Biofilm

                                                                      
BioFilm:
Q.What Are Biofilms?

Biofilms are a collective of one or more types of microorganisms that can grow on many different surfaces. Microorganisms that form biofilms include bacteria, fungi and protists.

One common example of a biofilm dental plaque, a slimy buildup of bacteria that forms on the surfaces of teeth. Pond scum is another example. Biofilms have been found growing on minerals and metals. They have been found underwater, underground and above the ground. They can grow on plant tissues and animal tissues, and on implanted medical devices such as catheters and pacemakers. Each of these distinct surfaces has a common defining feature: they are wet. These environments are "periodically or continuously suffused with water," according to a 2007 article published in Microbe Magazine. Biofilms thrive upon moist or wet surfaces.Biofilms have established themselves in such environments for a very long time. Fossil evidence of biofilms dates to about 3.25 billion years ago, according to a 2004 article published in the journal Nature Reviews Microbiology. For example, biofilms have been found in the 3.2 billion-year-old deep-sea hydrothermal rocks of the Pilbara Craton in Australia. Similar biofilms are found in hydrothermal environments such as hot springs and deep-sea vents.

Extra:

Bacteria collect on the teeth and along the edge of the gums in a cream-colored mass called plaque (Figure 2). The bacterial deposits that form plaque on teeth differ considerably from that on soft tissues because teeth are a non-shedding surface, allowing more time for the development of a “structure” consisting of multiple layers of bacteria. This plaque “structure” also serves as a biofilm, typically defined as an aggregate of microorganisms in which cells adhere to each other and/or to a solid substrate exposed to an aqueous surface. The bulk of the volume (~90%) of dental plaque biofilm is comprised of a gel-like matrix of extracellular polysaccharides produced by oral bacteria. These polysaccharides are what holds the biofilm together and triggers changes that make it increasingly difficult to remove over time: When a cell becomes a component of biofilm, one of the many changes it experiences is a shift in gene expression that makes it up to 1,000 times more resistant to antibodies, antibiotics, and antimicrobial compounds than its planktonic (single cell) counterparts

Figure 2. Dental Plaque Deposits.

Nutritional interactions:

The primary nutrients for oral microorganisms are host proteins and glycoproteins, and these are obtained mainly from saliva for organisms in supragingival plaque (for a review, see Jakubovics 2015b) and from gingival crevicular fluid (GCF) for those located in subgingival biofilms (Wei et al. 1999). Pure cultures of oral microorganisms grow poorly or not at all on these structurally complex substrates, and consortia of interacting species are needed for their catabolism. Proteins are broken down by the action of mixtures of proteases and peptidases, but the catabolism of glycoproteins (consisting of a protein backbone decorated with linear or branched oligosaccharide side chains) involves the sequential removal of terminal sugars from side chains before the protein backbone becomes accessible to proteolytic attack (Takahashi 2015). Oral bacteria express glycosidases with different specificities so that the concerted action of several species is necessary for the complete degradation of host glycoproteins (Bradshaw et al. 1994). Similarly, combinations of mutans streptococci, Streptococcus oralis and Fusobacterium nucleatum, degraded albumin more effectively than any of the three species alone (Homer & Beighton 1992). The biofilm matrix is another potential source for carbon and energy for interacting consortia of oral bacteria. Fructans and soluble glucans in dental plaque can be metabolized by combinations of bacteria that produce exo‐ and/or endo‐hydrolytic enzymes (Bergeron & Burne 2001, Koo et al. 2013). Individual bacteria are dependent therefore on the metabolic capability of other species for access to essential nutrients.

Further complex nutritional inter‐relationships develop in microbial communities when the products of metabolism of one organism (primary feeder) become the main source of nutrients for another (secondary feeder), resulting in the development of food chains or food webs (Hojo et al. 2009) (some examples are illustrated in Fig. 1). These food webs can result in the complete and energetically efficient catabolism of complex host molecules to the simplest end products of metabolism (e.g. CO2, CH4, H2S). Numerous synergistic metabolic interactions occur among bacteria in subgingival biofilms in order to enable them to degrade host proteins and glycoproteins as nutrient sources (ter Steeg et al. 1987, ter Steeg & van der Hoeven 1989). These interactions are discussed in more detail later in the section on “Ecological drivers towards dysbiosis and disease”
Nutritional inter‐dependencies such as those described above contribute to the temporal stability and resilience of oral microbial communities, while a consequence of the reliance of resident oral bacteria on the metabolism of these complex substrates is that species avoid direct competition for individual nutrients, and hence are able to co‐exist and maintain a stable equilibrium, also termed microbial homeostasis (Alexander 1971, Marsh 1989). This has been elegantly demonstrated in a computational study on KEGG pathway‐based metabolic distances between 11 oral bacteria that are known to interact (Mazumdar et al. 2013). Metabolism was a major factor driving the order of colonization, with specific metabolic pathways associated with different layers in the biofilm, resulting in a functionally structured community. However, in such a structured community, there was an optimal trade‐off between their resource sharing and functional synergy (Mazumdar et al. 2013).

Biofilm formation:

Biofilm formation begins when free-floating microorganisms such as bacteria come in contact with an appropriate surface and begin to put down roots, so to speak. This first step of attachment occurs when the microorganisms produce a gooey substance known as an extracellular polymeric substance (EPS), according to the Center for Biofilm Engineering at Montana State University. An EPS is a network of sugars, proteins and nucleic acids (such as DNA). It enables the microorganisms in a biofilm to stick together.

Attachment is followed by a period of growth. Further layers of microorganisms and EPS build upon the first layers. Ultimately, they create a bulbous and complex 3D structure, according to the Center for Biofilm Engineering. Water channels crisscross biofilms and allow for the exchange of nutrients and waste products, according to the article in Microbe.

Multiple environmental conditions help determine the extent to which a biofilm grows. These factors also determine whether it is made of only a few layers of cells or significantly more. "It really depends on the biofilm," said Robin Gerlach, a professor in the department of chemical and biological engineering at Montana State University-Bozeman. For instance, microorganisms that produce a large amount of EPS can grow into fairly thick biofilms even if they do not have access to a lot of nutrients, he said. On the other hand, for microorganisms that depend on oxygen, the amount available can limit how much they can grow. Another environmental factor is the concept of "shear stress." "If you have a very high flow [of water] across a biofilm, like in a creek, the biofilm is usually fairly thin. If you have a biofilm in slow flowing water, like in a pond, it can become very thick," Gerlach explained.

Finally, the cells within a biofilm can leave the fold and establish themselves on a new surface. Either a clump of cells breaks away, or individual cells burst out of the biofilm and seek out a new home. This latter process is known as "seeding dispersal," according to the Center for Biofilm Engineering.

Q.Why form a biofilm?

For microorganisms, living as a part of a biofilm comes with certain advantages. "Communities of microbes are usually more resilient to stress," Gerlach told Live Science. Potential stressors include the lack of water, high or low pH, or the presence of substances toxic to microorganisms such as antibiotics, antimicrobials or heavy metals.

There are many possible explanations for the hardiness of biofilms. For example, the slimy EPS covering can act as a protective barrier. It can help prevent dehydration or act as a shield against ultraviolet (UV) light. Also, harmful substances such as antimicrobials, bleach or metals are either bound or neutralized when they come into contact with the EPS. Thus, they are diluted to concentrations that aren't lethal well before they can reach various cells deep in the biofilm, according to a 2004 article in Nature Reviews Microbiology.

Still, it is possible for certain antibiotics to penetrate the EPS and make their way through a biofilm's layers. Here, another protective mechanism can come into play: the presence of bacteria that are physiologically dormant. In order to work well, all antibiotics require some level of cellular activity. So, if bacteria are physiologically dormant to begin with, there is not much for an antibiotic to disrupt.

Another mode of protection against antibiotics is the presence of special bacterial cells known as "persisters." Such bacteria do not divide and are resistant to many antibiotics. According to a 2010 article published in the journal Cold Spring Harbor Perspectives in Biology, "persisters" function by producing substances that block the targets of the antibiotics.

In general, microorganisms living together as a biofilm benefit from the presence of their various community members. Gerlach cited the example of autotrophic and heterotrophic microorganisms that live together in biofilms. Autotrophs, such as photosynthetic bacteria or algae, are able to produce their own food in the form of organic (carbon containing) material, while heterotrophs cannot produce their own food and require outside sources of carbon. "In these multi-organismal communities, they often cross feed," he said.