Formation of Biofilms

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Biofilms

1. Introduction

Microbial biofilms on surfaces cost the nation billions of dollars yearly in equipment damage,

product contamination, energy losses and medical infections. Conventional methods ofkilling bacteria (such as antibiotics, and disinfection) are often ineffective with biofilmbacteria. The huge doses of antimicrobials required to rid systems of biofilm bacteria areenvironmentally undesirable (and perhaps not allowed by environmental regulations) andmedically impractical (since what it would take to kill the biofilm bacteria would also kill thepatient!). So new strategies based on a better understanding of how bacteria attach, growand detach are urgently needed by many industries such as in dairy processing.

Conversely, microbial processes at surfaces also offer opportunities for positive industrialand environmental effects, such as bioremediating hazardous waste sites, biofilteringindustrial water, and forming biobarriers to protect soil and groundwater from contamination.

The ubiquity and significance of the biofilm phenomenon is confirmed by the ongoing, longterm industrial interest in the work at the Montana State University Center for BiofilmEngineering, whose associates represent a wide range of industries, including petroleum,specialty chemicals, health, household products, drinking water, mining, and utilities

Single-celled organisms generally exhibit two distinct modes of behavior. The first is the

familiar free floating, orplanktonic, form in which single cells float or swim independently insome liquid medium. The second is an attached state in which cells are closely packed andfirmly attached to each other and usually form a solid surface. A change in behavior istriggered by many factors, includingquorum sensing, as well as other mechanisms thatvary between species. When a cell switches modes, it undergoes aphenotypic shift inbehavior in which large suites of genes are up- and down- regulated.

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2. Definitions biofilms

An aggregate of microbes with a distinct architecture. A biofilm is like a tiny city in whichmicrobial cells, each only a micrometer or two long, form towers that can be hundreds ofmicrometers high. The "streets" between the towers are really fluid-filled channels thatbring in nutrients, oxygen and other necessities for live biofilm communities.

A microbial (bacterial, fungal, algae) community, enveloped by the extracellularbiopolymer which these microbial cells produce, that adheres to the interface of a liquidand a surface.

A complex structure adhering to surfaces that are regularly in contact with water,consisting of colonies of bacteria and usually other microorganisms such as yeasts, fungi,and protozoa that secrete a mucilaginous protective coating in which they are encased.

Biofilms can form on solid or liquid surfaces as well as on soft tissue in living organisms, andare typically resistant to conventional methods of disinfection. Dental plaque, the slimy

coating that fouls pipes and tanks, and algae mats on bodies of water are examples ofbiofilms. While biofilms are generally pathogenic in the body, they can be used beneficially intreating sewage, industrial waste, and contaminated soil.

3. Formation biofilms

The formation of biofilms is in many aspects a most intriguing process. How biofilms areformed and behave is told by the Montana State University and the Science & TechnicalEncyclopedia.

3.1 Montana State University: Center for Biofilm Engineering

Biofilm forms when bacteria adhere to surfaces in aqueous environments and begin to

excrete a slimy, glue-like substance that can anchor them to all kinds of material such as

metals, plastics, soil particles, medical implant materials, and tissue. A biofilm can be formed

by a single bacterial species, but more often biofilms consist of many species of bacteria, as

well as fungi, algae, protozoa, debris and corrosion products. Essentially, biofilm may form

on any surface exposed to bacteria and some amount of water. Once anchored to a surface,

biofilm microorganisms carry out a variety of detrimental or beneficial reactions (by human

standards), depending on the surrounding environmental conditions.

Figure 1. Biofilm formation

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Place a clean sterile glass slide in a stream of water containing at least minimal nutrients,and over the course of days or weeks a microbial ecosystem will form consisting of a varietyof microorganisms arranged in a complex relationship to one another and embedded in amass of extracellular polysaccharides of their own making.

The formation of this biofilm is far from a random process. To the contrary, the formation of abiofilm follows a course the nature of which can be predicted and recorded.

Figure 2. Stages in biofilm formation

Typically, within minutes, an organic monolayeradsorbs to the surface of the slidesubstrate. This changes the chemical and physical properties of the glass slide or other

substrate. These organic compounds are found to be polysaccharides or glycoproteins.These adsorbed materials condition the surface of the slide and appear to increase theprobability of the attachment of planktonic bacteria.

Free floating or planktonic bacteria encounter the conditioned surface and form a reversible,sometimes transient attachment often within minutes.

This attachment called adsorption is influenced by electrical charges carried on the bacteria,by Van der Waals forces and by electrostatic attraction although the precise nature of theinteraction is still a matter of intense debate. In some instances, as for example, in theassociation between a pathogen and the receptor sites of cells of its host there may be a

stereospecificity which though still reversible is stronger than that achieved strictly by ionic orelectrostatic forces.

If the association between the bacterium and its substrate persists long enough, other typesof chemical and physical structures may form which transform the reversible adsorption to apermanent and essentially irreversible attachment.

The final stage in the irreversible adhesion of a cell to an environmental surface is associatedwith the production ofextracellular polymer substances or EPS. Most of the EPS ofbiofilms are polymers containing sugars such as glucose, galactose, mannose, fructose,rhamnose, N-acetylglucosamine and others.

This layer of EPS and bacteria can now entrap particulate materials such as clay, organicmaterials, dead cells and precipitated minerals adding to the bulk and diversity of the biofilmhabitat. This growing biofilm can now serve as the focus for the attachment and growth ofother organisms increasing the biological diversity of the community.

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Figure 3. A full grown biofilm

The microbial inhabitants with in biofilms and microbial mats in a significant sense behave asmulticellular assemblages. Far from being the homogeneous populations usually assumed inplanktonic pure cultures, biofilms as simple as colonies on agar surfaces and as complex aspathogen interactions with host cells and the bacterial populations inhabiting the surface ofmud flats behave in many respects like the tissues of a multicellular organism.

The accompanying illustration presented here (figure 3)courtesy of the Center for BiofilmEngineering gives some concept of the structure and complexity of a typical biofilm.

At one time, most scientists thought that biofilms consisted of bacteria randomly distributed

within a uniform slimy matrix. However, once scientists began using new imaging tools likethe Confocal Scanning Laser Microscope, which allows viewing of hydrated living biofilms,they found that biofilm structures take a wide variety of forms depending on their age andgrowing conditions.

This structural complexity allows bacteria deep within the biofilm to have access to nutrientscarried by the convective flow of water. Some of the features typically seen in biofilms arelabeled in figure 3.

3. 2 Answers.com Sci Tech Encyclopedia - Biofilms

Formation of a biofilm begins with the attachment of free-floating microorganisms to asurface. These first colonists adhere to the surface initially through weak, reversible van derWaals forces. If the colonists are not immediately separated from the surface, they cananchor themselves more permanently using cell adhesion structures such as pili. Pili is afine, filamentous hairlike structure found on the surface of many gram-negative bacteria,shorter, thinner and straighter than flagella.The first colonists facilitate the arrival of other cells by providing more diverse adhesionsitesand beginning to build the matrix that holds the biofilm together. Some species are not ableto attach to a surface on their own but are often able to anchor themselves to the matrix ordirectly to earlier colonists. It is during this colonization that the cells are able to communicatevia quorum sensing. Once colonization has begun, the biofilm grows through a combinationof cell division and recruitment. The final stage of biofilm formation is known as development,

and is the stage in which the biofilm is established and may only change in shape and size.This development of biofilm allows for the cells to become more antibiotic resistant.

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3.2.1 Development stages biofilm

During biofilm development, a large number of phenomena occur simultaneously and interactover a large range of length and time scales. As a result of nutrient conversions, the biofilmexpands based on bacterial growth and production of extracellular polymers. Chemical

species need to be continuously transported to and from the biofilm system by physicalprocesses such as molecular diffusion and convection. Fluid flow drives biofilm growth byregulating the concentrations of available substrates and products. On the other hand, theflow also shears the biofilm surface, and determines biofilm detachment processes. All theselinked phenomena create a dynamic picture of the biofilm three-dimensional structure.

In the development of biofilms five stages of development can be recognized. Each stage ofdevelopment in the diagram is paired with a photomicrograph of a developing Pseudomonasaeruginosa biofilm. All photomicrographs are shown to same scale.

The five stages of biofilm development are the following:1. initial attachment

2. irreversible attachment3. maturation I4. maturation II5. dispersion

3.2.2 Biofilm dispersal

Dispersal of cells from the biofilm colony is an essential stage of the biofilm lifecycle.Dispersal enables biofilms to spread and colonize new surfaces. Enzymes that degrade thebiofilm extracellular matrix, such as dispersin B and deoxyribonuclease, may play a role inbiofilm dispersal. [2][3] Biofilm matrix degrading enzymes may be useful as anti-biofilm agents.

3.2.3 Properties

Biofilms are usually found on solid substrates submerged in or exposed to some aqueoussolution, although they can form as floating mats on liquid surfaces and also on the surfaceof leaves, particularly in high humidity climates. Given sufficient resources for growth, abiofilm will quickly grow to be macroscopic. Biofilms can contain many different types ofmicroorganism, e.g.bacteria, archaea,protozoa,fungi and algae; each group performingspecialized metabolic functions. However, some organisms will form monospecies filmsunder certain conditions.

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Figure 4. Biofilm of Staphylococcus aureus on a catheter

Researchers from the Helmholtz Center for Infection Research have investigated thestrategies used by biofilms. They discovered that biofilm bacteria apply chemical weapons inorder to defend themselves against disinfectants and antibiotics, phagocytes and ourimmune system.

The lead researcher, Dr. Carsten Matz, began a serious investigation in order to find whyphagocytes cannot annihilate the biofilm bacteria. He analyzed the marine bacteria, whichdefend themselves against the amoebae, the behavior of which copies the behavior ofphagocytes. The amoebae behave in the sea just like the immune cells in human body: theysearch for and feed on the bacteria. When bacteria are alone and separated in the water,they become an easy catch for the attackers. However, when they attach to a surface and

join other bacteria, the amoebae cannot assault them.

The researcher stated that biofilms may be seen as a source of new bioactive agents. Whenbacteria are organized in biofilms, they produce effective substances which individualbacteria are unable to produce alone.

3.2.4 Extracellular matrix

The biofilm is held together and protected by a matrix of excretedpolymeric compoundscalled EPS. EPS is an abbreviation for eitherextracellular polymeric substance orexopolysaccharide. This matrix protects the cells within it and facilitates communicationamong them through biochemical signals. Some biofilms have been found to contain water

channels that help distribute nutrientsand signalling molecules. This matrix is strongenough that under certain conditions, biofilms can become fossilized.

Bacteria living in a biofilm usually have significantly different properties from free-floatingbacteria of the same species, as the dense and protected environment of the film allowsthem to cooperate and interact in various ways. One benefit of this environment is increasedresistance to detergents and antibiotics, as the dense extracellular matrix and the outerlayer of cells protect the interior of the community. In some cases antibiotic resistance can beincreased 1000 fold.

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3.2.5 Examples of biofilms

Biofilm inYellowstone National Park. Longest raised mat area is about half a meter long.

Biofilms are ubiquitous. Nearly every species of microorganism, not only bacteria andarchaea, have mechanisms by which they can adhere to surfaces and to each other.

Biofilms can be found on rocks and pebbles at the bottom of most streams orriversand often form on the surface ofstagnant pools of water. In fact, biofilms areimportant components offood chainsin rivers and streams and are grazed by theaquatic invertebrates upon which many fish feed.

Biofilms grow in hot, acidic pools inYellowstone National Park (USA) and on glaciersinAntarctica.

Biofilms can grow in showers very easily since they provide a moist and warmenvironment for the biofilm to thrive.

Biofilms can develop on the interiors ofpipesleading to clogging andcorrosion.Biofilms on floors and counters can make sanitation difficult in food preparation areas.

Biofilms in cooling water systems are known to reduce heat transfer.

Bacterial adhesion to boat hulls serves as the foundation for biofouling of seagoingvessels. Once a film of bacteria forms, it is easier for other marine organisms such asbarnacles to attach. Such fouling can inhibit vessel speed by up to 20%, makingvoyages longer and requiring additional fuel. Time in dry dock for refitting andrepainting reduces the productivity of shipping assets, and the useful life of ships isalso reduced due to corrosion and mechanical removal (scraping) of marineorganisms from ships hulls.

Biofilms can also be harnessed for constructive purposes. For example, many

sewage treatmentplants include a treatment stage in which waste water passes overbiofilms grown on filters, which extract and digest organic compounds. In suchbiofilms, bacteria are mainly responsible for removal of organic matter (BOD); whilstprotozoa and rotifers are mainly responsible for removal of suspended solids (SS),including pathogens and other microorganisms. Slow sand filtersrely on biofilmdevelopment in the same way to filter surface water from lake, spring or river sourcesfor drinking purposes. What we regard as clean water is a waste material to thesemicrocellular organisms since they are unable to extract any further nutrition from thepurified water.

Biofilms can help eliminate petroleum oil from contaminated oceans or marinesystems. The oil is eliminated by thehydrocarbon-degrading activities of microbial

communities, in particular by a remarkable recently discovered group of specialists,the so-called hydrocarbonoclastic bacteria (HCB).

Biofilms are also present on the teeth of most animals as dental plaque, where theymay become responsible fortooth decayand gum disease.

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Biofilms are found on the surface of and inside plants. They can both contribute tocrop disease or, as in the case of nitrogen fixing Rhizobium on roots, exist symbioticallywith the plant. Examples of crop diseases related to biofilms include Citrus Canker,Pierce's Disease of grapes, and Bacterial Spot of plants such as peppers and tomatoes.

Biofilms are found in food processing plants such as in heaters and inevaporators and may cause biofilm recontamination in passing bulk fluid.

3.2.6. Biofilms and infectious diseases

Biofilms have been found to be involved in a wide variety of microbial infections in the body,by one estimate 80% of all infections. Infectious processes in which biofilms have beenimplicated include common problems such as urinary tract infections,catheterinfections,middle-ear infections, formation ofdental plaque, gingivitis, coatingcontact lenses, and lesscommon but more lethal processes such as endocarditis, infections incystic fibrosis, andinfections of permanent indwelling devices such as joint prostheses andheart valves. More

recently it has been noted that bacterial biofilms may impair cutaneous wound healing andreduce topical antibacterial efficacy in infected skin wounds. It has recently been shown thatbiofilms are present on the removed tissue of 80% of patients undergoing surgery for chronicsinusitis. The patients with biofilms were shown to have been denuded ofcilia andgobletcells, unlike the controls without biofilms who had normal cilia and goblet cell morphology.

Biofilms were also found on samples from two of 10 healthy controls mentioned. The speciesof bacteria from interoperative cultures did not correspond to the bacteria species in thebiofilm on the respective patient's tissue. In other words, the cultures were negative thoughthe bacteria were present.

New staining techniques are being developed to differentiate bacterial cells growing in living

animals, e.g. from tissues with allergy-inflammations.

According to the researchers from the Helmholtz Center for Infection Research, they wereable to discover the strategies that biofilm bacteria use.

In most cases biofilm bacteria develop in crowds and squat on areas where they create acommunity with other bacteria. They can develop on any surface to which bacteria can affixto. It is interesting to note that these biofilms cannot be destroyed by any disinfectantsand antibiotics, not even phagocytes and our immune system are able to annihilate thebiofilm bacteria.

This is one of the major problems in hospitals if biofilms create a community a catheter orimplant, surfaces where bacteria may cause a dangerous infection.

Researchers working at the Helmholtz Centre for Infection Research, located inBraunschweig, claim that they have spotted one of the main mechanisms that biofilms usein order to defend themselves against the attack of phagocytes.

Teamed up with colleagues from Australia, Great Britain and the United States, scientistsare now publishing theirdiscoveries in the popular specialist publication PLoS ONE. Thefinding states that biofilm bacteria apply chemical weapons to protect themselves.

Until recent findings, researchers could not understand the core of the biofilm problem - whyphagocytes cannot destroy the biofilm bacteria. Dr. Carsten Matz was the one to start aserious investigation of the problem. The model for his research was the marine bacteria,which face continuous threats in their environment. The threat comes from the amoebae,

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whose behavior resembles the behavior of phagocytes. The amoebae acts the same way inthe sea as the immune cells in human body, namely they look for and feed on the bacteria.

As long as bacteria are free and separated in the water, they turn into an easy target for theattackers. But when they become affixed to a surface and form a community with otherbacteria, the amoebae is unable to assault them.

"The surprising thing was that the amoebae attacking the biofilms were de-activated or even killed.

The bacteria are clearly not just building a fortress, they are also fighting back,"says CarstenMatz.

In order to protect themselves, bacteria use chemical weapons. Marine bacteria, for instance,uses a very effective molecule called pigment violacein. After the protection system isprepared, the biofilm bacteria turn color soft purple. In case the invaders consume only onecell of the biofilm, together with the pigment included, they are instantly paralyzed and thechemical weapon activates theirsuicide mechanism.

"I feel that these results could offer a change of perspective. Biofilms may no longer be seenjust as a problem; they may also be a source of new bioactive agents. When organized inbiofilms, bacteria produce highly effective substances which individual bacteria alone cannotproduce,"says Carsten Matz.

Researchers look forward to apply these molecules in order to deal with a certain group ofpathogens: parasites living in human body and causing serious infections, including sleepingillness and malaria. It is worth mentioning that amoeba represent olden relatives of thesepathogens, which is why weapons obtained by the biofilm bacteria may offer a great basis forthe development ofnew parasiticidal drugs.

Pseudomonas aeruginosa biofilms

The achievements of medical care in industrialised societies are markedly impaired due tochronic opportunistic infectionsthat have become increasingly apparent inimmunocompromised patients and the aging population. Chronic infections remain a majorchallenge for the medical profession and are of great economic relevance because traditionalantibiotictherapy is usually not sufficient to eradicate these infections. One major reason forpersistence seems to be the capability of the bacteria to grow within biofilms that protectsthem from adverse environmental factors. Pseudomonas aeruginosa is not only an importantopportunistic pathogen and causative agent of emerging nosocomial infections but can alsobe considered a model organism for the study of diverse bacterial mechanisms thatcontribute to bacterial persistence. In this context the elucidation of the molecular

mechanisms responsible for the switch from planctonic growth to a biofilm phenotype and therole of inter-bacterial communication in persistent disease should provide new insights in P.aeruginosapathogenicity, contribute to a better clinical management of chronically infectedpatients and should lead to the identification of new drug targets for the development ofalternative anti-infective treatment strategies.

Dental plaque

Dental plaque is the material that adheres to the teeth and consists of bacterial cells (mainlyStreptococcus mutans and Streptococcus sanguis), salivary polymers and bacterialextracellular products. Plaque is a biofilm on the surfaces of the teeth. This accumulation ofmicroorganisms subject the teeth and gingival tissues to high concentrations of bacterialmetabolites which results in dental disease.

Legionellosis

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Legionella bacteria are known to grow under certain conditions in biofilms, in which they areprotected against disinfectants. Workers in cooling towers, persons working inair conditionedrooms and people taking ashowerare exposed to Legionella by inhalation when the systemsare not well designed, constructed, or maintained.

4. Biofilms: development and controlA novel approach to control biofilms

A pilot plant-scale cheese-milk pasteurisation plant was designed and constructed to studythe development of biofilms ofStreptococcus thermophilus during pasteurisation of milk, andto investigate methods for preventing this growth from occurring. Under base run conditions,S. thermophilus grew on surfaces in the cool-down sections of the pilot plant, between 50and 35 C (bulk milk temperature), and could be detected in the product stream after 810 hproduction, reaching levels of 106 CFU ml1 after 16 h. Thermoduric bacteria also grew in theheating sections of the pilot plant, although to a lesser extent, as did non-thermoduricbacteria that originated in the raw milk. The novel application of temperature step changes,implemented periodically to the growth region ofS. thermophilus, successfully controlled the

development of biofilms of these organisms. The growth of bacteria on the heating side ofthe pilot plant was also prevented by the implementation of these same step changes. Theoptimum step change conditions required to achieve a 20-h production run withoutdetectable growth ofS. thermophilus comprised a step change to 55 C, applied for 10 min,with a 60-min interval between step changes.

References:

1. Montana State University: Center for biofilm engineering

2. Answers.com Sci Tech Encyclopedia - Biofilms

3. Temperature step changes: a novel approach to control biofilms ofStreptococcus

thermophilus in a pilot plant-scale cheese-milk pasteurisation plant .G. C. Knight, R. S. Nicol and T. A. McMeekin .a Centre for Food Safety and Quality, School of Agricultural Science and TasmanianInstitute of Agricultural Research, University of Tasmania, Hobart, Tasmania,Australiab Birubi Innovation Pty. Ltd., 1 Bevan St., Albert Park, Victoria 3206, Australia.

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Formation of Biofilms

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