What are Microbial Biofilms and Mats??
Biofilms and microbial mats can be defined as surface associated to layers of microbial cells embedded in extracellular polymeric substance. Biofilms cover solid surfaces, while mats cover sediments.
A microbial mat is a multi layered sheet of microorganisms mainly consisting of bacteria and archaea. Microbial slabs grow on submerged or moist and damp surfaces between different types of material, but few also survive in the wilderness. They are found in colonies in an environment ranging in temperature from –40 °C to 120 °C. A few or very less are found as endo-symbionts of animals.
A biofilm comprises of microorganisms in which cells stick to each other and often also to a surface and substratum. These adhering cells are integrated into a narrow extracellular layer consisting of extracellular polymers. The cells within and outside the biofilm produce the EPS (extracellular polymeric substances) components that are typically a polymeric amalgamation of extracellular polysaccharides, proteins, lipids, and DNA. Because they have three dimensional structure and function represents a community lifestyle for microorganisms they have been metaphorically described as “cities for microbes”.
The use and application of new innovations such as confocal scanning- laser microscopy and molecular-probes which studies about the microbial biofilms and microbial mats has dramatically changed our view towards these systems. In instance, it was an old belief that biofilms were essentially bacteria that had been suspended into a heterogeneous matrix that was limiting diffusion, which was wrong because of the fact that this matrix is actually very hydrated and specialised with channels. The microbial species which form the microcolonies and megacolonies have been shown to express genes which could not be not expressed when the organisms are free-swimming. Many dynamic models of microorganisms based on the investigations are therefore not necessarily applicable in organic films (for example, growth, gene exchange, biocide resistance). Recent studies of microbial mats have also uncovered a new startling phenomenon. For instance an active sulphate reduction has been measured in the oxic-zone. The impact of diurnal fluctuations in the environment cannot be underestimated and as species have been shown to migrate within the mat and use totally symbolically different metabolic pathways in response to light and oxygen concentration. There is focus primarily on some of the insights into the chemical and biological structure of biofilms and microbial mats and how the structure is affected by the physical and chemical-nature, species composition and species interactions. This begins with a comparison of microbial mats and biofilms and ends with some suggestions for future researchers.
Succession within Biofilm communities: The principle of ecological succession is one of the first ecological concepts. Traditionally, the theory has been applied to plant communities to offer explanations of how the diversity and structure of an ecosystem changes once it starts to establish or re-establish. For example, after a forest fire has occurred, succession theory has been used to describe the patterns of the plants which are able to colonise the habitat in the immediate aftermath of the event and also in the following generations, as some initial colonisers become locally extinct, while others come to dominate or are joined by future immigrant species.
Following this early work, in other systems such as phytoplankton the idea of ecological succession was studied. However, it is only really in the last decade or two that there has been significant research on how the theory might apply to microbial systems. Largely, this has been due to technological and methodological limitations, which are only now being broken down. The advent of molecular methods to analyse and quantify microbial communities has allowed for a revolution in microbial ecology. As such, we have had the instruments for examining succession issues in microbial ecosystems for the first time in the previous few decades.
In the past decade, there have been several studies looking at microbial ecological succession within biofilm communities. These previous works have examined community structure and its dynamics from the first establishing colonisers through towards mature biofilms. Some important signatures have been recorded in these communities that form the basis for the construction of conceptual models of the succession of a microbial community. These features include:
Community diversity increased rapidly during the first phase of biofilm establishment before dropping again in the intermediate stage as some of these initial colonising species become locally extinct.
In the third stage of biofilm growth, diversity is increasing again.
The total biomass increased with time in all phases but cannot grow unbounded. Additionally, when multiple successional trajectories under the same conditions are examined, the community structures are seen to be initially similar, before diverging slightly and then finally converging. The period of diverging similarity is seen to coincide roughly with the intermediate period of biofilm development, after the initial colonisation but before establishing into a mature biofilm.
Development of Microbial communities:
Micro colony formation, multiplication and adherence takes place after bacteria adhered and attached to the physical surface or biological tissue and this binding then becomes stable which results in formation of microbial colony. Multiplication of bacteria in the biofilm starts as a result of chemical and mechanical signals. The genetic mechanism of exopolysaccharide production gets activated when intensity of the signal cross certain threshold potential. Thus the bacterial cell divisions take occur within an implanted exopolysaccharide matrix using such a chemical signal, which eventually leads to the creation of microcolonies.
Three-Dimensional Structure Formation and Maturation
After micro colony formation stage of biofilm the expression of certain biofilm related genes take place. These gene products are needed for the EPS which is the main structure material of biofilm. It is reported that bacterial attachment by itself can trigger formation of extracellular and intracellular matrix. Matrix formation is followed by water filled channels formation for transport of nutrients and minerals within the biofilm. Researcher have proposed that these water channels are like a circulatory systems, distributing different nutrients to and removing waste materials from the communities in the micro-colonies of the biofilm.
After biofilm formation the research scholars have often noticed that bacteria leave the biofilms itself on regular basis. The bacteria can quickly multiply and disperse through this process. The unlocking of bacterial planktonic cells from the biofilm is a designed unlock with a natural pattern.. Sometimes occasionally due to some mechanical stress bacteria are detached from the colonies into the surrounding. But in most cases some bacteria stop multiplying and are detached into environment. Spread of biofilm producing cells occur either by detachment of new formed cells from growing cells or dispersion of biofilm aggregates due to flowing effects or due to quorum- sensing or by physical or chemical changes. The enzyme function causing alginate digestion is removed in cell biofilm. The manner of biofilm dispersion appears to alter phenotypic characteristics of organisms. Dispersed cells from the biofilm have the ability to retain certain properties of biofilm such as antibiotic in-sensitivity etc. The cells which are dispersed form biofilm as result of growth and multiplication may return quickly to their normal phenotype as it was before
In the process of biofilm formation many species of bacteria are able to communicate and interact with one another through a mechanism called quorum sensing, it is a method used by bacteria to communicate and interact with each other.
Biofilm Forming Bacteria
Nearly all (99.9%) of micro-organisms have the ability to form biofilm on a wide range of surfaces i.e. biological and inert surfaces. When micro-organisms bind to a surface, they produce extracellular polymeric substance (EPS) and form biofilm. Biofilms are creating a great problem for public health due to its resistant nature to antibiotics and disease associated with indwelling medical devices. It is found that H. influenza has the ability to form biofilm in human body and can escape from human immune system pretty easily. Biofilm forming capability has been reported in numerous species of bacteria such as P. aeruginosa, S. epidermidis, E. coli spp, S. aureus, E. cloacae, K. pneumoniae.(Table 1).
Biofilms and Antibiotic Resistance
Mechanisms of antibiotic and biocide resistance of biofilms are categorized into four classes which include 1. active molecule inactivation directly 2. altering body’s sensitivity to target of action, 3.reduction of the drug concentration before reaching to the target site and 4. efflux systems (Figure 2).
Biofilm antibiotic resistance level may vary among different sittings and the key factors responsible for this resistance may also differ. In terms of resistance, the fundamental proof reveals that traditional processes cannot explain the high resistance to biofilm related antibacterial agents, although this proof is not unavoidable in resistance in the growth of adherent cells. It is therefore hypothesised that the bacterium or biofilm attached resistance may have certain intrinsic processes and is accountable for traditional antibiotic resistance. Several mechanisms have been explored that are considered to be key factors in high resistance nature of biofilms. These mechanisms are (a) limited diffusion, (b) enzyme causing neutralizations, (c) heterogeneous functions, (d) slow growth rate, (e) presence of persistent (non-dividing) cells and (f) biofilm phenotype such adaptive mechanisms e.g. efflux pump and membrane alteration.