominent in marine service conditions, oil and gas facilities, aqueous systems and biomedical implants.
Biofilm fouling widely occurs in marine engineering, food processing, water supply, petroleum, papermaking, circulating cooling, power generation, medicine, dentistry and many other industries, substantially increasing equipment operation, maintenance and cleaning costs. Household environments also represent the most common setting for human exposure to biofilms.