Mesophilic Strains Research Articles

Evaluation of growth behavior of lactic acid bacteria isolated from kimchi via kinetic modeling

Lactic acid bacteria (LAB) are critical in kimchi fermentation. This study determined the effect of temperature on the growth of 11 predominant LAB (five psychrotrophic and six mesophilic strains) isolated from kimchi and analyzed their growth behavior using a kinetic model to understand changes in LAB communities during kimchi fermentation. The modified Gompertz model was more suitable for the growth of kimchi LAB rather than the logistic model in terms of RMSE, AICc, R2, adjusted R2, Af and Bf. Latilactobacillus sakei KCKM 0001 and Leuconostoc mesenteroides KCKM 0008 rapidly grew at all temperatures (5, 10, 15, 20, 25, 30, and 37 °C) among the strains analyzed for growth characteristics using the modified Gompertz model. Additionally, the growth of psychrotrophic strains was more active than that of the mesophilic strains, excluding L. sakei KCKM 0001 and Leu. mesenteroides KCKM 0008 at kimchi fermentation temperatures of 5–10 °C. The differences observed in the lag phase and maximum specific growth rate at each temperature showed a trend consistent with previously reported dominant LAB at kimchi fermentation temperatures. The predictive modeling results showed that the changes in LAB communities during kimchi fermentation could be explained by their growth behavior at different temperatures.

Molecular characterization of the PhiKo endolysin from Thermus thermophilus HB27 bacteriophage phiKo and its cryptic lytic peptide RAP-29.

In the era of increasing bacterial resistance to antibiotics, new bactericidal substances are sought, and lysins derived from extremophilic organisms have the undoubted advantage of being stable under harsh environmental conditions. The PhiKo endolysin is derived from the phiKo bacteriophage infecting Gram-negative extremophilic bacterium Thermus thermophilus HB27. This enzyme shows similarity to two previously investigated thermostable type-2 amidases, the Ts2631 and Ph2119 from Thermus scotoductus bacteriophages, that revealed high lytic activity not only against thermophiles but also against Gram-negative mesophilic bacteria. Therefore, antibacterial potential of the PhiKo endolysin was investigated in the study presented here. Enzyme activity was assessed using turbidity reduction assays (TRAs) and antibacterial tests. Differential scanning calorimetry was applied to evaluate protein stability. The Collection of Anti-Microbial Peptides (CAMP) and Antimicrobial Peptide Calculator and Predictor (APD3) were used to predict regions with antimicrobial potential in the PhiKo primary sequence. The minimum inhibitory concentration (MIC) of the RAP-29 synthetic peptide was determined against Gram-positive and Gram-negative selected strains, and mechanism of action was investigated with use of membrane potential sensitive fluorescent dye 3,3'-Dipropylthiacarbocyanine iodide (DiSC3(5)). The PhiKo endolysin is highly thermostable with melting temperature of 91.70°C. However, despite its lytic effect against such extremophiles as: T. thermophilus, Thermus flavus, Thermus parvatiensis, Thermus scotoductus, and Deinococcus radiodurans, PhiKo showed moderate antibacterial activity against mesophiles. Consequently, its protein sequence was searched for regions with potential antibacterial activity. A highly positively charged region was identified and synthetized (PhiKo105-133). The novel RAP-29 peptide lysed mesophilic strains of staphylococci and Gram-negative bacteria, reducing the number of cells by 3.7-7.1 log units and reaching the minimum inhibitory concentration values in the range of 2-31 μM. This peptide is unstructured in an aqueous solution but forms an α-helix in the presence of detergents. Moreover, it binds lipoteichoic acid and lipopolysaccharide, and causes depolarization of bacterial membranes. The RAP-29 peptide is a promising candidate for combating bacterial pathogens. The existence of this cryptic peptide testifies to a much wider panel of antimicrobial peptides than thought previously.

Genomics of Re-Emergent Aeromonas salmonicida in Atlantic Salmon Outbreaks.

Furunculosis, caused by Aeromonas salmonicida, poses a significant threat to both salmonid and non-salmonid fish in diverse aquatic environments. This study explores the genomic intricacies of re-emergent A. salmonicida outbreaks in Atlantic salmon (Salmo salar). Previous clinical cases have exhibited pathological characteristics, such as periorbital hemorrhages and gastrointestinal abnormalities. Genomic sequencing of three Chilean isolates (ASA04, ASA05, and CIBA_5017) and 25 previously described genomes determined the pan-genome, phylogenomics, insertion sequences, and restriction-modification systems. Unique gene families have contributed to an improved understanding of the psychrophilic and mesophilic clades, while phylogenomic analysis has been used to identify mesophilic and psychrophilic strains, thereby further differentiating between typical and atypical psychrophilic isolates. Diverse insertion sequences and restriction-modification patterns have highlighted genomic structural differences, and virulence factor predictions can emphasize exotoxin disparities, especially between psychrophilic and mesophilic strains. Thus, a novel plasmid was characterized which emphasized the role of plasmids in virulence and antibiotic resistance. The analysis of antibiotic resistance factors revealed resistance against various drug classes in Chilean strains. Overall, this study elucidates the genomic dynamics of re-emergent A. salmonicida and provides novel insights into their virulence, antibiotic resistance, and population structure.

Polyinfection in Fish Aeromoniasis: A Study of Co-Isolated Aeromonas Species in Aeromonas veronii Outbreaks.

We studied the phenotypic and genomic characteristics related to the virulence and antibiotic resistance of two Aeromonas strains, which were co-isolated before an outbreak of Aeromonas veronii among diseased seabass on Agathonisi Island, Greece, in April 2015. The first strain, AG2.13.2, is a potentially pathogenic mesophilic variant of Aeromonas salmonicida, and the second, AG2.13.5, corresponds to an Aeromonas rivipollensis related to A. rivipollensis KN-Mc-11N1 with an ANI value of 97.32%. AG2.13.2 lacks the type III secretion system just like other mesophilic strains of A. salmonicida. This characteristic has been associated with lower virulence. However, the genome of AG2.13.2 contains other important virulence factors such as type II and type VI secretion systems, and toxins such as rtxA, aerolysin aer/act, and different types of hemolysins. The strain also carries several genes associated with antibiotic resistance such as the tetE efflux pump, and exhibits resistance to tetracycline, ampicillin, and oxolinic acid. In an in vivo challenge test with gilthead seabream larvae, the A. veronii bv sobria strain AG5.28.6 exhibited the highest virulence among all tested strains. Conversely, both A. salmonicida and A. rivipollensis showed minimal virulence when administered alone. Interestingly, when A. veronii bv sobria AG5.28.6 was co-administered with A. rivipollensis, the larvae survival probability increased compared to those exposed to A. veronii bv sobria AG5.28.6 alone. This finding indicates an antagonistic interaction between A. veronii bv sobria AG5.28.6 and A. rivipollensis AG2.13.5. The co-administration of A. veronii bv sobria AG5.28.6 with Aeromonas salmonicida did not yield distinct survival probabilities. Our results validate that the primary pathogen responsible for European seabass aeromoniasis is Aeromonas veronii bv sobria.

Genome-Based Insights into the Thermotolerant Adaptations of Neobacillus endophyticus BRMEA1T

The bacterium <i>Neobacillus endophyticus</i> BRMEA1<sup>T</sup>, isolated from the medicinal plant <i>Selaginella involvens</i>, known as its thermotolerant can grow at 50°C. To explore the genetic basis for its heat tolerance response and its potential for producing valuable natural compounds, the genomes of two thermotolerant and four mesophilic strains in the genus <i>Neobacillus</i> were analyzed using a bioinformatic software platform. The whole genome was annotated using RAST SEED and OrthVenn2, with a focus on identifying potential heattolerance- related genes. <i>N. endophyticus</i> BRMEA1<sup>T</sup> was found to possess more stress response genes compared to other mesophilic members of the genus, and it was the only strain that had genes for the synthesis of osmoregulated periplasmic glucans. This study sheds light on the potential value of <i>N. endophyticus</i> BRMEA1<sup>T</sup>, as it reveals the mechanism of heat resistance and the application of secondary metabolites produced by this bacterium through whole-genome sequencing and comparative analysis.

Whole genome sequence analyses of thermotolerant Bacillus sp. isolates from food.

The Bacillus cereus group, also known as B. cereus sensu lato (B. cereus s.l.), is composed of various Bacillus species, some of which can cause diarrheal or emetic food poisoning. Several emerging highly heat-resistant Bacillus species have been identified, these include B. thermoamylovorans, B. sporothermodurans, and B. cytotoxicus NVH 391-98. Herein, we performed whole genome analysis of two thermotolerant Bacillus sp. isolates, Bacillus sp. B48 and Bacillus sp. B140, from an omelet with acacia leaves and fried rice, respectively. Phylogenomic analysis suggested that Bacillus sp. B48 and Bacillus sp. B140 are closely related to B. cereus and B. thuringiensis, respectively. Whole genome alignment of Bacillus sp. B48, Bacillus sp. B140, mesophilic strain B. cereus ATCC14579, and thermophilic strain B. cytotoxicus NVH 391-98 using the Mauve program revealed the presence of numerous homologous regions including genes responsible for heat shock in the dnaK gene cluster. However, the presence of a DUF4253 domain-containing protein was observed only in the genome of B. cereus ATCC14579 while the intracellular protease PfpI family was present only in the chromosome of B. cytotoxicus NVH 391-98. In addition, prophage Clp protease-like proteins were found in the genomes of both Bacillus sp. B48 and Bacillus sp. B140 but not in the genome of B. cereus ATCC14579. The genomic profiles of Bacillus sp. isolates were identified by using whole genome analysis especially those relating to heat-responsive gene clusters. The findings presented in this study lay the foundations for subsequent studies to reveal further insights into the molecular mechanisms of Bacillus species in terms of heat resistance mechanisms.

Comparative genomic analysis provides insights into taxonomy and temperature adaption of Aeromonas salmonicida.

Aeromonas salmonicida has long been known as psychrophiles since it is mainly isolated from cold water fish, and recent reports have revealed the existence of mesophilic strains isolated from warm sources. However, the genetic differences between mesophilic and psychrophilic strains remain unclear due to few complete genomes of mesophilic strain are available. In this study, six A. salmonicida (2 mesophilic and 4 psychrophilic) were genome-sequenced, and comparative analyses of 25 A. salmonicida complete genomes were conducted. The ANI values and phylogenetic analysis revealed that 25 strains formed three independent clades, which were referred as typical psychrophilic, atypical psychrophilic and mesophilic groups. Comparative genomic analysis showed that two chromosomal gene clusters, related to lateral flagella and outer membrane proteins (A-layer and T2SS proteins), and insertion sequences (ISAs4, ISAs7 and ISAs29) were unique to the psychrophilic groups, while the complete MSH type IV pili were unique to the mesophilic group, all of which may be considered as lifestyle-related factors. The results of this study not only provide new insights into the classification, lifestyle adaption and pathogenic mechanism of different strains of A.salmonicida, but also contributes to the prevention and control of disease caused by psychrophilic and mesophilic A. salmonicida.

Aeromonas salmonicida isolates from Canada demonstrate wide distribution and clustering among mesophilic strains.

All the 36 known species to date of the genus Aeromonas are mesophilic except the species Aeromonas salmonicida, which includes both psychrophilic and mesophilic subspecies. For 20 years, more and more mesophilic A. salmonicida strains have been discovered. Only A. salmonicida subsp. pectinolytica has officially been classified as a mesophilic subspecies. Most mesophiles have been isolated in hot countries. We present, for the first time, the characterization of two new mesophilic isolates from Quebec (Canada). Phenotypic and genomic characterizations were carried out on these strains, isolated from dead fish from a fish farm. Isolates 19-K304 and 19-K308 are clearly mesophiles, virulent to the amoeba Dictyostelium discoideum, a surrogate host, and close to strain Y577, isolated in India. To our knowledge, this is the first time that mesophilic strains isolated from different countries are so similar. The major difference between the isolates is the presence of plasmid pY47-3, a cryptic plasmid that sometimes presents in mesophilic strains. More importantly, our extensive phylogenetic analysis reveals two well-defined clades of mesophilic strains with psychrophiles associated with one of these clades. This helps to have a better understanding of the evolution of this species and the apparition of psychrophilic subspecies.

Production of polyhydroxyalkanoates by the thermophile Cupriavidus cauae PHS1

Thermophilic production of polyhydroxyalkanoate is considered a very promising way to overcome the problems that may arise when using mesophilic strains. This study reports the first thermophilic polyhydroxybutyrate-producing Cupriavidus species, which are known as the best polyhydroxybutyrate-producing microorganisms. Cupriavidus cauae PHS1 harbors a phbCABR cluster with high similarity to the corresponding proteins of C. necator H16 (80, 93, 96, and 97 %). This strain can produce polyhydroxybutyrate from a range of substrates, including acetate (5 g/L) and phenol (1 g/L), yielding 7.6 % and 18.9 % polyhydroxybutyrate, respectively. Moreover, the strain produced polyhydroxybutyrate at temperatures ranging from 25 to 50 °C, with the highest polyhydroxybutyrate content (47 °C) observed at 45 °C from gluconate. Additionally, the strain could incorporate 3-hydroxyvalerate (12.5 mol. %) into the polyhydroxybutyrate polymer using levulinic acid as a precursor. Thus, Cupriavidus cauae PHS1 may be a promising polyhydroxybutyrate producer as alternative for mesophilic polyhydroxybutyrate-producing Cupriavidus species.

Enhanced Hemolytic Activity of Mesophilic Aeromonas salmonicida SRW-OG1 Is Brought about by Elevated Temperatures.

Aeromonas salmonicida is a well-known cold-water pathogenic bacterium. Previously, we reported the first isolation of pathogenic A. salmonicida from diseased Epinephelus coioides, a kind of warm-water fish, and it was proved to be a putative mesophilic strain with potent pathogenicity to humans. In order to investigate the mechanisms underlying mesophilic growth ability and virulence, the transcriptome of A. salmonicida SRW-OG1 at 18, 28, and 37 °C was analyzed. The transcriptome of A. salmonicida SRW-OG1 at different temperatures showed a clear separation boundary, which might provide valuable information for the temperature adaptation and virulence regulation of A. salmonicida SRW-OG1. Interestingly, aerA and hlyA, the hemolytic genes encoding aerolysin and hemolysin, were found to be significantly up-regulated at 28 and 37 °C. Since aerolysin and hemolysin are the most well-known and -characterized virulence factors of pathogenic Aeromonas strains, the induction of aerA and hlyA was associated with the mesophilic virulence. Further study proved that the extracellular products (ECPs) purchased from A. salmonicida SRW-OG1 cultured at 28 and 37 °C showed elevated hemolytic activity and virulence than those at 18 °C. Moreover, the silence of aerA and hlyA led to significantly decreased hemolysis and virulence. Taken together, our results revealed that the mesophilic virulence of A. salmonicida SRW-OG1 might be due to the enhanced expression of aerA and hlyA induced by elevated temperatures.

Aeromonas salmonicida intraspecies divergence revealed by the various strategies displayed when grazed by Tetrahymena pyriformis.

Worldwide, Aeromonas salmonicida is a major bacterial pathogen of fish in both marine and freshwater environments. Despite psychrophilic growth being common for this species, the number of characterized mesophilic strains is increasing. Thus, this species may serve as a model for the study of intraspecies lifestyle diversity. Although bacteria are preyed upon by protozoan predators, their interaction inside or outside the phagocytic pathway of the predator can provide several advantages to the bacteria. To correlate intraspecies diversity with predation outcome, we studied the fate of psychrophilic and mesophilic strains of A. salmonicida cocultured with the ciliate Tetrahymena pyriformis. A total of three types of outcome were observed: digestion, resistance to phagocytosis, and pathogenicity. The psychrophilic strains are fully digested by the ciliate. In contrast, the mesophilic A. salmonicida subsp. pectinolytica strain is pathogenic to the ciliate. All the other mesophilic strains display mechanisms to resist phagocytosis and/or digestion, which allow them to survive ciliate predation. In some cases, passage through the phagocytic pathway resulted in a few mesophilic A. salmonicida being packaged inside fecal pellets. This study sheds light on the great phenotypic diversity observed in the complex range of mechanisms used by A. salmonicida to confront a predator.

Acetobacter senegalensis isolated from mango fruits: Its polyphasic characterization and adaptation to protect against stressors in the industrial production of vinegar: A review.

It has been more than a decade since Acetobacter senegalensis was isolated, identified and described as a thermotolerant strain of acetic acid bacteria. It was isolated from mango fruits in Senegal and used for industrial vinegar production in developing countries, mainly in sub-Saharan Africa. The strain was tested during several spirit vinegar fermentation processes at relatively high temperatures in accordance with African acclimation. The upstream fermentation process had significant stress factors, which are highlighted in this review so that the fermentation process can be better controlled. Due to its high industrial potential, this strain was extensively investigated by diverse industrial microbiologists worldwide; they concentrated on its microbiological, physiological and genomic features. A research group based in Belgium proposed an important project for the investigation of the whole-genome sequence of A. senegalensis. It would use a 454-pyrosequencing technique to determine and corroborate features that could give this strain significant diverse bio-industrial applications. For instance, its application in cocoa bean fermentation has made it a more suitable acetic acid bacterium for the making of chocolate than Acetobacter pasteurianus. Therefore, in this paper, we present a review that summarizes the current research on A. senegalensis at its microbial and genomic levels and also its specific bio-industrial applications, which can provide economic opportunities for African agribusiness. This review summarizes the physiological and genomic characteristics of Acetobacter senegalensis, a thermotolerant strain isolated from mango fruits and intended to be used in industrial vinegar fermentation processes. It also explores other bio-industrial applications such as cocoa fermentation. Vinegar fermentation is usually performed with mesophilic strains in temperate regions of the world. Developing countries, such as Senegal, import vinegar or make 'fake' vinegar by diluting acetic acid obtained from petrochemicals. The use of a thermotolerant Acetobacter senegalensis strain as a solid functional starter culture, as well as the design of a new adapted bioreactor, has significantly contributed to food security and the creation of small- to medium-sized enterprises that produce mango vinegar in West Africa.

To Be or Not to Be Mesophilic, That Is the Question for Aeromonas salmonicida.

The bacterium Aeromonas salmonicida has long been known to be one of the most feared pathogens in fish farming. However, the more we discover about this bacterial species, the more we question whether it is really exclusively an aquatic pathogen. In recent years, it has become obvious that this bacterial species includes a myriad of strains with various lifestyle and ecological niches, including the well-known strict psychrophiles, the first bacteria known of the species, and the newly described mesophilic strains. The mesophiles are able to grow at low temperatures, but even better at temperatures of approximately 37 °C, which strict psychrophiles cannot do. In this perspective article, we address some aspects surrounding this dual lifestyle in A. salmonicida, including the impact of mobile genetic elements, and how future research around this bacterial species may focus on the psychrophilic/mesophilic dichotomy, which makes A. salmonicida an increasingly interesting and relevant model for the study of speciation.

Characterization of sulfide oxidation and optimization of sulfate production by a thermophilic Paenibacillus naphthalenovorans LYH-3 isolated from sewage sludge composting

The sulfur-containing odor emitted from sludge composting could be controlled by sulfide oxidizing bacteria, yet mesophilic strains show inactivation during the thermophilic stage of composting. Aimed to investigate and characterize the thermotolerant bacterium that could oxidize sulfide into sulfate, a heterotrophic strain was isolated from sewage sludge composting and identified as Paenibacillus naphthalenovorans LYH-3. The effects of various environmental factors on sulfide oxidation capacities were studied to optimize the sulfate production, and the highest production rate (27.35% ± 0.86%) was obtained at pH 7.34, the rotation speed of 161.14 r/min, and the inoculation amount of 5.83% by employing Box-Behnken design. The results of serial sulfide substrates experiments indicated that strain LYH-3 could survive up to 400 mg/L of sulfide with the highest sulfide removal rate (88.79% ± 0.35%) obtained at 50 mg/L of sulfide. Growth kinetic analysis presented the maximum specific growth rate µm (0.5274 hr−1) after 22 hr cultivation at 50°C. The highest enzyme activities of sulfide quinone oxidoreductase (0.369 ± 0.052 U/mg) and sulfur dioxygenase (0.255 ± 0.014 U/mg) were both obtained at 40°C, and the highest enzyme activity of sulfite acceptor oxidoreductase (1.302 ± 0.035 U/mg) was assessed at 50°C. The results indicated that P. naphthalenovorans possessed a rapid growth rate and efficient sulfide oxidation capacities under thermophilic conditions, promising a potential application in controlling sulfur-containing odors during the thermophilic stage of sludge composting.

Extremophile enzyme optimization for low temperature and high salinity are fundamentally incompatible.

The evolutionary mechanisms behind cold and high-saline co-adaptation of proteins are not thoroughly understood. To explore how enzymes evolve in response to multiple environmental pressures we developed a novel in silico method to model the directed evolution of proteins, the Protein Evolution Parameter Calculator (PEPC). PEPC carries out single amino acid substitutions that lead to improvements in the selected user-defined parameters. To investigate the evolutionary relationship between increased flexibility and decreased isoelectric point, which are presumed indicators of cold and saline adaptation in proteins, we applied PEPC to a subset of core haloarchaea orthologous group (cHOG) proteins from the mesophilic Halobacterium salinarum NRC-1 and cold-tolerant Halorubrum lacusprofundi strain ATCC 49239. The results suggest that mutations that increase flexibility will also generally increase isoelectric point. These findings suggest that enzyme adaptation to low temperature and high salinity might be evolutionarily counterposed based on the structural characteristics of probable amino acid mutations. This may help to explain the apparent lack of truly psychrophilic halophiles in nature, and why microbes adapted to polar hypersaline environments typically have mesophilic temperature optima. A better understanding of protein evolution to extremely cold and salty conditions will aid in our understanding of where and how life is distributed on Earth and in our solar system.

Complete genome of the thermophilic purple sulfur Bacterium Thermochromatium tepidum compared to Allochromatium vinosum and other Chromatiaceae.

The complete genome sequence of the thermophilic purple sulfur bacterium Thermochromatium tepidum strain MCT (DSM 3771T) is described and contrasted with that of its mesophilic relative Allochromatium vinosum strain D (DSM 180T) and other Chromatiaceae. The Tch. tepidum genome is a single circular chromosome of 2,958,290 base pairs with no plasmids and is substantially smaller than the genome of Alc. vinosum. The Tch. tepidum genome encodes two forms of RuBisCO and contains nifHDK and several other genes encoding a molybdenum nitrogenase but lacks a gene encoding a protein that assembles the Fe-S cluster required to form a functional nitrogenase molybdenum-iron cofactor, leaving the phototroph phenotypically Nif-. Tch. tepidum contains genes necessary for oxidizing sulfide to sulfate as photosynthetic electron donor but is genetically unequipped to either oxidize thiosulfate as an electron donor or carry out assimilative sulfate reduction, both of which are physiological hallmarks of Alc. vinosum. Also unlike Alc. vinosum, Tch. tepidum is obligately phototrophic and unable to grow chemotrophically in darkness by respiration. Several genes present in the Alc. vinosum genome that are absent from the genome of Tch. tepidum likely contribute to the major physiological differences observed between these related purple sulfur bacteria that inhabit distinct ecological niches.

The effect of pH on the growth rate of Bacillus cereus sensu lato: Quantifying strain variability and modelling the combined effects of temperature and pH

In this study, the effect of pH, alone or in combination with temperature, on the maximum growth rate (μmax) of B. cereus sensu lato was investigated. In phase 1, the effect of pH at 30 °C was studied for 16 mesophilic strains and 2 psychrotrophic strains of Bacillus cereus sensu lato. The μmax vs. pH relationship was found to show a similar pattern for all the strains. Several pH models from literature were evaluated and the best performing ‘growth rate vs. pH’ model selected. A stochastic model was then developed to predict the maximum specific growth rate of mesophilic B. cereus at 30 °C as a function of pH, the intra-species variability being incorporated via considering the model parameters (e.g. pHmin) randomly distributed. The predicted maximum specific growth rates were acceptably close to independent published data. In phase 2, the combined effects of temperature and pH were studied. Growth rates were also generated at 15, 20 and 40 °C for a selection of strains and the pH model was fitted at each temperature. Interestingly, the results showed that the estimates for the pHmin parameter for mesophilic strains were lower at 20–30 °C than near the optimum temperature (40 °C), suggesting that experiments for the determination of this parameter should be conducted at lower-than-optimum temperatures. New equations were proposed for the relationship between temperature and the minimum pH-values, which were also consistent with the experimental growth boundaries. The parameters defining this equation quantify the minimum temperature for growth observed experimentally, the temperature of maximum enzyme stability and the maximum temperature for growth. Deviations from the Gamma hypothesis (multiplicative effects of environmental factors on the maximum specific growth rate) were observed near the growth limits, especially at 40 °C. To improve model performance, two approaches, one based on a minimum pH-term (doi: https://doi.org/10.3389/fmicb.2019.01510) and one based on an interaction term (doi: http://dx.doi.org/10.1016/S0168-1605(01)00640-7) were evaluated.

Phenotypic plasticity and evolution of thermal tolerance in bacteria from temperate and hot spring environments.

Phenotypic plasticity allows individuals to respond to the selective forces of a new environment, followed by adaptive evolution. We do not know to what extent phenotypic plasticity allows thermal tolerance evolution in bacteria at the border of their physiological limits. We analyzed growth and reaction norms to temperature of strains of two bacterial lineages, Bacillus cereus sensu lato and Bacillus subtilis sensu lato, that evolved in two contrasting environments, a temperate lagoon (T) and a hot spring (H). Our results showed that despite the co-occurrence of members of both lineages in the two contrasting environments, norms of reactions to temperature exhibited a similar pattern only in strains within the lineages, suggesting fixed phenotypic plasticity. Additionally, strains from the H environment showed only two to three degrees centigrade more heat tolerance than strains from the T environment. Their viability decreased at temperatures above their optimal for growth, particularly for the B. cereus lineage. However, sporulation occurred at all temperatures, consistent with the known cell population heterogeneity that allows the Bacillus to anticipate adversity. We suggest that these mesophilic strains survive in the hot-spring as spores and complete their life cycle of germination and growth during intermittent opportunities of moderate temperatures. The limited evolutionary changes towards an increase in heat tolerance in bacteria should alert us of the negative impact of climate change on all biological cycles in the planet, which at its most basic level depends on microorganisms.

Morphological Features and Cold-Response Gene Expression in Mesophilic Bacillus cereus Group and Psychrotolerant Bacillus cereus Group under Low Temperature.

At low temperatures, psychrotolerant B. cereus group strains exhibit a higher growth rate than mesophilic strains do. However, the different survival responses of the psychrotolerant strain (BCG34) and the mesophilic strain (BCGT) at low temperatures are unclear. We investigated the morphological and genomic features of BCGT and BCG34 to characterize their growth strategies at low temperatures. At low temperatures, morphological changes were observed only in BCGT. These morphological changes included the elongation of rod-shaped cells, whereas the cell shape in BCG34 was unchanged at the low temperature. A transcriptomic analysis revealed that both species exhibited different growth-related traits during low-temperature growth. The BCGT strain induces fatty acid biosynthesis, sulfur assimilation, and methionine and cysteine biosynthesis as a survival mechanism in cold systems. Increases in energy metabolism and fatty acid biosynthesis in the mesophilic B. cereus group strain might explain its ability to grow at low temperatures. Several pathways involved in carbohydrate mechanisms were downregulated to conserve the energy required for growth. Peptidoglycan biosynthesis was upregulated, implying that a change of gene expression in both RNA-Seq and RT-qPCR contributed to sustaining its growth and rod shape at low temperatures. These results improve our understanding of the growth response of the B. cereus group, including psychrotolerant B. cereus group strains, at low temperatures and provide information for improving bacterial inhibition strategies in the food industry.

Microbe profile: Aeromonas salmonicida: an opportunistic pathogen with multiple personalities.

The bacterial species Aeromonas salmonicida is a fish pathogen. Feared by fish farmers everywhere on Earth over the past century, this species has turned out to be more diverse than initially suspected. While some psychrophilic subspecies cannot grow at temperatures above 25 °C or 30 °C, other mesophilic strains growing up to 37 °C and above are now characterized. Adding to the surprising diversity of this species, some of the mesophilic strains infect mammals and birds. The remarkable diversity is explained in part by the presence of numerous mobile genetic elements, which sculpt and modify the genome of the various strains of this species.