Variations in the fluorescence intensity of intact DAPI-stained bacteria and their implications for rapid bacterial quantification

Practical observations on the use of fluorescent reporter systems in Clostridioides difficile

Fluorescence microscopy is a valuable tool to study a broad variety of bacterial cell components and dynamics thereof. For Clostridioides difficile, the fluorescent proteins CFP^opt, mCherry^Opt and phiLOV2.1, and the self-labelling tags SNAP^Cd and HaloTag, hereafter collectively referred as fluorescent systems, have been described to explore different cellular pathways. In this study, we sought to characterize previously used fluorescent systems in C. difficile cells. We performed single cell analyses using fluorescence microscopy of exponentially growing C. difficile cells harbouring different fluorescent systems, either expressing these separately in the cytosol or fused to the C-terminus of HupA, under defined conditions. We show that the intrinsic fluorescence of C. difficile cells increases during growth, independent of sigB or spo0A. However, when C. difficile cells are exposed to environmental oxygen autofluorescence is enhanced. Cytosolic overexpression of the different fluorescent systems alone, using the same expression signals, showed heterogeneous expression of the fluorescent systems. High levels of mCherry^Opt were toxic for C. difficile cells limiting the applicability of this fluorophore as a transcriptional reporter. When fused to HupA, a C. difficile histone-like protein, the fluorescent systems behaved similarly and did not affect the HupA overproduction phenotype. The present study compares several commonly used fluorescent systems for application as transcriptional or translational reporters in microscopy and summarizes the limitations and key challenges for live-cell imaging of C. difficile. Due to independence of molecular oxygen and fluorescent signal, SNAP^Cd appears the most suitable candidate for live-cell imaging in C. difficile to date.

Optimal fluorescent-dye staining time for the real-time detection of microbes: a study of Saccharomyces cerevisiae

AIMS

To provide information on the time-dependent behaviour of microbe staining by fluorescent dyes in the order of seconds, which is important in terms of the recent rapid and online techniques for microbe measurements and/or environmental microbe analysis.

METHODS AND RESULTS

For combinations of yeast (Saccharomyces cerevisiae) and typical dyes, including DAPI (4',6-diamidino-2-phenylindole) and Auramine-O, a suspension of yeast cells in ultrapure water was injected into a dye solution in a micro cuvette placed inside a spectrofluorometer and the fluorescence intensity of the resulting solution was measured at 1 s intervals, starting immediately after the mixing and continued until the time for the maximum intensity using various concentrations of yeast and dyes. The relaxation time τ, which corresponds to ~63·2% of the maximum fluorescence intensity, was shown to decrease to below 1 s with increasing DAPI concentration, whereas it remained constant for 2-3 s with increasing Auramine-O concentration, for example at a yeast concentration of 100 µg ml^-1 .

CONCLUSIONS

For the conditions of yeast >10 µg ml^-1 , DAPI >1 µg ml^-1 and Auramine-O >0·1 µg ml^-1 , τ could be adjusted to below 5 s to achieve a rapid and stable staining.

SIGNIFICANCE AND IMPACT OF THE STUDY

Design and operating conditions for rapid and online measurements of microbes can be optimized.

Label-Free SERS Discrimination and In Situ Analysis of Life Cycle in Escherichia coli and Staphylococcus epidermidis

Surface enhanced Raman spectroscopy (SERS) has been proven suitable for identifying and characterizing different bacterial species, and to fully understand the chemically driven metabolic variations that occur during their evolution. In this study, SERS was exploited to identify the cellular composition of Gram-positive and Gram-negative bacteria by using mesoporous silicon-based substrates decorated with silver nanoparticles. The main differences between the investigated bacterial strains reside in the structure of the cell walls and plasmatic membranes, as well as their biofilm matrix, as clearly noticed in the corresponding SERS spectrum. A complete characterization of the spectra was provided in order to understand the contribution of each vibrational signal collected from the bacterial culture at different times, allowing the analysis of the bacterial populations after 12, 24, and 48 h. The results show clear features in terms of vibrational bands in line with the bacterial growth curve, including an increasing intensity of the signals during the first 24 h and their subsequent decrease in the late stationary phase after 48 h of culture. The evolution of the bacterial culture was also confirmed by fluorescence microscope images.

Growth behaviors of bacteria in biofouling cake layer in a dead-end microfiltration system

The growth behaviors of three bacterial species, i.e. Escherichia coli, Pseudomonas putida and Aquabaculum hongkongensis, in biofouling cake layer (attached form) were investigated using an unstirred dead-end continuous microfiltration system, and were compared with those in suspended form. Results showed that all the three bacteria had larger average growth rates in suspended form than in attached form under high substrates levels. Under oligotrophic conditions, the average growth rates in the attached form were faster than those in the suspended form, especially for A. hongkongensis. The growth behaviors analysis presented the same results due to all the tested bacteria had higher maximum growth rate and saturation constant in suspended form than attached form, indicating the dominant growth mode would be shifted from attached form to suspended form with substrate concentration increase. Finally, total filtration resistance determined in the experiments increased significantly with the bacterial growth in filtration system.

Micro-thermal field-flow fractionation of bacteria

The retention of Staphylococcus epidermidis bacteria cells, achieved with the use of micro-thermal field-flow fractionation and described in this paper, represents the first experimental proof that the separation and characterization of the bio-macromolecules and biological particles is possible by exploiting Ludwig-Soret effect of thermal diffusion. The experiments were carried out under gentle experimental conditions preventing the denaturation of the bacteria. Lift forces, appearing at high linear velocities of the carrier liquid, generated the focusing mechanism of the retention which resulted in high-speed and high-performance separation performed in less than 10 min.

Sedimentation field-flow fractionation device cleaning, decontamination and sterilization procedures for cellular analysis

In Sedimentation FFF (SdFFF) practice, it is known that a large number of cell elutions create aging phenomena of the separator, thereby reducing recovery and modifying elution characteristics. Systematic cleaning procedures are developed to enhance channel lifetime, together with microbial decontamination processes. Cells can be therefore reproducibly eluted for a large number of analyses and collected under sterile conditions, if needed. This is one of the most valuable aspect if further culture or transplantation is required. Decontamination was performed using, as contaminant probe, Staphylococcus aureus, highly adherent pathogenic bacteria that eluted from SdFFF as aggregates.

Adherence of Listeria monocytogenes strains to stainless steel coupons

An assay was developed to measure the number of Listeria monocytogenes cells adhering to stainless steel, and was used to investigate the adherence of 111 strains of the organism, which included representatives with respect to serotype, carriage of plasmids, source and persistence in the food processing environment. Growth and adherence curves of four L. monocytogenes strains over 48 h were obtained. While the growth curves of all four micro-organisms were seen to reach similar levels at stationary phase, there was still substantial variation among the adherence curves. In addition, a scatter-graph of growth vs adherence counts at 24 h showed poor correlation. These factors indicated that interstrain variation in adherence at stationary phase is due to factor(s) intrinsic to each strain of L. monocytogenes. Persistent strains were found to adhere in significantly greater numbers than sporadic strains, and variation was also found among serotypes, with serotype 1/2c showing significantly greater adherence than serotypes 1/2a and 4b; 4b strains were significantly higher than those of 1/2a strains. No significant difference was found between strains according to source or plasmid carriage.