Taking on Pathogenic Biofilms: An Apple a Day?

When finally ready to commit, as a college undergraduate, to a specific area of biological science,  I chose microbiology because of a fascination with infectious disease and its causation and cure. And let’s be clear, my interest was primarily in bacteriology—the big microbes. (The  virologists I knew back then teased that I wasn’t smart enough to study viruses, the small microbes; we’ll save that debate for another day.)

Bacterial biofilms were just beginning to come on the microbiological radar back in the 1980s, and were not yet part of the microbiology curriculum. However, we now know that biofilms are important, both as pathogenic organisms and in good health. This research on a means of interrupting pathogenic biofilms, while not harming commensal biofilms, both caught my attention and provided some fascinating information on the state of biofilms and biofilm research.

Biofilms are groups or large colonies of bacterial cells that adhere to and form coatings on living and nonliving surfaces. Such surfaces can include human and animal tissues, the floor and walls of a shower or even a medical device, such as a heart valve or indwelling catheter.

Bacteria can exist in an individual state, either floating or swimming in liquid, or in biofilms—rather a microbial metropolis. Free, individual floating/swimming  bacterial cells are called planktonic bacteria, and are physiologically distinct from the members of the large groups that form biofilms.

In an amazingly smart and devious means of survival, when bacteria get together in these large biofilms, they turn off certain genes and turn on others. The cues that cause shifts from planktonic or free-living cells to biofilm formation vary and include changes in nutrition and even exposure to sub-inhibitory concentrations of antibiotics (1,2).

When a cell switches to the biofilm mode of growth, it undergoes a phenotypic shift in behavior in which large suites of genes are differentially regulated (3).

Bacterial biofilms often produce and coat themselves with an extracellular polymeric substance (EPS) composed of DNA, protein and polysaccharide, which produces a slime-like material.

Biofilms can be very refractive to treatment. The EPS coating can prevent penetration of inhibitory agents to the cells. In addition the phenotypic shift seen with biofilm bacteria can decrease susceptibility to antibiotics.

Enter the apple.

In an Infection and Immunity article published in December 2011, Jin-Hyung Lee et al. examined the ability of phloretin, a flavonoid found in apples and strawberries, to inhibit biofilm formation (4). Phloretin is a type of polyphenol, known to have numerous biological functions, including antioxidative, anticarcinogenic and estrogenic activities. Phloretin is also believed to inhibit cardiovascular disease.

Lee et al. compared a number of flavonoids for the ability to induce nontoxic biofilm inhibition against enterohemorrhagic E. coli O157:h7. The flavonoids tested included curcumin, vitamin C, vitamin E, 6-aminoflavone, 6-hydroxyflavone, apigenin, daidzein, flavone, genistein, chrysin and phloretin. Phloretin was far and away the best inhibitor of biofilm formation, in a dose-dependent manner.

It is important for biofilm inhibitors to be nontoxic, as toxicity can lead to bacterial drug resistance. Phloretin was tested for toxicity by measuring the rate of growth of planktonic (free-living) E. coli O157:H7 cells. Growth rates of planktonic cells were nearly identical with or without phloretin present—so this flavonoid inhibits biofilm growth but not the growth free-living bacterial cells.

Not all biofilms are bad, and a successful drug against pathogenic biofilms would ideally not harm good, in fact essential, commensal biofilms. In these studies, four strains of the commensal organism E. coli K12, along with another nonpathogenic E. coli strain, were tested with phloretin. Phloretin caused no inhibition in tests with these five commensal bacterial strains.

The researchers next looked for a mechanistic explanation of phloretins ability to inhibit pathogenic biofilms. DNA microarray and qRT-PCR experiments identified phloretin suppression of autoinducer-2 importer genes (lsrACDEF) of E. coli O157:H7 biofilm cells;  in other words, flavonoids can interfer with bacterial quorum sensing  signaling. Biofilm communities rely on intercellular signaling and communication for survival.

Recent reports on biofilm mechanisms have shown that fimbriae (both curli and pili) are important in E. coli O157:H7 biofilm formation (1). In this study, phloretin reduced curli genes and fimbriae production, a potential mechanism for the role of phloretin in inhibiting O157:H7 biofilm formation.

In another exciting result from this paper, the authors report that phloretin shows anti-inflammatory properties in both in vitro and in vivo inflammatory colitis models.

In conclusion, while no one is suggesting you scrub your shower with an apple, you might consider having one for lunch. That “apple a day” maxim is gaining support with each new study.

Oregon State University is home to the Linus Pauling Institute on flavonoids, for those that seek more information.


    1. Karatan, E.,  Watnick, P. (2009). “Signals, regulatory networks, and materials that build and break bacterial biofilms“. Microbiology and Molecular Biology Reviews 73 310–47. PMID 19487730.
    2. Hoffman LR, D’Argenio DA, MacCoss MJ, Zhang Z, Jones RA, Miller SI (August 2005). “Aminoglycoside antibiotics induce bacterial biofilm formation“. Nature 436 1171–5.  PMID 16121184.
    3. An D, Parsek MR (2007). “The promise and peril of transcriptional profiling in biofilm communities“. Current Opinion in Microbiology 10 292–6. PMID 17573234.
    4. Lee JH, Regmi SC, Kim JA, Cho MH, Yun H, Lee CS, & Lee J (2011). Apple flavonoid phloretin inhibits Escherichia coli O157:H7 biofilm formation and ameliorates colon inflammation in rats. Infection and immunity, 79 (12), 4819-27 PMID: 21930760


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Kari Kenefick

Kari Kenefick

Kari has been a science writer/editor for Promega since 1996. Prior to that she enjoyed working in veterinary microbiology/immunology, and has an M.S. in Bacteriology, U of WI-Madison. Favorite topics include infectious disease, inflammation, aging, exercise, nutrition and personality traits. When not writing, she enjoys training her dogs in agility and obedience. About the practice of writing, as we say for cell-based assays, "add-mix-measure".

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