Biofilms in the Food Environment (original) (raw)

2011, Encyclopedia of Earth Sciences Series

Biofilms in the Food Environment conformations. The EPS are long chains with molecular masses of (0.5-2.0) × 10 6 Da, which can associate in a variety of ways. Electrostatic forces and hydrogen bonds are the dominant forces that govern these interactions. The increased EPS production in biofilms may be a result of a stress response as in the case of colonic acid production in E. coli. The amount of EPS produced depends on the nutrients present. Synthesis of EPS is promoted by excess carbon sources with limiting nitrogen, potassium, and phosphorus. Bacterial mutants that are unable to produce EPS are unable to form biofilms. They may, however, be able to attach to surfaces. Exopolysaccharides allow for the binding of large amounts of water and contribute to mechanical stability of the biofilm, allowing it to withstand shear forces (Sutherland 2001). The EPS from several organisms have been characterized. Danese and others (2000) reported that the wcaF gene product was required for the production of colonic acid. Colonic acid was not required for initial attachment like the polysaccharides of Shewannella putrefaciens and Vibrio cholerae. It was, however, necessary for the establishment of the complex three-dimensional (3-D) structure of the E. coli biofilm. Mutants that could not produce colonic acid were still able to attach to abiotic surfaces (Danese and others 2000). Alginate is the primary component of P. aeruginosa biofilms. The algC gene involved in the production of alginate is transcribed at a higher rate (∼fourfold) in P. aeruginosa cells grown in biofilms compared to planktonic cells (Davies and others 1993). Genes in the intercellular adhesion locus (icaADBC) in Staphylococcus aureus and Staphylococcus epidermis encode genes involved in the synthesis of β-1-6-linked poly-Nacetylglucosamine referred to as PNAG. Staphylococcus strains deficient in PNAG production do not exhibit mushroom-like colonies and wide water channel-like strains, which produce larger amounts of PNAG. Quorum Sensing Microbial biofilms provide a suitable environment for cell-to-cell signaling due to the large cell density. This quorum sensing occurs in a densitydependent manner via low-molecular-weight signaling compounds. The concentration of these compounds depends on population density. When a critical concentration is reached certain genes are turned on or off in the bacterial cells. There are several different molecules involved in cell-to-cell signaling. The most common autoinducer in Gram-negative microorganism is N-acyl-homoserine lactones (AHL). 2-Heptyl-3-hydroxy-4-quinolone (PQS) is involved in quorum sensing in P. aeruginosa. Amino acids and short posttranslationally modified peptides are used by Gram-positive Biofilms in the Food Industry 11 microorganisms. Autoinducer-2 (AI-2) first discovered in Vibrio is produced by both Gram-positive and Gram-negative microorganisms (Van Houdt and others 2004). P. aeruginosa possesses two cell-to-cell signaling systems: lasR-lasI and rhlR-rhlI. The lasI gene product directs the synthesis of autoinducer N-(3-oxododecanoyl)-L-homoserine lactone. The lasR gene product requires a certain level of homoserine lactones to activate the transcription of virulence genes in this organism. The system directs the production of N-butryl homoserine lactone, which causes the transcription of virulence genes and RpoS. Davies and others (1998) examined biofilm production in knockout mutants, which were unable to produce either of the autoinducers described above. The biofilm produced by mutant cells was 80% thinner than the wild-type and the cells were densely packed. This phenotype was attributed to lasI gene. When the lasI gene product N-(3oxododecanoyl)-L-homoserine lactone was added to lasI mutant, biofilms that were similar to the wild-type were formed. These authors concluded that the quorum sensing molecule N-(3-oxododecanoyl)-L-homoserine lactone is required for biofilm formation. No significant difference was observed between the EPS of the mutant and wild-type P. aeruginosa cells (Davies and others 1998). Van Houdt and others (2004) isolated 68 strains of Gram-negative bacteria from a plant processing fresh vegetables. These strains were examined for their ability to form biofilms. Various degrees of biofilm-forming ability were observed and all strains were significantly better at forming biofilms than E. coli DH5α. The 68 strains were examined for their ability to produce AHL, PQS, or AI-2. No bacteria tested produced PQS, 26 isolates produced AI-2, and 5 isolates were positive for AHL production. These strains were identified as Vibrio diazotrophicus, Serratia plymuthica (2), and Panthoea agglomerans (2). The authors did not find a correlation between biofilm formation and the production of autoinducers (Van Houdt and others 2004). Microscopic Examination of Biofilms Biofilms are often not very homogeneous, resulting in a specimen that is difficult to visualize. Thick biofilms may be problematic because the bacteria in lower layers cannot be observed or quantified and organisms in the upper layers may be lost if harsh fixation and staining techniques are used. Additional difficulties may occur depending on the surface on which the biofilm is located. Opaque and irregularly shaped surfaces require optics with a large depth of field. A variety of microscopic techniques such