Desiccation survival in Salmonella enterica, Escherichia coli and Enterococcus faecium related to initial cell concentration and cellular components

Salmonella enterica is well-known for its ability to survive and persist in low-moisture environments. Previous studies have indicated a link between the initial cell concentration and the population of Salmonella that survive upon desiccation and subsequent storage; however, how the initial cell concentration affects survival is unknown. This study examined the basis of this phenomena and whether it occurred in other microorganisms, specifically Shiga toxigenic Escherichia coli (STEC), and Enterococcus faecium . Salmonella, STEC, and E. faecium were grown as lawns on TSAYE and harvested using buffered peptone water (BPW). To determine recovery at different initial cell levels, cultures were diluted to 9, 7, and 5 log CFU/mL and applied to filters. Filters were dried for 24 h, then stored for 28 d at 25°C/33% RH. During storage, cells were recovered from filters using BPW and cultivated on TSAYE. Both Salmonella and E. coli , but not E. faecium , showed non-proportional recovery. Less viability remained with lower initial starting population after 24 h desiccation such that ≥10 log CFU/mL were recovered when 11 log CFU/mL was desiccated, but <3 log CFU/mL were recovered when 5 log CFU/mL was desiccated. Once dried, persistence did not appear affected by initial cell concentration. When dead cells (heat-treated) were added to the diluent, recovery of Salmonella was proportional with respect to the initial cell concentration. To further examine the response on desiccation, Salmonella was diluted in BPW containing one of 11 different test cell components related to quorum sensing or known to affect desiccation resistance to assess recovery and persistence. Of the 11 additions only cell debris fractions, cell-free extract, and peptidoglycan improved recovery of Salmonella . Desiccation survival appears related to cell wall components, however, the exact mechanism affecting survival remains unknown.

Effect of Initial Cell Concentration on Bio-Oxidation of Pyrite before Gold Cyanidation

Bio-oxidation of refractory sulfidic gold minerals has been applied at the commercial scale as a pre-treatment to improve gold yields and reduce chemical consumption during gold cyanidation. In this study, the effect of initial cell concentration on the oxidation of pyritic gold ore was evaluated with four aerated bioreactors at 30 °C with 10% pulp density and pH maintained at 1.4 with NaOH. Results of NaOH consumption and changes in soluble Fe and S concentrations indicated that increasing the initial cell concentration from 2.3 × 107 to 2.3 × 1010 cells mL−1 enhanced pyrite oxidation during the first week. However, by day 18 the reactor with the lowest initial cell concentration showed profound performance enhancement based on soluble Fe and S concentrations, sulfide-S and pyrite contents in the residues, and subsequent gold leaching of the bio-oxidation residues by cyanidation. Overall, the results showed that the cell concentration was clearly beneficial during the initial stages of oxidation (first 7–8 days).

L-asparaginase Production by Leucosporidium scottii in a Bench-Scale Bioreactor With Co-production of Lipids

L-asparaginase (ASNase) is a therapeutical enzyme used for treatment of acute lymphoblastic leukemia. ASNase products available in the market are produced by bacteria and usually present allergic response and important toxicity effects to the patients. Production of ASNase by yeasts could be an alternative to overcome these problems since yeasts have better compatibility with the human system. Recently, it was found that Leucosporidium scottii, a psychrotolerant yeast, produces ASNase. In order to advance the production of ASNase by this yeast, the present study aimed to select suitable process conditions able to maximize the production of this enzyme in a bench-scale bioreactor. Additionally, the accumulation of lipids during the enzyme production process was also determined and quantified. Experiments were carried out with the aim of selecting the most appropriate conditions of initial cell concentration (1.0, 3.5, and 5.6 g L–1), carbon source (sucrose and glycerol, individually or in mixture) and oxygen transfer rate (kLa in the range of 1.42–123 h–1) to be used on the production of ASNase by this yeast. Results revealed that the enzyme production increased when using an initial cell concentration of 5.6 g L–1, mixture of sucrose and glycerol as carbon source, and kLa of 91.72 h–1. Under these conditions, the enzyme productivity was maximized, reaching 35.11 U L–1 h–1, which is already suitable for the development of scale-up studies. Additionally, accumulation of lipids was observed in all the cultivations, corresponding to 2–7 g L–1 (32–40% of the cell dry mass), with oleic acid (C18:1) being the predominant compound (50.15%). Since the L-asparaginase biopharmaceuticals on the market are highly priced, the co-production of lipids as a secondary high-value product during the ASNase production, as observed in the present study, is an interesting finding that opens up perspectives to increase the economic feasibility of the enzyme production process.

A novel optical panel photobioreactor for cultivation of microalgae

In this study, a novel optical panel photobioreactor (OPPBR) equipped with a V-cut/or flat optical panel (OP) and a light source, i.e., light-emitting diodes (LEDs) was developed. The performance of this OPPBR was assessed using cultures of Chlorella vulgaris. Growth rates of biomass were compared in bioreactors operated separately using a V-cut OP, or a flat-plate OP both equipped with LEDs or a fluorescent light source without any OP. The experiments were conducted at neutral pH (7.2 ± 0.3) with an initial cell concentration of 0.15 ± 0.05 g L−1, at 23 ± 1 °C under dark and light cycles of 8 and 16 h, respectively, using LEDs and fluorescent lamps for 11 days. The results demonstrated that the amount of biomass produced using the V-cut OP was three times higher than the flat-plate OP and five times higher than without the OP. Parametric studies demonstrated that a distance of 3 mm between the OP and the LEDs produced the highest illumination uniformity, i.e., 65.7% for the flat-plate OP and 87.6% for the V-cut OP. The OPPBR system can be scaled up and could be used to enhance biomass production using an LED and OP combination.

The Growth ofMonoraphidiumsp. andScenedesmussp. Cells in the Presence of Thorium

Toxicity of thorium byMonoraphidiumsp. andScenedesmussp. was studied. Microalgal cultures were inoculated in ASM-1 medium in presence and absence of thorium. Its effect was monitored by direct counting on Fuchs-Rosenthal chamber and with software. The toxicity of thorium over the species was observed for concentrations over 50.0 mg/L. After 30 days,Monoraphidiumcells decreased their concentration from4.23×106to4.27×105and8.57×105 cells/mL, in the presence of 50.0 and 100.0 mg/L of thorium, respectively.Scenedesmussp. cells were more resistant to thorium: for an initial cell concentration of7.65×104 cells/mL it was observed a change to5.25×105and5.12×105 cells/mL, in the presence of thorium at 50.0 and 100.0 mg/L, respectively. This is an indication that low concentrations of the radionuclide favored the growth, and thatScenedesmuscells are more resistant to thorium thanMonoraphidiumcells. The software used for comparison with direct count method proved to be useful for the improvement of accuracy of the results obtained, a decrease in the uncertainty and allowed recording of the data. The presence of thorium suggests that low concentrations have a positive effect on the growth, due to the presence of the nitrate, indicating its potential for ecotoxicological studies.

Effect of culture age, protectants, and initial cell concentration on viability of freeze-dried cells ofMetschnikowia pulcherrima

The effect of freeze-drying using different lyoprotectants at different concentrations on the viability and biocontrol efficacy of Metschnikowia pulcherrima was evaluated. The effects of initial yeast cell concentration and culture age on viability were also considered. Yeast cells grown for 36 h were more resistant to freeze-drying than were 48 h cells. An initial concentration of 108cells·mL–1favoured the highest survival after freeze-drying. When maltose (25%, m/v) was used as protectant, a high cell viability was obtained (64.2%). Cells maintained a high viability after 6 months of storage at 4 °C. The biocontrol efficacy of freeze-dried cells was similar to the activity of fresh cells on ‘Gala’ apples and was slightly lower on ‘Golden Delicious’ apples. After optimizing freeze-drying conditions, the viability of M. pulcherrima cells was similar to that obtained in other studies. The results constitute a first step towards the commercial development of M. pulcherrima as a biocontrol agent.