Ralitsa Georgieva

Identification and in vitro characterisation of Lactobacillus plantarum strains from artisanal Bulgarian white brined cheeses.

Lactobacillus plantarum strains were isolated from fully ripened, white brined Bulgarian home‐made cheeses. Strains were derived from phenotypically homogenous Lactobacillus group and were identified as L. plantarum based on both phenotypic and molecular identification (species‐specific and multiplex PCR) methods. Heterogeneity of L. plantarum isolates was evaluated by Rep‐PCR analysis.

Antimicrobial activity and antibiotic susceptibility of Lactobacillus and Bifidobacterium spp. intended for use as starter and probiotic cultures

Antimicrobial activity and antibiotic susceptibility were tested for 23 Lactobacillus and three Bifidobacterium strains isolated from different ecological niches. Agar-well diffusion method was used to test the antagonistic effect (against Staphylococcus aureus, Escherichia coli, Bacillus cereus and Candida albicans) of acid and neutralized (pH 5.5) lyophilized concentrated supernatants (cell-free supernatant; CFS) and whey (cell-free whey fractions; CFW) from de Man–Rogosa–Sharpe/trypticase-phytone-yeast broth and skim milk. Acid CFS and CFW showed high acidification rate-dependent bacterial inhibition; five strains were active against C. albicans. Neutralized CFS/CFW assays showed six strains active against S. aureus (L. acidophilus L-1, L. brevis 1, L. fermentum 1, B. animalis subsp. lactis L-3), E. coli (L. bulgaricus 6) or B. cereus (L. plantarum 24-4В). Inhibition of two pathogens with neutralized CFS (L. bulgaricus 6, L. helveticus 3, L. plantarum 24-2L, L. fermentum 1)/CFW (L. plantarum 24-5D, L. plantarum 24-4В) was detected. Some strains maintained activity after pH neutralization, indicating presence of active substances. The antibiotics minimum inhibitory concentrations (MICs) were determined by the Epsilometer test method. All strains were susceptible to ampicillin, gentamicin, erythromycin and tetracycline. Four lactobacilli were resistant to one antibiotic (L. rhamnosus Lio 1 to streptomycin) or two antibiotics (L. acidophilus L-1 and L. brevis 1 to kanamycin and clindamycin; L. casei L-4 to clindamycin and chloramphenicol). Vancomycin MICs > 256 μg/mL indicated intrinsic resistance for all heterofermentative lactobacilli. The antimicrobially active strains do not cause concerns about antibiotic resistance transfer and could be used as natural biopreservatives in food and therapeutic formulations.

Total reflection X-ray fluorescence analysis of trace elements in Bulgarian bottled mineral waters of low and high mineral content

The trace elements S, Cl, K, Ca, Mn, Fe, Ni, Cu, Zn, As, Br, Rb and Ba in Bulgarian bottled mineral waters of low and high total dissolved solids (TDS) content were determined by total reflection X-ray fluorescence (TXRF) analysis using gallium as internal standard. The water samples were analysed after simple preparation consisting in three-fold sequential pipetting/drying of 5 μL portions of the water sample mixed with the internal standard on the quartz holders. The precision of the obtained results is characterised by an RSD of 5–16%. The lower limits of detection (LLDs) in waters of TDS content of 160–420 mg L−1 were in the low µg L−1 range, whereas those in the water with TDS content of 2900 mg L−1 were higher by a factor of 5–15. There is a good agreement between the results for the trace element content obtained by TXRF and inductively coupled plasma optical emission spectrometry (ICP-OES) for mineral waters with TDS content in the range from 160 to 2900 mg L−1.

A study of the thermal denaturation of the S-layer protein from Lactobacillus salivarius

Surface layer (S-layer) proteins display an intrinsic self-assembly property, forming monomolecular crystalline arrays, identified in outermost structures of the cell envelope in many organisms, such as bacteria and archaea. Isolated S-layer proteins also possess the ability to recrystallize into regular lattices, being used in biotechnological applications, such as controlling the architecture of biomimetic surfaces. To this end, the stability of the S-layer proteins under high-temperature conditions is very important. In this study, the S-layer protein has been isolated from Lactobacillus salivarius 16 strain of human origin, and purified by cation-exchange chromatography. Using circular dichroism (CD) spectroscopy, we have investigated the thermal denaturation of the S-layer protein. The far- and near-UV CD spectra have been collected, and the temperature dependence of the CD signal in these spectral domains has been analyzed. The variable temperature results show that the secondary and tertiary structures of the S-layer protein change irreversibly due to the heating of the sample. After the cooling of the heated protein, the secondary and tertiary structures are partially recovered. The denaturation curves show that the protein unfolding depends on the sample concentration and on the heating rate. The secondary and tertiary structures of the protein suffer changes in the same temperature range. We have also detected an intermediate state in the protein denaturation pathway. Our results on the thermal behavior of the S-layer protein may be important for the use of S-layer proteins in biotechnological applications, as well as for a better understanding of the structure and function of S-layer proteins.

Feasibility of ETV-ICP-OES for Characterization of Archaeological Glasses

In the present study the possibilities of electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) for characterization of archaeological glasses were investigated. The objects of our research were fragments of a colorless late antique Roman flat window glass (fifth century A.D.) and colored medieval glass bracelets (eleventh–twelfth century A.D.) excavated in the region of Pernik (West Bulgaria). The finely ground glass samples were analyzed directly and CHF3 was used as evaporation and transport modifier. Dried aqueous standard solutions and certified reference materials with different matrix (glass, fly ash, and stream sediment) were used as calibration standards. No matrix effects were observed by the optimized conditions. Measurements were performed using common calibration curves obtained with all appropriate calibration standards and major, minor, and trace element concentrations were determined. ETV-ICP-OES analytical data were used to establish the type of glass, the fluxing agents, the typical coloring and decoloring elements, and the recipe norm.

Adhesion Ability of Lactobacillus Plantarum AC131

ABSTRACT Adhesion of lactic acid bacteria (LAB) to mucosal surfaces and their ability to prevent pathogenic invasion of the epithelium are among the most beneficial properties of probiotic lactobacilli. The molecular mechanisms of lactobacillus adhesion have not been fully studied and understood yet. Therefore, a current task is to reveal the genetic and biological factors that determine the adhesion of probiotic LAB cultures. A dairy product originating strain AC131 was selected for the present study, based on its broad-spectrum antimicrobial activity. It was identified as Lactobacillus plantarum by 16S rDNA sequence analysis. The presence of genes Map, Mub and EF-Tu, as genetic determinants of the adhesion factors in Lactobacillus plantarum AC131, was established by PCR analyses. In addition, in vitro tests for adhesive ability of the strain AC131 were carried out. Positive results in tests with two different cell lines were obtained and the strain AC131 was classified as strongly adhesive to HeLa and moderate adhesive to HT-29 cell line. Our results provide a promising basis for further characterization of Lactobacillus plantarum AC131, as a potential probiotic.

Do bacteria have an electric permanent dipole moment

In the scientific literature in the last 40 years, some data for the permanent dipole moment and the electric polarizability of Escherichia coli can be found [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991]. In this paper the data based mainly on electro-optic investigation is considered as much as some dipolophoretic (most often called dielectrophoretic) studies. Serious grounds are found to doubt the conclusions made for the electric dipole moments of bacteria by one of the authors of this paper (SPS) and by some other researchers. This concerns both the permanent dipole moment and the electric charge dependent polarizabilities of E. coli. Here, along with the discussion of the old experimental data, new experimental data are shown for a strain of E. coli HB101. The conclusions from the analysis of the old and the new experimental data is that they do not provide correct evidence for the presence of a permanent dipole moment. It seems that all statements for the existence of electric permanent dipole moment in bacteria [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991; S.P. Stoylov, S. Sokerov, I. Petkanchin, N. Ibroshev, Dokl. AN URSS 180 (1968) 1165; N.A. Tolstoy, A.A. Spartakov, A.A. Trusov, S.A. Schelkunova, Biofizika 11 (1966) 453; V. Morris, B. Jennings, J. Chem. Soc. Faraday Trans. II 71 (1975) 1948; V. Morris, B. Jennings, J. Colloid Interface Sci. 55 (1978) 313; S.P. Stoylov, V.N. Shilov, S.S. Dukhin, S. Sokerov, I. Petkanchin, in: S.S. Dukhin (Ed.), Electro-optics of Colloids, Naukova Dumka, Kiev, 1977 (in Russian).] based on electro-optic studies are result of incorrect interpretation. Therefore, they should be further ignored.

Modulation of Escherichia coli biofilm growth by cell-free spent cultures from lactobacilli

E. coli biofilms cause serious problems in medical practice by contaminating surfaces and indwelling catheters. Due to the rapid development of antibiotic resistance, alternative approaches to biofilm suppression are needed. This study addresses whether products released by antagonistic bacteria — Lactobacillus isolates from vaginal and dairy-product samples could be useful for controlling E. coli biofilms. The effects of diluted cell-free supernatants (CFS) from late-exponential Lactobacillus cultures on the growth and biofilm production of Escherichia coli were tested. Most of the CFS applied as 10−2 had no impact on bacterial growth, biofilm development however was influenced even by 10−4 of CFS. Initial screening by crystal violet assay showed that biofilm modulation varied between different CFS and E. coli combinations from inhibition to activation; however three of the tested CFS showed consistency in biofilm suppression. This was not due to antibacterial activity since Live/Dead fluorescence labeling showed insignificant differences in the amount of dead cells in control and treated samples. Some E. coli strain-specific mechanisms of response to the three CFS included reduction in hydrophobicity and motility. Released exoploysaccharides isolated from the three CFS stimulated sessile growth, but proteinase K reduced their inhibitory activities implying participation of protein or peptide biofilm suppression factor(s).

Efficient removal of cyclobutane pyrimidine dimers in barley: differential contribution of light-dependent and dark DNA repair pathways.

Barley stress response to ultraviolet radiation (UV) has been intensively studied at both the physiological and morphological level. However, the ability of barley genome to repair UV-induced lesions at the DNA level is far less characterized. In this study, we have investigated the relative contribution of light-dependent and dark DNA repair pathways for the efficient elimination of cyclobutane pyrimidine dimers (CPDs) from the genomic DNA of barley leaf seedlings. The transcriptional activity of barley CPD photolyase gene in respect to the light-growth conditions and UV-C irradiation of the plants has also been analyzed. Our results show that CPDs induced in the primary barley leaf at frequencies potentially damaging DNA at the single-gene level are removed efficiently and exclusively by photorepair pathway, whereas dark repair is hardly detectable, even at higher CPD frequency. A decrease of initially induced CPDs under dark is observed but only after prolonged incubation, suggesting the activation of light-independent DNA damage repair and/or tolerance mechanisms. The green barley seedlings possess greater capacity for CPD photorepair than the etiolated ones, with efficiency of CPD removal dependent on the intensity and quality of recovering light. The higher repair rate of CPDs measured in the green leaves correlates with the higher transcriptional activity of barley CPD photolyase gene. Visible light and UV-C radiation affect differentially the expression of CPD photolyase gene particularly in the etiolated leaves. We propose that the CPD repair potential of barley young seedlings may influence their response to UV-stress.

Thermal properties and microstructure of Roman flat window glass found in Bulgaria

Abstract The aim of this study is to carry out investigations in order to establish the composition and technological behaviour of the Roman glass dated from IV to VI c. For this reason the glass transition temperature (Tg) at 380 °C, the softening point at 490 °C and crystallization ability were determined. The fragment has been investigated by a classical chemical analysis, an electron probe microanalysis, differential thermal analysis, infrared spectroscopy, transmission electron microscopy and X-ray diffraction. According to the chemical composition, the fragment is soda-lime–silica glass that is typical for the production of this region. Microheterogeneities were found which are originated from the preparation method and from the corrosion caused by soil influence. As a result of additional thermal treatment the crystal phases were also established.