151
surface attachment (Geesey 2001). Microscopic observations of Pseudomonas
aeruginosa biofilms indicated that motility promotes not only movement along the
surface but also initial interactions with it (O’Toole and Kolter 1998). Although,
motility is needed for initial surface interactions, further cellular attachments to the
surface may be sterically hindered by the flagella besides its movement. Accordingly,
after a bacterium attaches to a surface the motility genes are repressed. A small non
coding RNA (sRNA), RsmZ was shown to be involved in switch machinery from
planktonic to biofilm lifestyle in Pseudomonas aeruginosa. RsmZ is positively
regulated by sensor kinases LadS and negatively regulated by sensor kinase
RetS. RetS helps in maintaining Pseudomonas aeruginosa in planktonic state necessary for expression of type three secretion system, during acute infection, by activating a mRNA binding protein RsmA. LadS on other hand increase the transcription
of RsmZ, which binds to RsmA and inactivates it. This RsmZ mediated inhibition
of RsmA results in inhibition of Type three secretion systems by switching to biofilm formation associated with chronic infections (Ventre et al. 2006).
Bacillus subtilis under biofilm forming conditions switches from flagellated,
motile cells to long chains of non motile cells growing in parallel. This switch in
lifestyle is governed by a global transcriptional regulator, SinR that suppresses the
transcription of genes responsible for matrix production in motile cells to indirectly
enhance cell separation and motility. Under biofilm forming condition, SinI, YibF
and YmcA antagonisze SinR activity leading to loss of motility, and cell chain formation along with matrix production (Kearns et al. 2005; Branda et al. 2006).
7.2.2 Initial Adhesion to Surfaces: Reversible Linkages
Reversible adhesion stages include loose aggregation of bacterium that may dissociate and revert back to free floating forms. Electrostatic interactions between the
substratum, the bacteria and surface topography are the driving forces, during the
initial stages of biofilm formation on abiotic surfaces that are facilitated by the tip
adhesins on bacterial pili. Hydrodynamic forces stemming from viscosity of
medium influence the probability of making initial interaction within the substratum. Furthermore, presence or absence of chemotactic molecules greatly affects the
initial interactions between the substratum and microbial proteins. Bacteria with
increased hydrophobicity have reduced repulsion between the substratum and the
bacterium (Zobell 1943; van Loosdrecht et al. 1990).
Both the nature of the surface and the environmental conditions influence the
reversible attachment. The environmental conditions such as pH and the ionic force
of the medium or the temperature are important players in biofilm formation
(Donlan 2002). Hydrophobic surfaces such as Teflon or plastic are more prone to
colonization by bacteria as compared to hydrophilic surfaces like glass and metal.
Moreover, adsorption and desorption of nutrients at the surface makes a conditioning film that may influence bacterial initial attachment depending on the properties
of the organic molecules (Olsen et al. 1989).
7 Control of Bacterial Biofilms for Mitigating Antimicrobial Resistance
surface attachment (Geesey 2001). Microscopic observations of Pseudomonas
aeruginosa biofilms indicated that motility promotes not only movement along the
surface but also initial interactions with it (O’Toole and Kolter 1998). Although,
motility is needed for initial surface interactions, further cellular attachments to the
surface may be sterically hindered by the flagella besides its movement. Accordingly,
after a bacterium attaches to a surface the motility genes are repressed. A small non
coding RNA (sRNA), RsmZ was shown to be involved in switch machinery from
planktonic to biofilm lifestyle in Pseudomonas aeruginosa. RsmZ is positively
regulated by sensor kinases LadS and negatively regulated by sensor kinase
RetS. RetS helps in maintaining Pseudomonas aeruginosa in planktonic state necessary for expression of type three secretion system, during acute infection, by activating a mRNA binding protein RsmA. LadS on other hand increase the transcription
of RsmZ, which binds to RsmA and inactivates it. This RsmZ mediated inhibition
of RsmA results in inhibition of Type three secretion systems by switching to biofilm formation associated with chronic infections (Ventre et al. 2006).
Bacillus subtilis under biofilm forming conditions switches from flagellated,
motile cells to long chains of non motile cells growing in parallel. This switch in
lifestyle is governed by a global transcriptional regulator, SinR that suppresses the
transcription of genes responsible for matrix production in motile cells to indirectly
enhance cell separation and motility. Under biofilm forming condition, SinI, YibF
and YmcA antagonisze SinR activity leading to loss of motility, and cell chain formation along with matrix production (Kearns et al. 2005; Branda et al. 2006).
7.2.2 Initial Adhesion to Surfaces: Reversible Linkages
Reversible adhesion stages include loose aggregation of bacterium that may dissociate and revert back to free floating forms. Electrostatic interactions between the
substratum, the bacteria and surface topography are the driving forces, during the
initial stages of biofilm formation on abiotic surfaces that are facilitated by the tip
adhesins on bacterial pili. Hydrodynamic forces stemming from viscosity of
medium influence the probability of making initial interaction within the substratum. Furthermore, presence or absence of chemotactic molecules greatly affects the
initial interactions between the substratum and microbial proteins. Bacteria with
increased hydrophobicity have reduced repulsion between the substratum and the
bacterium (Zobell 1943; van Loosdrecht et al. 1990).
Both the nature of the surface and the environmental conditions influence the
reversible attachment. The environmental conditions such as pH and the ionic force
of the medium or the temperature are important players in biofilm formation
(Donlan 2002). Hydrophobic surfaces such as Teflon or plastic are more prone to
colonization by bacteria as compared to hydrophilic surfaces like glass and metal.
Moreover, adsorption and desorption of nutrients at the surface makes a conditioning film that may influence bacterial initial attachment depending on the properties
of the organic molecules (Olsen et al. 1989).
7 Control of Bacterial Biofilms for Mitigating Antimicrobial Resistance