A biofilm is primarily made up of bacteria and other microbes that attach to surfaces including living tissues and medical devices.

What is a Biofilm?

A biofilm is a group of germs—mostly bacteria—that stick to a surface and surround themselves with a slimy protective layer they make. This layer helps them survive harsh conditions like disinfectants, antibiotics, and the immune system.

A biofilm starts when bacteria or other germs stick to a surface, like a pipe, countertop, or medical device. Once they’re attached, they begin to grow and make a slimy, glue-like coating that helps them stick together and stay protected.

This coating is called a matrix, and it’s made from substances the bacteria produce themselves—mainly sugars, proteins, and bits of DNA. These materials create a kind of shield that helps the bacteria survive and spread, even when they’re exposed to disinfectants, antibiotics, or the body’s immune system.

As the biofilm gets thicker and more organized, it becomes a complex living community. Inside, bacteria can communicate with each other, share nutrients, and even swap genes that make them more dangerous, including genes that help them resist antibiotics. This makes infections involving biofilms much harder to treat and control, especially in places like hospitals, food factories, and water systems.

In public health and infectious disease control, biofilms matter because they are often the hidden source of persistent or recurrent infections, particularly in healthcare and environmental settings. Once a biofilm forms on a surface—whether it’s a medical device, water pipe, or food processing conveyor belt—it becomes extremely difficult to eliminate. Biofilms also facilitate the exchange of genetic material between bacteria, including genes for antibiotic resistance and virulence, increasing the risk of hard-to-treat infections.

Two well-documented examples where biofilms contribute to significant infectious disease threats are Listeria monocytogenes and Legionella pneumophila.

Examples of Biofilm-Related Infections: Listeria

Listeria monocytogenes is a foodborne pathogen known for its ability to grow at refrigeration temperatures and form robust biofilms on stainless steel, plastic, and other surfaces commonly found in food processing facilities.

Biofilm and Listeria Persistence

These biofilms protect Listeria from standard cleaning agents, allowing it to persist and contaminate food products over long periods. This persistence has been implicated in several deadly listeriosis outbreaks.

Key characteristics of Listeria biofilms

• Form on stainless steel, plastic, and rubber
• Resistant to common disinfectants
• Survive at low temperatures
• Enable chronic contamination of food products
• Associated with high fatality in vulnerable groups
• Require specialized cleaning protocols

Notable Listeria outbreaks linked to biofilms

• 2011 U.S. cantaloupe outbreak
• 2018 South Africa processed meats outbreak
• 2020 enoki mushroom outbreak in the U.S.
• 2023 deli meat and cheese outbreaks
• 2025 pre-packaged sandwiches and nutritional supplement shakes

Preventing Listeria outbreaks requires understanding the resilience of biofilms and applying mechanical, enzymatic, and chemical strategies to break them down.

Examples of Biofilm-Related Infections: Legionella

Legionella pneumophilais a respiratory pathogen known for its ability to cause a severe pneumonia that can lead to death in vulnerable populations. It was first discovered at an outbreak of pneumonia at a convention in Philadelphia in 1976 leading to the name Legionnaires Disease. Legionella pneumophila colonizes biofilms in water systems, evading chlorine and other disinfectants. It can be protected inside amoebae living within a biofilm.

Settings Where Legionella Biofilms Thrive

• Cooling towers and evaporative condensers
• Hot tubs, spas, and whirlpools
• Hospital and hotel plumbing systems

Mechanisms of Biofilm Resistance

Structural protection

Bacteria release extracellular polymeric substances, abbreviated as EPS. EPS are sticky, slimy materials that bacteria make and release outside their cells. Composed of sugars, proteins, and pieces of DNA, EPS acts like glue, helping the bacteria stick to surfaces and to each other. Together, they can form a structure—or matrix—that holds the biofilm together. The EPS matrix:

• Slows diffusion of biocides
• Shields inner bacteria from environmental stress
• Creates microenvironments with different pH and oxygen levels

Cellular dormancy

Some bacteria within biofilms enter a dormant state, making them less susceptible to antimicrobial agents.

• Low metabolic activity reduces drug uptake
• Persistent cells survive high-dose treatments
• Can resurge after treatment ends

Genetic exchange

Close proximity of bacteria facilitates horizontal gene transfer within biofilms.

• Spread of resistance genes
• Exchange of virulence factors
• Increased adaptability to stress

Host immune evasion

Biofilms reduce immune system recognition and clearance.

• Reduced antigen presentation
• Neutrophils and macrophages struggle to penetrate
• Prolonged inflammation without clearance

Protozoa as reservoirs

For Legionella, protozoa in biofilms serve as a protected niche, aiding replication and persistence.

Public Health Implications

• Outbreaks linked to contaminated food and water, including hot tubs
• Hospital-acquired infections involving devices and plumbing
• Challenges in eradicating pathogens from industrial systems

Control and Prevention Strategies

• Mechanical removal through scrubbing or flushing
• Use of biofilm-specific enzymatic or chemical agents
• Systemic monitoring and environmental sampling
• Regular maintenance of high-risk systems

Best practices for food industry

• Clean-in-place systems with validated protocols
• Routine microbial surface testing
• Design of equipment to minimize crevices
• Staff training on contamination risks
• Use of biofilm-disrupting sanitizers