Biomedical Implications of the Porosity of Microbial Biofilms - Porous Media: Applications in Biological Systems and Biotechnology - page 130

alginate lyase may play a role in the release of cells from the solid surfaces of
biofilms.
Detachment affected by physical shear forces was investigated in greater
detail. Brading et al. (1995) emphasized the importance of physical forces in
detachment, stating that the three main processes for detachment are ero-
sion/shearing (continuous removal of small portions of the biofilm), sloughing
(rapid and massive removal), and abrasion (detachment due to collision of
particles from the bulk fluid with the biofilm). The mode of cell dispersal
apparently affects the phenotypic characteristics of the organisms. Eroded
or sloughed aggregates from the biofilm are likely to retain certain poten-
tial biofilm characteristics, for example, antimicrobial resistance, whereas cells
that have been shed as a result of growth may quickly revert to the planktonic
phenotype.

4.3 Infectious Microbial Biofilms—Structural
and Biological Characteristics
4.3.1 Bacterial Biofilms
Biofilms may be composed of either single or multiple microbial species and
are formed on a range of biotic and abiotic surfaces. Though mixed-species
biofilms mostly dominate, single-species biofilms do exist in some infections as
well as on the surface of medical implants (Adal and Farr 1996). Pseudomonas
aeruginosa was particularly investigated as single-species, biofilm-forming
Gram-negative bacterial cells. Other Gram-negative bacteria, for example,
Pseudomonas fluorescens , Escherichia coli , and Vibrio cholera have also been
studied in detail. Gram-positive biofilm-forming bacteria studied included
S. epidermidis , Staphylococcus aureus , and the Enterococci .

4.3.1.1 Biofilms Composed of Gram-Negative Bacteria

The first stage in the formation of Gram-negative biofilms is primarily regu-
lated by the initial attachment. Biofilm formation is assumed to begin when
suspended bacterial cells sense environmental conditions, triggering a shift to
life on a surface (Pratt and Kolter 1998; Stoodley et al. 1999; O'Toole et al.
2000a). These environmental signals may be different for different organisms,
for example, P. aeruginosa and P. fluorescens will form biofilms under almost
any condition enabling growth (O'Toole and Kolter 1998) while E. coli K-12
and V. cholera will not form biofilms in minimal medium unless supplemented
with amino acids (Pratt and Kolter 1998; Watnick et al. 1999). In addition
to the nutritional impact, other environmental factors that may affect biofilm
formation include temperature, osmolarity, pH, iron, and oxygen (O'Toole and
Kolter 1998; Stoodley et al. 1999).