Enumerating bacterial cells on bioadhesive coated slides

Viability assessment of Bifidobacterium longum ATCC 15707 on non-dairy foods using quantitative fluorescence microscopy

This study demonstrates an effective technique for separating and purifying viable bacteria from samples that interfere with viability staining. The viability of Bifidobacterium longum ATCC 15707 was assessed using Percoll Buoyant Density Gradient Centrifugation (PBDC) to separate bacteria from complex non-dairy food matrices and Quantitative Fluorescence Microscopy (QFM) to determine individual cells using LIVE/DEAD BacLight bacterial viability staining. Water agar (3%) was used to retain cells of B. longum and offered a lower fluorescence background with BacLight viability staining, compared with fixation on polycarbonate (PC) black membrane. The effect of drying temperatures and non-dairy foods on viability of B. longum was assessed. B. longum coated on oat, peanut or raisin was separated by filtration, low- and high-speed centrifugation, flotation and sedimentation buoyant density centrifugation. Purified cells were subsequently deposited on water agar for rehydration followed by LIVE/DEAD BacLight viability staining and enumeration. Conventional plate counting was also conducted to compare viability results. Finally, this method was applied to assess cell membrane damages of B. longum incorporated onto non-dairy foods during 24 h drying. Furthermore, viability assessment of B. longum coated onto oat, peanut, or raisin was much lower by plate counting compared to viability staining. Drying appeared to have a greater impact when viability was assessed by plate counting compared to viability staining. IMPORTANCE: Enumeration of viable beneficial bacteria from function foods presents a significant bottleneck for product development and quality control. Interference with microscopic and/or fluorescent techniques by ingredients, time required to incubate plated microbes, and the transient nature of the colony forming unit make rapid assessment of viable bacteria difficult. Viability assessment of Bifidobacterium longum ATCC 15707 by Percoll Buoyant Density Gradient Centrifugation with LIVE/DEAD BacLight viability staining on water agar (3%) was in agreement with serial dilution enumeration. Without the need for incubation viability assessment by staining provided a more rapid means to assess the impact of drying on the viability of B. longum coated onto oat, peanut or raisin.

Lactobacillus hordei dextrans induce Saccharomyces cerevisiae aggregation and network formation on hydrophilic surfaces

Water kefir granules are supposed to mainly consist of dextrans produced by Lactobacillus (L.) hilgardii. Still, other microorganisms such as L. hordei, L. nagelii, Leuconostoc (Lc.) citreum and Saccharomyces (S.) cerevisiae are commonly isolated from water kefir granules, while their contribution to the granule formation remains unknown. We studied putative functions of these microbes in granule formation, upon development of a simplified model system containing hydrophilic object slides, which mimics the hydrophilic surface of a growing kefir granule. We found that all tested lactic acid bacteria produced glucans, while solely those isolated from the four different L. hordei strains induced yeast aggregation on the hydrophilic slides. Therefore, structural differences between these glucans were investigated with respect to their size distributions and their linkage types. Beyond the finding that all glucans were identified as dextrans, those of the four L. hordei strains were highly similar among each other regarding portions of linkage types and size distributions. Thus, our study suggests the specific size and structural organization of the dextran produced by L. hordei as the main cause for inducing S. cerevisiae aggregation and network formation on hydrophilic surfaces and thus as crucial initiation of the stepwise water kefir granule growth.

Anaerobic decomposition of humic substances by Clostridium from the deep subsurface

Decomposition of humic substances (HSs) is a slow and cryptic but non-negligible component of carbon cycling in sediments. Aerobic decomposition of HSs by microorganisms in the surface environment has been well documented; however, the mechanism of anaerobic microbial decomposition of HSs is not completely understood. Moreover, no microorganisms capable of anaerobic decomposition of HSs have been isolated. Here, we report the anaerobic decomposition of humic acids (HAs) by the anaerobic bacterium Clostridium sp. HSAI-1 isolated from the deep terrestrial subsurface. The use of (14)C-labelled polycatechol as an HA analogue demonstrated that the bacterium decomposed this substance up to 7.4% over 14 days. The decomposition of commercial and natural HAs by the bacterium yielded lower molecular mass fractions, as determined using high-performance size-exclusion chromatography. Fourier transform infrared spectroscopy revealed the removal of carboxyl groups and polysaccharide-related substances, as well as the generation of aliphatic components, amide and aromatic groups. Therefore, our results suggest that Clostridium sp. HSAI-1 anaerobically decomposes and transforms HSs. This study improves our understanding of the anaerobic decomposition of HSs in the hidden carbon cycling in the Earth's subsurface.

Improving reliability of live/dead cell counting through automated image mosaicing

Cell counting is one of the basic needs of most biological experiments. Numerous methods and systems have been studied to improve the reliability of counting. However, at present, manual cell counting performed with a hemocytometer still represents the gold standard, despite several problems limiting reproducibility and repeatability of the counts and, at the end, jeopardizing their reliability in general. We present our own approach based on image processing techniques to improve counting reliability. It works in two stages: first building a high-resolution image of the hemocytometer's grid, then counting the live and dead cells by tagging the image with flags of different colours. In particular, we introduce GridMos (http://sourceforge.net/p/gridmos), a fully-automated mosaicing method to obtain a mosaic representing the whole hemocytometer's grid. In addition to offering more significant statistics, the mosaic "freezes" the culture status, thus permitting analysis by more than one operator. Finally, the mosaic achieved can thus be tagged by using an image editor, thus markedly improving counting reliability. The experiments performed confirm the improvements brought about by the proposed counting approach in terms of both reproducibility and repeatability, also suggesting the use of a mosaic of an entire hemocytometer's grid, then labelled trough an image editor, as the best likely candidate for the new gold standard method in cell counting.

Visualizing aquatic bacteria by light and transmission electron microscopy

The understanding of the functional role of aquatic bacteria in microbial food webs is largely dependent on methods applied to the direct visualization and enumeration of these organisms. While the ultrastructure of aquatic bacteria is still poorly known, routine observation of aquatic bacteria by light microscopy requires staining with fluorochromes, followed by filtration and direct counting on filter surfaces. Here, we used a new strategy to visualize and enumerate aquatic bacteria by light microscopy. By spinning water samples from varied tropical ecosystems in a cytocentrifuge, we found that bacteria firmly adhere to regular slides, can be stained by fluorochoromes with no background formation and fast enumerated. Significant correlations were found between the cytocentrifugation and filter-based methods. Moreover, preparations through cytocentrifugation were more adequate for bacterial viability evaluation than filter-based preparations. Transmission electron microscopic analyses revealed a morphological diversity of bacteria with different internal and external structures, such as large variation in the cell envelope and capsule thickness, and presence or not of thylakoid membranes. Our results demonstrate that aquatic bacteria represent an ultrastructurally diverse population and open avenues for easy handling/quantification and better visualization of bacteria by light microscopy without the need of filter membranes.