Biofilms nutrient availability

Although reaction-diffusion limitation and the presence of nutritionally restricted phenotypes are obviously important determinants of biofilm drug resistance, neither, either separately or in combination, provides a complete explanation of the phenomena. Cells on the periphery of the biofilm, subject to nutrient fluxes similar to planktonic organisms would succumb to antibacterial concentrations that are effective against the planktonic cells. Cell-death at the periphery would lead to increased nutrient availability for deeper-lying cells. These would, in turn, grow faster and adopt a more susceptible... 

As the biofilm develops, the nutrient availability to the bulk biofilm may become affected. The biofilm, despite its voids and channels, offers a further resistance to mass transfer. The cells within the biofilm consume nutrients that diffuse through the biofilm in response to the difference in concentration between nutrients at the biofilm surface and the cells attached to the conditioning layer. As a consequence, it is entirely possible that cells in the region of the solid surface are likely to become starved of nutrients. The properties of the biofilm may be different, therefore, in the layers where nutrient is available compared with the regions where there is little or no nutrient. For instance, the lack of oxygen may encourage anaerobic species to develop (some bacteria can exist as aerobes or anaerobes), with attendant changes to the quality of the biofilm. 

The leveling off of the biofilm growth is attributed to a balance between growth and removal, depending on nutrient availability and temperature on the one hand, and shear forces caused by water flow on the other. 

Microorganisms have an optimum growth temperature, when, provided that there are sufficient nutrients available, the growth is maximum. The optimum temperature is different for different species, on account of various metabolic characteristics. It is usually in the range of 20-50" C, with many in the range of 35-40"" C. Fig. 8 shows the very pronounced effect of a relatively small temperature change on the development of a biofilm of Escherichia coliP ... 

FIGURE 12.8. The reduction of nutrient availability due to biofilm growth... 

These equations are based on the assumption that at any given moment the population of micro-organisms (bacteria) in a culture will multiply as long as either there is nutrient available or, the concentration of the inhibitory product is not limiting. The rate of multiplication within the biofilm will vary according to these criteria. Belkhadir et al [1988] described the fundamental growth phases in a biofilm. The growth of active biomass is assumed to have an order of zero in relation to the nutrient and an order of unity in relation to the active bacteria so that ... 

Although these data show the trends associated with changes in system variables the age and quality of the biofilm will also be a factor, i.e. whether the biofilm is open and "fluffy" or dense and compact. Diffusion of biocide into the biofilm will be facilitated by the existence of "pores" containing water within the biofilm matrix. The quality of the biofilm will be very much a function of the conditions under which it was laid down, particularly in terms of nutrient availability and flow rate (see Chapter 12). Apparent variations in biocidal efficiency reported in the literature may in fact be due to the different morphologies of biofilms of the same strain, but grown under different conditions. 

When the biofilm is formed and developed, Le., in stages 1 to 3 in Figure 4.4, the outer cells will start to consume the nutrient available to them more rapidly than the cells located deeper within the biofilm, so that the activity and growth rate of the latter are considerably reduced [37]. Therefore, while the outer cells increase in number, the biofilm starts to act hke a net to trap more and more particles, organic or inorganic. This will increase the thickness of the biofilm even further. 

The resident microbes within the mouth readily form biofilms on teeth. A biofilm consists of a population of bacteria coexisting in an orderly structure at the interface of a solid and a liquid [14] and, within a biofilm, bacteria living in colonies encapsulated in a matrix of extracellular polymer. Oral biofilms are known to vary extensively in structure throughout the colony, with regions of densely packed microorganisms surrounded by open water channels. Each type of bacteria exists in reasonably defined environments which are influenced by surrounding cells, distance from the outer surface and local structure, all of which influence availability of nutrients and ambient pH. 

Biofilm is being researched for potential use as an in situ barrier technology. It is not yet commercially available. Biofilm is an organic material consisting of microorganisms embedded in a self-made polymer matrix. It is formed when bacteria are introduced into soil and stimulated with specific nutrients. 

Gradients of nutrients and oxygen in biofilms additionally promote high diversity, which may ultimately result in functional differences of the bacterial community in biofilms compared with free-floating bacteria. Additionally, increased species diversity may provide spatial and temporal niches not available within monocultures or may create microenvironments within the biofilm (Gieseke et al., 2001 Whiteley et al., 2001). These thoughts reinforce the need for community-level biofilm studies as opposed to monocultures. 

Environmental conditions related to rock surface exposure have profound effects on biofilm development, as metabolic activity and growth are directly connected to the availability of water, energy sources and nutrients, as well as to conditions of temperature and irradiation. Another important factor for establishment of subaerial biofilms is the resistance of the supporting substrate to environmentally and biologically influenced disintegration and dissolution (wear-down). Rapidly weathering rock surfaces (e.g. porous sandstone in an intertidal coastal zone) show little or... 

After colonization of the surface, the microorganisms utilize the available nutrients to grow, multiply, and synthesize both intracellular products and extracellular polymeric substances that constitute the substance of the biofilm. Significant amounts of biofilm can be produced under ideal conditions, and even if planktonic cells are no longer present in the flowing water, the sessile cells already on the surface can provide the basis for biofilm development. 

After the colonization, there is a period of rapid growth, where the conditions, in terms of temperature, availability of nutrients, and their transport to the biofilm, are conducive to sustained growth. 

It is to be expected that the availability of nutrients is crucial to the development of biofilms. The effects of... 

Fig. 12.8 shows how in an experimental system, once the biofilm has started to develop following the initiation period the available nutrients in terms of glucose, are depleted [Kaur 1990]. The quality of the nutrient in terms of CH ratio also influences the growth of biofilms [Bott and Gunatillaka 1983]. 

These data show that low ozone concentrations for up to 3 hours, are sufficient to remove a large proportion of the biofilm. When the addition of ozone was stopped however, the remaining biofilm on the substrate rapidly regrew, suggesting the cells that were not removed were not damaged and because of the removal of the outer layers of biofilm, had full access to the available nutrients. 

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