Atomic force microscopy imaging of dried or living bacteria

Photodynamic effect of TPP encapsulated in polystyrene nanoparticles toward multi-resistant pathogenic bacterial strains: AFM evaluation

Photodynamic inactivation (PDI) is a promising approach for the efficient killing of pathogenic microbes. In this study, the photodynamic effect of sulfonated polystyrene nanoparticles with encapsulated hydrophobic 5,10,15,20-tetraphenylporphyrin (TPP-NP) photosensitizers on Gram-positive (including multi-resistant) and Gram-negative bacterial strains was investigated. The cell viability was determined by the colony forming unit method. The results showed no dark cytotoxicity but high phototoxicity within the tested conditions. Gram-positive bacteria were more sensitive to TPP-NPs than Gram-negative bacteria. Atomic force microscopy was used to detect changes in the morphological properties of bacteria before and after the PDI treatment.

Engineering of Antimicrobial Surfaces by Using Temporin Analogs to Tune the Biocidal/antiadhesive Effect

Proliferation of resistant bacteria on biomaterials is a major problem leading to nosocomial infections. Due to their broad-spectrum activity and their ability to disrupt bacterial membranes through a rapid membranolytic mechanism, antimicrobial peptides (AMPs) are less susceptible to the development of bacterial resistance and therefore represent good candidates for surface coating strategies to prevent biofilm formation. In this study, we report on the covalent immobilization of temporin-SHa, a small hydrophobic and low cationic antimicrobial peptide exhibiting broad-spectrum activity, and (SHa) analogs on modified gold surfaces. Several analogs derived from SHa with either a carboxamidated ([K³]SHa, d-[K³]SHa) or a carboxylated C-terminus ([K³]SHa-COOH) were used to achieve peptide grafting on gold surfaces modified by a thiolated self-assembled monolayer (SAM). Surface functionalization was characterized by polarization modulation infrared reflection absorption spectroscopy (PM-RAIRS) and X-ray photoemission spectroscopy (XPS). The antibacterial properties of the temporin-functionalized surfaces were tested against the Gram-positive Listeria ivanovii. Direct visualization of the peptide effects on the bacterial membrane was investigated by scanning electron microscopy equipped with a field emission gun (SEM-FEG). All active temporin analogs were successfully grafted and display significant antibacterial activity (from 80 to 90% killing efficiency) in addition to a 2-fold decrease of bacterial adhesion when all d-SHa analogs were used.

Biocidal Properties of a Glycosylated Surface: Sophorolipids on Au(111)

Classical antibacterial surfaces usually involve antiadhesive and/or biocidal strategies. Glycosylated surfaces are usually used to prevent biofilm formation via antiadhesive mechanisms. We report here the first example of a glycosylated surface with biocidal properties created by the covalent grafting of sophorolipids (a sophorose unit linked by a glycosidic bond to an oleic acid) through a self-assembled monolayer (SAM) of short aminothiols on gold (111) surfaces. The biocidal effect of such surfaces on Gram+ bacteria was assessed by a wide combination of techniques including microscopy observations, fluorescent staining, and bacterial growth tests. About 50% of the bacteria are killed via alteration of the cell envelope. In addition, the roles of the sophorose unit and aliphatic chain configuration are highlighted by the lack of activity of substrates modified, respectively, with sophorose-free oleic acid and sophorolipid-derivative having a saturated aliphatic chain. This system demonstrates thus the direct implication of a carbohydrate in the destabilization and disruption of the bacterial cell envelope.

Effects of photodynamic therapy on Enterococcus faecalis biofilms

Microbial biofilms are involved in almost all infectious pathologies of the oral cavity. This has led to the search for novel therapies specifically aimed at biofilm elimination. In this study, we used atomic force microscopy (AFM) to visualize injuries and to determine surface roughness, as well as confocal laser scanning microscopy (CLSM) to enumerate live and dead bacterial cells, to determine the effects of photodynamic therapy (PDT) on Enterococcus faecalis biofilms. The AFM images showed that PDT consisting of methylene blue and a 670-nm diode laser (output power 280 mW during 30 s) or toluidine blue and a 628-nm LED light (output power 1000 mW during 30 s) induced severe damage, including cell lysis, to E. faecalis biofilms, with the former also causing an important increase in surface roughness. These observations were confirmed by the increase in dead cells determined using CLSM. Our results highlight the potential of PDT as a promising method to achieve successful oral disinfection.

Temporin-SHa peptides grafted on gold surfaces display antibacterial activity

Development of resistant bacteria onto biomaterials is a major problem leading to nosocomial infections. Antimicrobial peptides are good candidates for the generation of antimicrobial surfaces because of their broad-spectrum activity and their original mechanism of action (i.e. rapid lysis of the bacterial membrane) making them less susceptible to the development of bacterial resistance. In this study, we report on the covalent immobilisation of temporin-SHa on a gold surface modified by a thiolated self-assembled monolayer. Temporin-SHa (FLSGIVGMLGKLF amide) is a small hydrophobic and low cationic antimicrobial peptide with potent and very broad-spectrum activity against Gram-positive and Gram-negative bacteria, yeasts and parasites. We have analysed the influence of the binding mode of temporin-SHa on the antibacterial efficiency by using a covalent binding either via the peptide NH2 groups (random grafting of α- and ε-NH2 to the surface) or via its C-terminal end (oriented grafting using the analogue temporin-SHa-COOH). The surface functionalization was characterised by IR spectroscopy (polarisation modulation reflection absorption IR spectroscopy) while antibacterial activity against Listeria ivanovii was assessed by microscopy techniques, such as atomic force microscopy and scanning electron microscopy equipped with a field emission gun. Our results revealed that temporin-SHa retains its antimicrobial activity after covalent grafting. A higher amount of bound temporin-SHa is observed for the C-terminally oriented grafting compared with the random grafting (NH2 groups). Temporin-SHa therefore represents an attractive candidate as antimicrobial coating agent.

Infection of orthopedic implants with emphasis on bacterial adhesion process and techniques used in studying bacterial-material interactions

Staphylococcus comprises up to two-thirds of all pathogens in orthopedic implant infections and they are the principal causative agents of two major types of infection affecting bone: septic arthritis and osteomyelitis, which involve the inflammatory destruction of joint and bone. Bacterial adhesion is the first and most important step in implant infection. It is a complex process influenced by environmental factors, bacterial properties, material surface properties and by the presence of serum or tissue proteins. Properties of the substrate, such as chemical composition of the material, surface charge, hydrophobicity, surface roughness and the presence of specific proteins at the surface, are all thought to be important in the initial cell attachment process. The biofilm mode of growth of infecting bacteria on an implant surface protects the organisms from the host immune system and antibiotic therapy. The research for novel therapeutic strategies is incited by the emergence of antibiotic-resistant bacteria. This work will provide an overview of the mechanisms and factors involved in bacterial adhesion, the techniques that are currently being used studying bacterial-material interactions as well as provide insight into future directions in the field.

Contribution of surface β-glucan polysaccharide to physicochemical and immunomodulatory properties of Propionibacterium freudenreichii

Propionibacterium freudenreichii is a bacterial species found in Swiss-type cheeses and is also considered for its health properties. The main claimed effect is the bifidogenic property. Some strains were shown recently to display other interesting probiotic potentialities such as anti-inflammatory properties. About 30% of strains were shown to produce a surface exopolysaccharide (EPS) composed of (1→3,1→2)-β-D-glucan due to a single gene named gtfF. We hypothesized that functional properties of P. freudenreichii strains, including their anti-inflammatory properties, could be linked to the presence of β-glucan. To evaluate this hypothesis, gtfF genes of three β-glucan-producing strains were disrupted. These knockout (KO) mutants were complemented with a plasmid harboring gtfF (KO-C mutants). The absence of β-glucan in KO mutants was verified by immunological detection and transmission electron microscopy. We observed by atomic force microscopy that the absence of β-glucan in the KO mutant dramatically changed the cell's topography. The capacity to adhere to polystyrene surface was increased for the KO mutants compared to wild-type (WT) strains. Anti-inflammatory properties of WT strains and mutants were analyzed by stimulation of human peripheral blood mononuclear cells (PBMCs). A significant increase of the anti-inflammatory interleukin-10 cytokine production by PBMCs was measured in the KO mutants compared to WT strains. For one strain, the role of β-glucan in mice gut persistence was assessed, and no significant difference was observed between the WT strain and its KO mutant. Thus, β-glucan appears to partly hide the anti-inflammatory properties of P. freudenreichii; which is an important result for the selection of probiotic strains.

Antibacterial action of dispersed single-walled carbon nanotubes on Escherichia coli and Bacillus subtilis investigated by atomic force microscopy

Single-walled carbon nanotubes (SWCNTs) exhibit strong antibacterial activities. Direct contact between bacterial cells and SWCNTs may likely induce cell damages. Therefore, the understanding of SWCNT-bacteria interactions is essential in order to develop novel SWCNT-based materials for their potential environmental, imaging, therapeutic, and military applications. In this preliminary study, we utilized atomic force microscopy (AFM) to monitor dynamic changes in cell morphology and mechanical properties of two typical bacterial models (gram-negative Escherichia coli and gram-positive Bacillus subtilis) upon incubation with SWCNTs. The results demonstrated that individually dispersed SWCNTs in solution develop nanotube networks on the cell surface, and then destroy the bacterial envelopes with leakage of the intracellular contents. The cell morphology changes observed on air dried samples are accompanied by an increase in cell surface roughness and a decrease in surface spring constant. To mimic the collision between SWCNTs and cells, a sharp AFM tip of 2 nm was chosen to introduce piercings on the cell surface. No clear physical damages were observed if the applied force was below 10 nN. Further analysis also indicates that a single collision between one nanotube and a bacterial cell is unlikely to introduce direct physical damage. Hence, the antibacterial activity of SWCNTs is the accumulation effect of large amount of nanotubes through interactions between SWCNT networks and bacterial cells.

Effects of poly(L-lysine) substrates on attached Escherichia coli bacteria

Poly(L-lysine) (PLL) is a cationic polymer that is often used for attaching and immobilizing cells to glass substrates for further investigation by, e.g., AFM techniques. Because of their small size, bacterial attachment is most easily done using thick air-dried PLL coatings--though thinner PLL coatings are also used and are commercially available. Nevertheless, the antimicrobial activity of PLL is well-established. Accordingly, we have investigated the physiological effects of suspended PLL and of PLL coatings on individual Escherichia coli bacteria through the pole-to-pole oscillations of cytoplasmic MinD-GFP fusion proteins. For planktonic bacteria, suspended PLL concentrations at the micromolar level quenched MinD-GFP oscillations and inhibited bacterial growth. On coverslips with PLL coatings prepared by short exposures of the slides to PLL solutions, followed by rinsing, only a fraction of available bacteria attached after hours of settling time. Min oscillations in the attached bacteria, however, were strong and only moderately slowed. On thick PLL coatings, prepared by drying drops on the slides followed by a brief rinse with deionized water, cells attached well within 15 min. With thick coatings, average oscillation periods for bacteria increased significantly, and considerable cell-to-cell variability was also observed; subsequent replacement of buffer with distilled water led to much larger period increases and/or fading of fluorescence intensity. We demonstrate that Min oscillations are a useful metric for bacteria attached to adhesion layers. We suggest that thick PLL coatings should probably be avoided for bacterial attachment, and that even thin PLL coatings can have significant effects on bacterial physiology.

Microscopic observation of bacteria: review highlighting the use of environmental SEM

Throughout the years, various methods have been adopted to investigate bacteria involved in root canal infection and apical periodontitis. This paper reviews the most commonly used microscopic techniques and discusses their possibilities, limitations and sample preparation. In particular, a recently developed variant of scanning electron microscope (SEM), referred to as environmental SEM (ESEM), is highlighted due to its potential impact across the diverse field of biomaterials research. The performance of this ESEM technique for bacterial observation of endodontic pathogens was illustrated by a practical approach. The paper concludes with a discussion on the possible use of ESEM for testing endodontic treatment modalities under environmental conditions in situ.

Predation, death, and survival in a biofilm: Bdellovibrio investigated by atomic force microscopy

Biofilms are complex microbial communities that are resistant to attack by bacteriophages and to removal by drugs and chemicals. Here we use atomic force microscopy (AFM) to image the attack on Escherichia coli biofilms by Bdellovibrio bacteriovorus 109J. Bdellovibrio is a small, predatory bacterium that invades and devours other Gram-negative bacteria. We demonstrate that under dilute nutrient conditions, bdellovibrios can prevent the formation of simple bacterial biofilms and destroy established biofilms; under richer conditions the prey bacteria persist and are not eradicated, but may be shifted toward solution populations. Using AFM we explore these bacterial interactions with more detail and accuracy than available by more traditional staining assays or optical microscopy. AFM also allows us to investigate the nanoscale morphological changes of the predator, especially those related to motility. This demonstration of Bdellovibrio's successful predation in a biofilm inspires us to consider ways that it might be used productively for industrial, medical, agricultural, and biodefensive purposes.

Atomic Force Microscopy of Bacterial Communities

This chapter discusses atomic force microscopy (AFM) for the benefit of microbiologists who are interested in using this technique to examine the structures and dynamics of bacteria. AFM is a powerful technique for imaging biological samples at the nanometer to micrometer scale under nondestructive conditions. In order to be imaged with AFM, bacteria must be supported by a surface, which presents challenges because many laboratory strains of bacteria are planktonic. Still, in nature many bacteria live at surfaces and interfaces. This chapter discusses the benefits and difficulties of different methods that have been used to support bacteria on surfaces for AFM imaging and presents two methods in detail used to successfully grow and image bacteria at solid-liquid and solid-air interfaces. Using these methods it is possible to study bacterial morphology and interactions in a native state. These explorations by AFM have important applications to the study of different kinds of bacteria, interfacial bacterial communities, and biofilms.

Investigations into the life cycle of the bacterial predator Bdellovibrio bacteriovorus 109J at an interface by atomic force microscopy

Atomic force microscopy was used to image Bdellovibrio bacteriovorus 109J, a gram-negative bacterial predator that consumes a variety of other gram-negative bacteria. In predator-prey communities grown on filters at hydrated air-solid interfaces, repeated cycles of hunting, invasion, growth, and lysis occurred readily even though the cells were limited to near two-dimensional movement. This system allowed us to image the bacteria directly without extensive preparation or modification, and many of the cells remained alive during imaging. Presented are images of the life cycle in two species of prey organisms, both Escherichia coli (a small prey bacterium that grows two-dimensionally on a surface) and Aquaspirillum serpens (a large prey bacterium that grows three-dimensionally on a surface), including high-resolution images of invaded prey cells called bdelloplasts. We obtained evidence for multiple invasions per prey cell, as well as significant heterogeneity in morphology of bdellovibrios. Mutant host-independent bdellovibrios were observed to have flagella and to excrete a coating that causes the predators to clump together on a surface. Most interestingly, changes in the texture of the cell surface membranes were measured during the course of the invasion cycle. Thus, coupled with our preparation method, atomic force microscopy allowed new observations to be made about Bdellovibrio at an interface. These studies raise important questions about the ways in which bacterial predation at interfaces (air-solid or liquid-solid) may be similar to or different from predation in solution.