Elsevier

Water Research

Volume 39, Issue 20, December 2005, Pages 5142-5152
Water Research

Effect of mechanical stress on biofilms challenged by different chemicals

https://doi.org/10.1016/j.watres.2005.09.028Get rights and content

Abstract

In this study a methodology was applied in order to ascertain the mechanical stability of biofilms, by using a stainless-steel (SS) rotating device immersed in a biological reactor where biofilms formed by Pseudomonas fluorescens were allowed to grow for 7 days at a Reynolds number of agitation of 2400. The biofilms developed with this system were characterised in terms of amount of total, extracellular and intracellular proteins and polysaccharides, amount of mass, metabolic activity and mechanical stability, showing that the biofilms were active, had a high content of extracellular constituents and an inherent mechanical stability. In order to assess the role of chemical agents on the mechanical stability, the biofilms were exposed to chemical agents followed by mechanical treatments by submission to increase Reynolds number of agitation. Seven different chemical agents were tested (two non-oxidising biocides, three surfactants and two oxidising biocides) and their effects on the biofilm mechanical stability were evaluated. The increase in the Reynolds number increased the biofilm removal, but total biofilm removal was not found for all the conditions tested. For the experiment without chemical addition (only mechanical treatment), the biofilm remaining on the surface was about 76%. The chemical treatment followed by the subsequent mechanical treatment did not remove all the biofilms from the surface. The biofilm remaining on the SS cylinder ranged from 3% to 62%, depending on the chemical treatment, showing that the chemical treatment is far from being a cause that induces massive biofilm detachment and even the synergistic chemical and mechanical treatments did not promote biofilm removal. Some chemical agents promoted an increase in the biofilm mechanical stability such as glutaraldehyde (GTA), benzalkonium chloride (BC), except for the lower concentration tested, and sodium dodecyl sulphate (SDS), except for the higher concentration tested. Treatments that promoted biofilm removal, to an extent similar to the control experiment (without chemical treatment), were BC, for the lower and the higher concentration of SDS. Cetyltrimethyl ammonium bromide (CTAB), ortho-phthalaldehyde (OPA), sodium hydroxide (NaOH) and sodium hypochlorite (SHC) promoted the weakening of the biofilm mechanical stability.

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).

References (30)

  • J. Azeredo et al.

    The role of exopolymers produced by Sphingomonas paucimobilis in biofilm formation and composition

    Biofouling

    (2000)
  • C. Campanac et al.

    Interactions between biocide cationic agents and bacterial biofilms

    Antimicrob. Agents Chemother.

    (2002)
  • T.E. Cloete et al.

    The chemical control of biofouling in industrial water systems

    Biodegradation

    (1997)
  • M. Dubois et al.

    Colorimetric method for determination of sugars and related substances

    Anal. Chem.

    (1956)
  • Flemming, H.-C., 1996. The forces that keep biofilms together. In: Biodeterioration and Biodegradation, DECHEMA...
  • Cited by (0)

    View full text