Microcolonies

Microbial cells transported with the stream of fluid above the surface interact with conditioning films. Immediately after attachment, microorganisms initiate production of slimy adhesive substances, predominantly exopolysaccharides (EPS) that assist the formation of microcolonies and microbial films. EPS create bridges for microbial cells to the substratum and permit negatively charged bacteria to adhere to both negatively and positively charged surfaces. EPS may also control interfacial chemistry at the mineral/biofilm interface. 

Ferrari BC, SJ Binnerup, M Gillings (2005) Microcolony cultivation on a soil substrates membrane system selects for previously uncultured soil bacteria. Appl Environ Microbiol 71 8714-8720. 

Similar work was performed by Maquelin et al.6 in 2000. They used con-focal Raman microspectroscopy to obtain spectra directly from microbial microcolonies on solid culture medium. The spectra were obtained after 6 h of culmring and were of most commonly encountered organisms. While depth studies showed varying quantitative levels of success, the qualitative (identification) of various bacterial strains was deemed a success. 

Most biofilm bacteria grow as a monospecies, in dense microcolonies, adjacent to other species of cooperative organisms, all held within a less dense matrix. 

Biofilm matrices act as a barrier to prevent the diffusion of oxygen into the biomass, and thus deep within established biofilms the conditions are often anaerobic. Matrices equally prevent the diffusion of chlorine and other biocides, thus making it difficult to kill the innermost sessile microcolonies. 

Conventional microbiological identification of isolates from patients can normally be obtained with a total turnaround time of 48-96 h. Ibelings et al. [106] and Maquelin et al. [46] developed alternatively a Raman spectroscopic approach for the identification of clinically relevant Candida species from smears and microcolonies in peritonitis patients taking at least overnight (smears) or about 6h (microcolonies). Hereby, a prediction accuracy of 90% was obtained for Raman spectroscopy in combination with multivariate statistical data analysis. 

Fig. 11.11. Illustration of the use of gel microdroplets for sensing growth at the level of one cell growing into a two-cell microcolony. By staining the cells within microdroplets with, for example, a DNA-specific fluorochrome, a small subpopulation of cells dividing more or less rapidly than most could be detected in a flow histogram of microdroplet fluorescence. From Weaver (1990).

Attachment of bacteria. At low ionic strength of the medium — as in many freshwaters — bacteria-surface interactions are controlled by the effects of van der Waals attraction and electrostatic repulsion. At high ionic strength — as in seawater — steric interactions between the outer cell surface macromolecules and the substratum gain in importance (van Loosdrecht et al., 1989 Rijnaarts etal., 1999). Additionally, flagellar and twitching motility of bacteria was found to be essential in the process of attachment by bacteria onto surfaces (Pratt and Kolter, 1998 O Toole and Kolter, 1998). It seems that extracellular polysaccharides of bacteria are not involved in the adhesion process itself. However, bacterial extracellular polysaccharides are necessary for the development of a biofilm and for the formation of microcolonies (Allison and Sutherland, 1987 Hoyle et al., 1993). 

If this can be applied to rather circular shapes (Saccharomyces cerevisiae [50], mammalian cells, either isolated or aggregated [26,78,79), microcolonies [80], etc.), it seems more difficult to characterize filamentous species by this size parameter. As mentioned before size and shape quantifications are intimately connected. 

Exclusion or reduced access of biocide to underlying bacterial cell Depends on (i) nature of biocide (ii) binding capacity of glycocalyx towards biocide (iii) rate of growth of microcolony relative to biocide diffusion rate... 

Fig. 10.9. Mycodeterioration of barrier concrete by Aspergillus niger in a microcosm cotton-wool-like microcolonies of fungus (black arrows) arising after one year on a sample of barrier concrete and associated with microcracks on the concrete surface (white arrows) (Fomina, unpublished). Scale bar = 1 mm.

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