Effect of mechanical stress on biofilms challenged by different chemicals
Introduction
Bacterial biofilms associated with surfaces are complex three-dimensional structures where bacteria are embedded in a matrix chiefly composed of extracellular polymeric substances (EPS) (Campanac et al., 2002). A better understanding of biofilm behaviour is particularly important due to the many serious problems associated with their presence (Simões et al., 2003b). The EPS matrix provides biofilm mechanical stability by filling and forming the space between the bacterial cells, keeping them together (Körstgens et al., 2001). Once developed, biofilms are harder to be removed completely (Simões et al., 2003b). Chemical agents and mechanical forces are parameters often involved simultaneously in the sanitation and removal of biofilms, since the application of sole chemical agents tends to leave the biofilm intact when no mechanical treatment is implemented in the control process (Flemming, 1996). Mechanical stability is an important factor in determining the structure and function of biofilm systems and this parameter plays a key role in the removal and/or control of biofilms in engineered systems (Poppele and Hozalski, 2003). So far, very limited studies have been conducted regarding the mechanical stability of biofilms (Körstgens et al., 2001; Ohashi and Harada, 1994, Ohashi and Harada, 1996; Ohashi et al., 1999; Poppele and Hozalski, 2003; Simões et al., 2003a, Simões et al., 2005b; Stoodley et al., 1999a). Moreover, studies concerning the effect of chemical agents on this biofilm parameter are even fewer. Physical forces acting on the biofilm can also influence the biofilm structure (Hall-Stoodley and Stoodley, 2002). One of the most important factors affecting biofilm structure and behaviour is the velocity field of the fluid in contact with the microbial layer (Pereira et al., 2002; Stoodley et al., 1999b; Vieira et al., 1993). The hydrodynamic conditions will determine the rate of transport of cells and nutrients to the surface, as well as the magnitude of shear forces acting on a developing biofilm.
In this paper, a reactor system that allows the formation and subsequent exposure of biofilms to different chemical and mechanical stresses is described. With this system, it is possible to assess the synergistic action of chemical and mechanical treatment on biofilm removal and to characterise the intrinsic biofilm mechanical stability.
Section snippets
Microorganism and culture conditions
Pseudomonas fluorescens (ATCC 13525T) was the microorganism used to produce biofilm. These bacteria are good biofilm producers and are one of the several microorganisms found in biofilms formed in industrial environments (Pereira et al., 2002). Their growth conditions were 27±1 °C, pH 7, and glucose as the carbon source (Oliveira et al., 1994). The bacterial planktonic culture was grown in a chemostat, consisting in a 0.5 l glass reactor, continuously fed with a sterile concentrated nutrient
Characterisation of the biofilm formed on the rotating device
Fig. 1 shows a SS cylinder before the biofilm formation process (Fig. 1a) and a SS cylinder covered with biofilm after 7 days of growth (Fig. 1b).
This figure clearly shows that the surface of the SS cylinder was completely covered with a thick and slimy biofilm that seems to be strongly adhered to the surface. Some characteristics of the biofilms formed on the cylinders of the rotating device, namely the biofilm activity, mass, protein and polysaccharide content, are presented in Table 2. This
Discussion
The characteristics of the biofilms formed on the SS cylinders (Table 2), namely the respiratory activity, biofilm mass and total content of proteins and polysaccharides, are similar to the ones observed in biofilms formed in a flow cell system under turbulent flow (Simões et al., 2003a), specifically the significant content of extracellular proteins and polysaccharides found in the composition of the biofilm matrix. The evidence of the slimy matrix of the biofilm depicted in Fig. 1 acquired
Conclusions
The system presented in this work provided an approach to investigate the influence of several parameters on the mechanical stability of biofilms, leading to a better understanding of biofilms in different environments and the development of biofilm control strategies. The characterisation of the biofilms showed that the system tested allowed the formation of a great amount of biofilm that covered the surface of the SS cylinder, the biofilms being metabolically active, vastly comprising EPS and
Acknowledgements
The authors acknowledge the financial support provided by IBQF, and the Portuguese Foundation for Science and Technology (Post-Doc Grant—Manuel Simões).
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