Atomic force microscopy studies of bioprocess engineering surfaces - imaging, interactions and mechanical properties mediating bacterial adhesion

The detrimental effect of bacterial biofilms on process engineering surfaces is well documented. Thus, interest in the early stages of bacterial biofilm formation; in particular bacterial adhesion and the production of anti-fouling coatings has grown exponentially as a field. During this time, Atomic force microscopy (AFM) has emerged as a critical tool for the evaluation of bacterial adhesion. Due to its versatility AFM offers not only insight into the topographical landscape and mechanical properties of the engineering surfaces, but elucidates, through direct quantification the topographical and biomechnical properties of the foulants The aim of this review is to collate the current research on bacterial adhesion, both theoretical and practical, and outline how AFM as a technique is uniquely equipped to provide further insight into the nanoscale world at the bioprocess engineering surface.

Proteins dominate in the surface layers formed on materials exposed to extracellular polymeric substances from bacterial cultures

The chemical compositions of the surface conditioning layers formed by different types of solutions (from isolated EPS to whole culture media), involving different bacterial strains relevant for biocorrosion were compared, as they may influence the initial step in biofilm formation. Different substrata (polystyrene, glass, steel) were conditioned and analyzed by X-ray photoelectron spectroscopy. Peak decomposition and assignment were validated by correlations between independent spectral data and the ubiquitous presence of organic contaminants on inorganic substrata was taken into account. Proteins or peptides were found to be a major constituent of all conditioning layers and polysaccharides were not present in appreciable concentrations; the proportion of nitrogen which may be due to DNA was lower than 15%. There was no significant difference between the compositions of the adlayers formed from different conditioning solutions, except for the adlayers produced with tightly bound EPS extracted from D. alaskensis.

Start-up of granule-based denitrifying reactors with multiple magnesium supplementation strategies

In the present work, the effect of Mg(2+) supplementation on the start-up of a denitrification process and the granulation of denitrifying sludge was investigated in three upflow anaerobic sludge blanket (UASB) reactors. The reactors R1 and R2 were continuously and intermittently, respectively, supplied with 50 mg L(-1) Mg(2+), whereas R0 was used as the control. The nitrogen loading rate (NLR) and organic loading rate (OLR) gradually increased, and extremely high values were obtained (36.0 kgN m(-3) d(-1) and 216.0 kgCOD m(-3) d(-1), respectively). Granulation occurred in R1 first, but the reactor capacities were comparable. Suffering from starvation, the R0-R2 performances were comparable. At the end of the experiment, the average diameter of the granules in R0, R1, and R2 were 1.67, 1.72 and 1.68 mm, respectively, and the settling velocities of the granules in R1 and R2 were 1.14-fold the speed of R0. The specific denitrifying activity (SDA) of the sludge from the reactors supplied with Mg(2+) was greater than the reactor without Mg(2+). Intermittent Mg(2+) supplementation was identified as the best choice to be utilized to cultivate denitrifying granules, which was consistent with kinetic analysis.

"Biofilmology": a multidisciplinary review of the study of microbial biofilms

The observation of biofilm formation is not a new phenomenon. The prevalence and significance of biofilm and aggregate formation in various processes have encouraged extensive research in this field for more than 40 years. In this review, we highlight techniques from different disciplines that have been used to successfully describe the extracellular, surface and intracellular elements that are predominant in understanding biofilm formation. To reduce the complexities involved in studying biofilms, researchers in the past have generally taken a parts-based, disciplinary specific approach to understand the different components of biofilms in isolation from one another. Recently, a few studies have looked into combining the different techniques to achieve a more holistic understanding of biofilms, yet this approach is still in its infancy. In order to attain a global understanding of the processes involved in the formation of biofilms and to formulate effective biofilm control strategies, researchers in the next decade should recognise that the study of biofilms, i.e. biofilmology, has evolved into a discipline in its own right and that mutual cooperation between the various disciplines towards a multidisciplinary research vision is vital in this field.

Efficacy of trypsin in enhancing assessment of bacterial colonisation of vascular catheters

Since the number of organisms isolated from a medical device is crucial in assessing the likelihood of device-associated infection, we examined whether incubation of catheters in trypsin before sonication can increase the yield of superficially colonised vascular catheters in vitro and those removed from patients. Polyurethane and silicone catheters were individually colonised in vitro with individual clinical isolates including Staphylococcus aureus and Escherichia coli. Equal numbers of 1 cm segments of colonised catheters were then individually incubated either in a trypsin-containing solution or a control solution without trypsin. Each solution containing the segment was then sonicated and cultured quantitatively. In the clinical arm, indwelling catheters removed from patients were also cut into 1 cm segments that were equally suspended in the trypsin-containing or control solution and then sonicated and cultured quantitatively. Trypsin-based sonication enhanced the detection of S. aureus on colonised polyurethane and silicone catheters in vitro by 14- and 30-fold, respectively (P = 0.03 and P = 0.04), and the detection of E. coli on colonised polyurethane and silicone catheters by 3- and 6-fold, respectively (P = 0.04 and P = 0.05). Compared with sonication alone, trypsin followed by sonication resulted in 10% increase in the detectability of significant colonisation of indwelling catheters removed from patients and 11% increase in the mean colony counts of colonising organisms (P = 0.04). Exposure of catheters to trypsin before sonication improves the sensitivity of sonication and enhances the accuracy of assessing significant catheter colonisation.

Adhesion and biofilm formation on polystyrene by drinking water-isolated bacteria

This study was performed in order to characterize the relationship between adhesion and biofilm formation abilities of drinking water-isolated bacteria (Acinetobacter calcoaceticus, Burkholderia cepacia, Methylobacterium sp., Mycobacterium mucogenicum, Sphingomonas capsulata and Staphylococcus sp.). Adhesion was assessed by two distinct methods: thermodynamic prediction of adhesion potential by quantifying hydrophobicity and the free energy of adhesion; and by microtiter plate assays. Biofilms were developed in microtiter plates for 24, 48 and 72 h. Polystyrene (PS) was used as adhesion substratum. The tested bacteria had negative surface charge and were hydrophilic. PS had negative surface charge and was hydrophobic. The free energy of adhesion between the bacteria and PS was > 0 mJ/m(2) (thermodynamic unfavorable adhesion). The thermodynamic approach was inappropriate for modelling adhesion of the tested drinking water bacteria, underestimating adhesion to PS. Only three (B. cepacia, Sph. capsulata and Staphylococcus sp.) of the six bacteria were non-adherent to PS. A. calcoaceticus, Methylobacterium sp. and M. mucogenicum were weakly adherent. This adhesion ability was correlated with the biofilm formation ability when comparing with the results of 24 h aged biofilms. Methylobacterium sp. and M. mucogenicum formed large biofilm amounts, regardless the biofilm age. Given time, all the bacteria formed biofilms; even those non-adherents produced large amounts of matured (72 h aged) biofilms. The overall results indicate that initial adhesion did not predict the ability of the tested drinking water-isolated bacteria to form a mature biofilm, suggesting that other events such as phenotypic and genetic switching during biofilm development and the production of extracellular polymeric substances (EPS), may play a significant role on biofilm formation and differentiation. This understanding of the relationship between adhesion and biofilm formation is important for the development of control strategies efficient in the early stages of biofilm development.

Bacterial factors influencing adhesion of Pseudomonas aeruginosa strains to a poly(ethylene oxide) brush

Most bacterial strains adhere poorly to poly(ethylene oxide) (PEO)-brush coatings, with the exception of a Pseudomonas aeruginosa strain. The aim of this study was to find factors determining whether P. aeruginosa strains do or do not adhere to a PEO-brush coating in a parallel plate flow chamber. On the basis of their adhesion, a distinction could be made between three adhesive and three non-adhesive strains of P. aeruginosa, while bacterial motilities and zeta potentials were comparable for all six strains. However, water contact angles indicated that the adhesive strains were much more hydrophobic than the non-adhesive strains. Furthermore, only adhesive strains released surfactive extracellular substances, which may be engaged in attractive interactions with the PEO chains. Atomic force microscopy showed that the adhesion energy, measured from the retract curves of a bacterial-coated cantilever from a brush coating, was significantly more negative for adhesive strains than for non-adhesive strains (P<0.001). Through surface thermodynamic and extended-DLVO (Derjaguin, Landau, Verwey, Overbeek) analyses, these stronger adhesion energies could be attributed to acid-base interactions. However, the energies of adhesion of all strains to a brush coating were small when compared with their energies of adhesion to a glass surface. Accordingly, even the adhesive P. aeruginosa strains could be easily removed from a PEO-brush coating by the passage of a liquid-air interface. In conclusion, cell surface hydrophobicity and surfactant release are the main factors involved in adhesion of P. aeruginosa strains to PEO-brush coatings.

Structural effects of the Azospirillum lipopolysaccharides in cell suspensions

The structural influence of Azospirillum lipopolysaccharides (LPS) and lipopolysaccharide-protein complexes (LPPC) on carrot, erythrocyte, and bacterial cell suspensions was explored. The structural potentialities of O-specific polysaccharide fragments of LPS and protein fractions of LPPC were also evaluated. An ability to induce the formation of three kinds of structures in the cell suspensions was revealed depending on the chemical composition of the preparations used. The first and the second ones were connected with effects of cell aggregation (a relatively fast process) and agglutination (a relatively slow process). The third one resulted in phase separation of erythrocyte suspensions (a medium-speed process), with segregating the cells to a separate homogeneous liquid phase.

Influence of extracellular polymeric substances on deposition and redeposition of Pseudomonas aeruginosa to surfaces

In this study, the role of extracellular polymeric substances (EPS) in the initial adhesion of EPS-producing Pseudomonas aeruginosa SG81 and SG81R1, a non-EPS-producing strain, to substrata with different hydrophobicity was investigated. The release of EPS by SG81 was concurrent with a decrease in surface tension of a bacterial suspension from 70 to 45 mJ m(-2) that was absent for SG81R1. Both strains adhered faster and in higher numbers to a hydrophilic than to a hydrophobic substratum, but the initial deposition rates and numbers of adhering bacteria in a stationary-end point were highest for the non-EPS-producing strain SG81R1, regardless of substratum hydrophobicity. Both strains adhered less to substrata pre-coated with isolated EPS of strain SG81. Furthermore, it was investigated whether bacteria, detached by passing air-bubbles, had left behind 'footprints' with an influence on adhesion of newly redepositing bacteria. Redeposition on glass was highest for non-EPS-producing SG81R1 and decreased linearly with the number of times these cycles of detachment and deposition were repeated to become similar to the redeposition of SG81 after six cycles. This indicates that P. aeruginosa SG81 leaves the substratum surface nearly completely covered with EPS after detachment, while SG81R1 releases only minor amounts of surface active EPS, completely covering the substratum after repeated cycles of detachment and adhesion. Atomic force microscopy showed a thick and irregular EPS layer (up to 32 nm) after the first detachment cycle of EPS-producing strain SG81, whereas the putatively non-EPS-producing strain SG81R1 left a 9 nm thin layer after one cycle. X-ray photoelectron spectroscopy indicated that the bacterial footprints consisted of uronic acids, the prevalence of which increased with the number of detachment and deposition cycles.

Identification and characterization of the omaA gene encoding the major outer membrane protein of Azospirillum brasilense

The major outer membrane protein (MOMP) of Azospirillum brasilense was purified and degenerate oligonucleotides were constructed on the basis of partial internal amino acid sequences. PCR products were obtained using total DNA of A. brasilense as template. One of these, a 766-bp fragment, was DIG-labelled and used in Southern hybridization against A. brasilense DNA and a genomic library of A. brasilense in Escherichia coli. A clone containing a 20-kb EcoRI insert in pLAFR3 was identified by PCR screening. From this insert, an EcoRI-SalI fragment of approximately 3.5-kb was subcloned in pUC19. The gene encoding the A. brasilense MOMP was sequenced and analyzed. The deduced amino acid sequence contains a putative signal peptide of 23 residues, followed by 367 amino acids of the mature protein with a molecular mass of 38,753 Da. The deduced amino acid sequence shows similarity to certain bacterial porins.

Limitation of adhesion and growth of Listeria monocytogenes on stainless steel surfaces by Staphylococcus sciuri biofilms

The adhesion and subsequent development of Listeria monocytogenes on stainless steel was studied in the absence and in the presence of a Staphylococcus sciuri biofilm. In the three growth media studied, the percentage of adherent cells was reduced to nearly the same extent by the presence of 1-day biofilms of Staph. sciuri for the two strains of L. monocytogenes studied. One-day biofilms of Staph. sciuri exhibited the same exopolysaccharide content per square centimetre, although they colonized from 3.5 to 35% of the stainless steel depending on the growth media. This suggests that extracellular substances rather than cell-to-cell interactions were involved in the decreased adhesion. After 3 days of culture, Staphylococcus biofilms prevented the adherent L. monocytogenes population from increasing within the biofilm, leading to an average logarithmic cfu difference of 0.9-2.7 between the pure and mixed culture. A competition for nutrients by Staph. sciuri was observed in one of the three media. A role for extracellular polysaccharides produced by the Staphylococcus biofilm in preventing the adhesion of L. monocytogenes and in modifying the balance existing between its planktonic and biofilm phase is hypothesized. A higher proportion of L. monocytogenes cells was observed in the planktonic phase in mixed cultures, suggesting that the extracellular substances produced by Staph sciuri biofilms and involved in the decreased adhesion of L. monocytogenes could modify the balance existing between planktonic and biofilm populations. In addition, co-cultures of L. monocytogenes and Staph. sciuri in broth showed competition for nutrients for Staph. sciuri in one of the three media.

Physiology of dairy-associated Bacillus spp. over a wide pH range

Bacillus species isolated from alkaline wash solutions used for cleaning in place in South African dairy factories have been suggested to contaminate product contact surfaces of dairy processing equipment and result in post-pasteurization spoilage of milk and milk products. Growth and attachment of such Bacillus isolates under alkaline and acidic conditions have not been previously described. Therefore, the in vitro growth temperature and pH ranges, attachment abilities and hydrophobicity, and enzyme production capabilities of four Bacillus isolates (tentatively identified as B. subtilis115, B. pumilus122, B. licheniformis137 and B. cereus144) previously isolated from the alkaline wash solutions in a South African dairy were examined. Growth pH ranges were determined in buffered Standard One-like Nutrient Broth and in unbuffered 1% Milk Medium at pH values ranging from 3 to 12. Growth and attachment to stainless steel surfaces and production of protease and lipase enzymes were determined in 1% Milk Medium at pH 4, 7 and 10. Colony hydrophobicity of each isolate by the Direction of Spreading Method (DOS) was also determined at pH 4, 7 and 10. In addition, Arrhenius plots were used to examine the growth temperature ranges of the isolates. All isolates grew at pH values ranging from 4.5 to 9.5 in buffered Standard One-like Nutrient Broth, and from pH 4 to 10 in 1% Milk Medium. All isolates also attached to stainless steel at pH 4, 7 and 10 in 1% Milk Medium. Generally the attachment of B. subtilis115, B. pumilus122 and B. lichenformis137 to stainless steel surfaces was enhanced at pH 4 and 10, compared to pH 7. By contrast, the best attachment of B. cereus144 cells to stainless steel surfaces was at pH 7. Planktonic and attached cells of all isolates produced proteolytic enzymes at pH 7 and 10, but not at pH 4. Similarly, planktonic and attached cells of B. subtilis115, B. pumilus122 and B. licheniformis137 produced lipolytic enzymes at pH 7 and 10, and weak lipolysis was observed at pH 4. The Bacillus cereus144 isolate showed no lipolytic activity at pH 10. All isolates exhibited low hydrophobic properties at all pH values even though attachment to stainless steel at the same pH values occurred. None of the isolates grew below 11 degrees C or above 56 degrees C, and optimum growth temperatures were in the high mesophilic range (36-44 degrees C).

Aggregation in Azospirillum brasilense: effects of chemical and physical factors and involvement of extracellular components

A medium for consistent induction of aggregation of Azospirillum brasilense cells was developed and used to study the effects of chemical and physical factors as well as extracellular components involved in this phenomenon. Growth of A. brasilense strain Cd in a high C:N medium using fructose and ammonium chloride as C and N sources, respectively, resulted in flocculation visible to the naked eye after 24 h. No cell aggregates were formed after 72 h growth in low C:N medium. Aggregating cells, but not cells grown under low C:N, accumulated high amounts of poly-beta-hydroxybutyrate and the cell envelope contained a well-defined electron-dense layer outside the outer membrane. Suspending the aggregates in 0.2 or 0.5 M urea was the only treatment effective for disrupting aggregates. The concentration of exopolysaccharide produced by four different strains of A. brasilense, differing in their capacity to aggregate, strongly correlated with the extent of aggregation. Electrophoretic protein profiles from different fractions of aggregating and non-aggregating cells were compared. Differences were observed in the pattern of low-molecular-mass proteins and in the polar flagellin that has previously been proposed to be involved in adhesion processes. However, a mutant lacking both lateral and polar flagella showed the strongest aggregation. The involvement of polysaccharides and/or proteins in aggregation of A. brasilense is discussed.

Initiation of biofilm formation in Pseudomonas fluorescens WCS365 proceeds via multiple, convergent signalling pathways: a genetic analysis

Populations of surface-attached microorganisms comprising either single or multiple species are commonly referred to as biofilms. Using a simple assay for the initiation of biofilm formation (e.g. attachment to an abiotic surface) by Pseudomonas fluorescens strain WCS365, we have shown that: (i) P. fluorescens can form biofilms on an abiotic surface when grown on a range of nutrients; (ii) protein synthesis is required for the early events of biofilm formation; (iii) one (or more) extracytoplasmic protein plays a role in interactions with an abiotic surface; (iv) the osmolarity of the medium affects the ability of the cell to form biofilms. We have isolated transposon mutants defective for the initiation of biofilm formation, which we term surface attachment defective (sad). Molecular analysis of the sad mutants revealed that the ClpP protein (a component of the cytoplasmic Clp protease) participates in biofilm formation in this organism. Our genetic analyses suggest that biofilm formation can proceed via multiple, convergent signalling pathways, which are regulated by various environmental signals. Finally, of the 24 sad mutants analysed in this study, only three had defects in genes of known function. This result suggests that our screen is uncovering novel aspects of bacterial physiology.

A transcriptional regulator of the LuxR-UhpA family, FlcA, controls flocculation and wheat root surface colonization by Azospirillum brasilense Sp7

Genetic complementation of a spontaneous mutant, impaired in flocculation, Congo red binding, and colonization of root surface, led to the identification of a new regulatory gene in Azospirillum brasilense Sp7, designated flcA. The deduced amino acid sequence of flcA shared high similarity with a family of transcriptional activators of the LuxR-UphA family. The most significant match was with the AgmR protein, an activator for glycerol metabolism in Pseudomonas aeruginosa. Derivatives of Sp7 resulting from site-directed Tn5 mutagenesis in the flcA coding sequence were constructed by marker exchange. Characterization of the resulting mutant strains showed that flcA controls the production of capsular polysaccharides, the flocculation process in culture, and the colonization of the root surface of wheat. This study provides new information on the genetic control of the mechanism of plant root colonization by Azospirillum.