Bacterial Generation Times Research Articles

Symbiotic fermentation study of Acetobacter orientalis and lactic acid bacteria for lactobionic acid enriched yoghurt production

Symbiotic fermentation of lactic acid bacteria and Acetobacter orientalis intended to produce lactobionic acid-enriched yoghurt was studied. Commercial lactic acid bacteria (1%) and A. orientalis (10, 15, 20%) were inoculated in milk and incubated at 30°C for 16 h. Incubation parameters, including acidity, lactose conversion, and bacterial colony count, were analysed every 6th hour. Secondary fermentation parameters like acidification rate, bacterial growth rate, generation time and conversion rate of lactose to lactic acid and lactobionic acid were also estimated. The incubation time and A. orientalis level had significant (P < 0.05) effects on the incubation parameters of the lactobionic acid-enriched yoghurt during the incubation period. This study suggests 20% A. orientalis, 1% lactic bacteria inoculation and 30°C/16 h incubation for yoghurt with higher lactobionic acid content.

Existence of log-phase Escherichia coli persisters and lasting memory of a starvation pulse.

The vast majority of a bacterial population is killed when treated with a lethal concentration of antibiotics. The time scale of this killing is often comparable with the bacterial generation time before the addition of antibiotics. Yet, a small subpopulation typically survives for an extended period. However, the long-term killing dynamics of bacterial cells has not been fully quantified even in well-controlled laboratory conditions. We constructed a week-long killing assay and followed the survival fraction of Escherichia coli K12 exposed to a high concentration of ciprofloxacin. We found that long-term survivors were formed during exponential growth, with some cells surviving at least 7 d. The long-term dynamics contained at least three time scales, which greatly enhances predictions of the population survival time compared with the biphasic extrapolation from the short-term behavior. Furthermore, we observed a long memory effect of a brief starvation pulse, which was dependent on the (p)ppGpp synthase relA Specifically, 1 h of carbon starvation before antibiotics exposure increased the surviving fraction by nearly 100-fold even after 4 d of ciprofloxacin treatment.

Media Modification Lactobacillus casei Strain Shirota with Skim Milk Enriched Green Coconut Water and Corn Starch

Bacteria need a medium that can play an important role in their growth. One example of a medium is the medium De Mann Rogosa and Sharpe (MRS) which is a specific medium for the growth of lactic acid bacteria. The use of MRS media on an industrial scale is ineffective due to its relatively high price. The purpose of this study was to find out the effect of skim milk media enriched with green coconut water and cornstarch on the growth of Lactobacillus casei bacteria. The type of research conducted is experimental research with post test only control group design where there are two groups, namely the control group and the experimental group. The experimental group consisted of 3 types of samples, namely skim milk and green coconut water, skim milk and cornstarch, as well as skim milk and green coconut water and cornstarch. The comparison of treatment levels for each sample was 1:2 and 1:3 and analyzed, including bacterial growth curves, growth speed and bacterial generation time. From each concentration, the results found that the addition of green coconut water and cornstarch had a significant effect on the growth of Lactobacillus casei (P˂0.01) to the speed of growth and generation time. It was concluded that the higher the concentration added it can accelerate the growth of Lactobacillus casei.

Phage strategies facilitate bacterial coexistence under environmental variability.

Bacterial communities are often exposed to temporal variations in resource availability, which exceed bacterial generation times and thereby affect bacterial coexistence. Bacterial population dynamics are also shaped by bacteriophages, which are a main cause of bacterial mortality. Several strategies are proposed in the literature to describe infections by phages, such as “Killing the Winner”, “Piggyback the loser” (PtL) or “Piggyback the Winner” (PtW). The two temperate phage strategies PtL and PtW are defined by a change from lytic to lysogenic infection when the host density changes, from high to low or from low to high, respectively. To date, the occurrence of different phage strategies and their response to environmental variability is poorly understood. In our study, we developed a microbial trophic network model using ordinary differential equations (ODEs) and performed ‘in silico’ experiments. To model the switch from the lysogenic to the lytic cycle, we modified the lysis rate of infected bacteria and their growth was turned on or off using a density-dependent switching point. We addressed whether and how the different phage strategies facilitate bacteria coexistence competing for limiting resources. We also studied the impact of a fluctuating resource inflow to evaluate the response of the different phage strategies to environmental variability. Our results show that the viral shunt (i.e. nutrient release after bacterial lysis) leads to an enrichment of the system. This enrichment enables bacterial coexistence at lower resource concentrations. We were able to show that an established, purely lytic model leads to stable bacterial coexistence despite fluctuating resources. Both temperate phage models differ in their coexistence patterns. The model of PtW yields stable bacterial coexistence at a limited range of resource supply and is most sensitive to resource fluctuations. Interestingly, the purely lytic phage strategy and PtW both result in stable bacteria coexistence at oligotrophic conditions. The PtL model facilitates stable bacterial coexistence over a large range of stable and fluctuating resource inflow. An increase in bacterial growth rate results in a higher resilience to resource variability for the PtL and the lytic infection model. We propose that both temperate phage strategies represent different mechanisms of phages coping with environmental variability. Our study demonstrates how phage strategies can maintain bacterial coexistence in constant and fluctuating environments.

Replication Dynamics for Six Gram-Negative Bacterial Species during Bloodstream Infection.

ABSTRACTBloodstream infections (BSI) are a major public health burden due to high mortality rates and the cost of treatment. The impact of BSI is further compounded by a rise in antibiotic resistance among Gram-negative species associated with these infections. Escherichia coli, Serratia marcescens, Klebsiella pneumoniae, Enterobacter hormaechei, Citrobacter freundii, and Acinetobacter baumannii are all common causes of BSI, which can be recapitulated in a murine model. The objective of this study was to characterize infection kinetics and bacterial replication rates during bacteremia for these six pathogens to gain a better understanding of bacterial physiology during infection. Temporal observations of bacterial burdens of the tested species demonstrated varied abilities to establish colonization in the spleen, liver, or kidney. K. pneumoniae and S. marcescens expanded rapidly in the liver and kidney, respectively. Other organisms, such as C. freundii and E. hormaechei, were steadily cleared from all three target organs throughout the infection. In situ replication rates measured by whole-genome sequencing of bacterial DNA recovered from murine spleens demonstrated that each species was capable of sustained replication at 24 h postinfection, and several species demonstrated <60-min generation times. The relatively short generation times observed in the spleen were in contrast to an overall decrease in bacterial burden for some species, suggesting that the rate of immune-mediated clearance exceeded replication. Furthermore, bacterial generation times measured in the murine spleen approximated those measured during growth in human serum cultures. Together, these findings provide insight into the infection kinetics of six medically important species during bacteremia.

Genome-Wide Analyses of the Temperature-Responsive Genetic Loci of the Pectinolytic Plant Pathogenic Pectobacterium atrosepticum.

Temperature is one of the critical factors affecting gene expression in bacteria. Despite the general interest in the link between bacterial phenotypes and environmental temperature, little is known about temperature-dependent gene expression in plant pathogenic Pectobacterium atrosepticum, a causative agent of potato blackleg and tuber soft rot worldwide. In this study, twenty-nine P. atrosepticum SCRI1043 thermoregulated genes were identified using Tn5-based transposon mutagenesis coupled with an inducible promotorless gusA gene as a reporter. From the pool of 29 genes, 14 were up-regulated at 18 °C, whereas 15 other genes were up-regulated at 28 °C. Among the thermoregulated loci, genes involved in primary bacterial metabolism, membrane-related proteins, fitness-corresponding factors, and several hypothetical proteins were found. The Tn5 mutants were tested for their pathogenicity in planta and for features that are likely to remain important for the pathogen to succeed in the (plant) environment. Five Tn5 mutants expressed visible phenotypes differentiating these mutants from the phenotype of the SCRI1043 wild-type strain. The gene disruptions in the Tn5 transposon mutants caused alterations in bacterial generation time, ability to form a biofilm, production of lipopolysaccharides, and virulence on potato tuber slices. The consequences of environmental temperature on the ability of P. atrosepticum to cause disease symptoms in potato are discussed.

Antibacterial activity of caffeine against plant pathogenic bacteria.

The objective of the present study was to evaluate the antibacterial properties of a plant secondary metabolite - caffeine. Caffeine is present in over 100 plant species. Antibacterial activity of caffeine was examined against the following plant-pathogenic bacteria: Ralstonia solanacearum (Rsol), Clavibacter michiganesis subsp. sepedonicus (Cms), Dickeya solani (Dsol), Pectobacterium atrosepticum (Pba), Pectobacterium carotovorum subsp. carotovorum (Pcc), Pseudomonas syringae pv. tomato (Pst), and Xanthomonas campestris subsp. campestris (Xcc). MIC and MBC values ranged from 5 to 20 mM and from 43 to 100 mM, respectively. Caffeine increased the bacterial generation time of all tested species and caused changes in cell morphology. The influence of caffeine on the synthesis of DNA, RNA and proteins was investigated in cultures of plant pathogenic bacteria with labelled precursors: [(3)H]thymidine, [(3)H]uridine or (14)C leucine, respectively. RNA biosynthesis was more affected than DNA or protein biosynthesis in bacterial cells treated with caffeine. Treatment of Pba with caffeine for 336 h did not induce resistance to this compound. Caffeine application reduced disease symptoms caused by Dsol on chicory leaves, potato slices, and whole potato tubers. The data presented indicate caffeine as a potential tool for the control of diseases caused by plant-pathogenic bacteria, especially under storage conditions.

Combinatorial active-site variants confer sustained clavulanate resistance in BlaC β-lactamase from Mycobacterium tuberculosis.

Bacterial resistance to β-lactam antibiotics is a global issue threatening the success of infectious disease treatments worldwide. Mycobacterium tuberculosis has been particularly resilient to β-lactam treatment, primarily due to the chromosomally encoded BlaC β-lactamase, a broad-spectrum hydrolase that renders ineffective the vast majority of relevant β-lactam compounds currently in use. Recent laboratory and clinical studies have nevertheless shown that specific β-lactam-BlaC inhibitor combinations can be used to inhibit the growth of extensively drug-resistant strains of M. tuberculosis, effectively offering new tools for combined treatment regimens against resistant strains. In the present work, we performed combinatorial active-site replacements in BlaC to demonstrate that specific inhibitor-resistant (IRT) substitutions at positions 69, 130, 220, and/or 234 can act synergistically to yield active-site variants with several thousand fold greater in vitro resistance to clavulanate, the most common clinical β-lactamase inhibitor. While most single and double variants remain sensitive to clavulanate, double mutants R220S-K234R and S130G-K234R are substantially less affected by time-dependent clavulanate inactivation, showing residual β-lactam hydrolytic activities of 46% and 83% after 24 h incubation with a clinically relevant inhibitor concentration (5 μg/ml, 25 µM). These results demonstrate that active-site alterations in BlaC yield resistant variants that remain active and stable over prolonged bacterial generation times compatible with mycobacterial proliferation. These results also emphasize the formidable adaptive potential of inhibitor-resistant substitutions in β-lactamases, potentially casting a shadow on specific β-lactam-BlaC inhibitor combination treatments against M. tuberculosis.

Phylogenetic and structural response of heterotrophic bacteria to dissolved organic matter of different chemical composition in a continuous culture study

Dissolved organic matter (DOM) and heterotrophic bacteria are highly diverse components of the ocean system, and their interactions are key in regulating the biogeochemical cycles of major elements. How chemical and phylogenetic diversity are linked remains largely unexplored to date. To investigate interactions between bacterial diversity and DOM, we followed the response of natural bacterial communities to two sources of phytoplankton-derived DOM over six bacterial generation times in continuous cultures. Analyses of total hydrolysable neutral sugars and amino acids, and ultrahigh resolution mass spectrometry revealed large differences in the chemical composition of the two DOM sources. According to 454 pyrosequences of 16S ribosomal ribonucleic acid genes, diatom-derived DOM sustained higher levels of bacterial richness, evenness and phylogenetic diversity than cyanobacteria-derived DOM. These distinct community structures were, however, not associated with specific taxa. Grazing pressure affected bacterial community composition without changing the overall pattern of bacterial diversity levels set by DOM. Our results demonstrate that resource composition can shape several facets of bacterial diversity without influencing the phylogenetic composition of bacterial communities, suggesting functional redundancy at different taxonomic levels for the degradation of phytoplankton-derived DOM.

Assessing the Probability of Detection of Horizontal Gene Transfer Events in Bacterial Populations

Experimental approaches to identify horizontal gene transfer (HGT) events of non-mobile DNA in bacteria have typically relied on detection of the initial transformants or their immediate offspring. However, rare HGT events occurring in large and structured populations are unlikely to be detected in a short time frame. Population genetic modeling of the growth dynamics of bacterial genotypes is therefore necessary to account for natural selection and genetic drift during the time lag and to predict realistic time frames for detection with a given sampling design. Here we draw on statistical approaches to population genetic theory to construct a cohesive probabilistic framework for investigation of HGT of exogenous DNA into bacteria. In particular, the stochastic timing of rare HGT events is accounted for. Integrating over all possible event timings, we provide an equation for the probability of detection, given that HGT actually occurred. Furthermore, we identify the key variables determining the probability of detecting HGT events in four different case scenarios that are representative of bacterial populations in various environments. Our theoretical analysis provides insight into the temporal aspects of dissemination of genetic material, such as antibiotic resistance genes or transgenes present in genetically modified organisms. Due to the long time scales involved and the exponential growth of bacteria with differing fitness, quantitative analyses incorporating bacterial generation time, and levels of selection, such as the one presented here, will be a necessary component of any future experimental design and analysis of HGT as it occurs in natural settings.

Mixed response in bacterial and biochemical variables to simulated sand mining in placer-rich beach sediments, Ratnagiri, West coast of India

We investigated the influence on bacterial community and biochemical variables through mechanical disturbance of sediment-akin to small-scale mining in Kalbadevi beach, Ratnagiri, a placer-rich beach ecosystem which is a potential mining site. Changes were investigated by comparing three periods, namely phase I before disturbance, phase II just after disturbance, and phase III 24 h after disturbance as the bacterial generation time is ≤7 h. Cores from dune, berm, high-, mid-, and low-tide were examined for changes in distribution of total bacterial abundance, total direct viability (counts under aerobic and anaerobic conditions), culturability and biochemical parameters up to 40 cm depth. Results showed that bacterial abundance decreased by an order from 10(6) cells g(-1) sediment, while, viability reduced marginally. Culturability on different-strength nutrient broth increased by 155% during phase II. Changes in sedimentary proteins, carbohydrates, and lipids were marked at berm and dune and masked at other levels by tidal influence. Sedimentary ATP reduced drastically. During phase III, Pearson's correlation between these variables evolved from non-significant to significant level. Thus, simulated disturbance had a mixed effect on bacterial and biochemical variables of the sediments. It had a negative impact on bacterial abundance, viability and ATP but positive impact on culturability. Viability, culturability, and ATP could act as important indicators reflecting the disturbance in the system at short time intervals. Culturability, which improved by an order, could perhaps be a fraction that contributes to restoration of the system at bacterial level. This baseline information about the potential mining site could help in developing rational approach towards sustainable harnessing of resources with minimum damage to the ecosystem.

Short-term and seasonal dynamics of bacterial biomass production and amino acid turnover in the water column of an intertidal ecosystem, the Wadden Sea

Intertidal flat systems exhibit systematic temporal patterns of suspended particulate matter (SPM) and associated properties (particulate organic carbon, chlorophyll) with enhanced con- centrations towards the current velocity maximum (CVM) and reduced concentrations towards slack water. It is unclear how these tidal patterns are reflected in organic matter processing by particle- associated (PA) and free-living (FL) heterotrophic bacteria. Therefore, we studied the abundance of PA and FL bacteria, bacterial biomass production (BP) and turnover of dissolved free amino acids (DFAA) during tidal cycles in a 13 to 16 m deep back-barrier intertidal flat system of the East Frisian Wadden Sea, Germany, in January, April, July and November 2005. Pronounced tidal patterns of microbial parameters were found with distinct differences around the CVM and at slack water. Tidal patterns varied seasonally with higher BP rates and bacterial generation times of 1 to 2 d in April and July. There was a trend of enhanced rates of BP from surface to bottom in April and July with an increasing significance of PA bacteria towards the bottom. Very high depth-integrated daily rates of BP and oxygen undersaturation suggest that this system is net-heterotrophic. A normalization proce- dure revealed systematic tidal patterns of most parameters irrespective of the seasonal situation. Our findings thus demonstrate the presence of tidal patterns of parameters reflecting microbial processes and thus complement previous findings of tidal patterns of SPM-associated properties. The tidally varying microbial activities indicate that the rhythmic tidal currents promote organic matter decom- position by heterotrophic bacteria in intertidal flat systems.

Study of Antibacterial Activity of ZnZnbisporphyrin Complexes and its Free Components on Staphylococcus aureus by Microcalorimetry

The antibacterial effect of Zn(II), tetraphenyl porphyrin (TPP), propdioxyl bridged tetraphenyl bisporphyrin 1, and its metallobisporphyrin complexes (ZnMnbisporphyrin 2 and ZnZnbisporphyrin 3) towards Staphylococcus aureus growth was investigated by microcalorimetry at 37 degrees C. Differences in their capacities to inhibit the growth metabolism of S. aureus were observed. By analyzing the power-time curves, crucial parameters such as the rate constant of bacterial growth (k), inhibitory ratio (I), and generation time (t (G)) were determined. The growth rate constant (k) of S. aureus (in the log phase) in the presence of the drugs decreased linearly with increasing concentrations of the complexes. The sequence of the antibacterial activities of these compounds tested was 3 > 2 > 1 > Zn(II) > TPP. ZnZnbisporphyrin 3 is proposed to benefit from the synergetic effects of Zn(II) and 1.

Antibacterial Activities of Novel Diselenide-bridged bis(Porphyrin)s on Staphylococcus aureus Investigated by Microcalorimetry

The actions of two novel diselenide-bridged bis(porphyrin)s (1 and 2) on Staphylococcus aureus growth was investigated by microcalorimetry at 37.00 degrees C, compared with that of Na2SeO3. Differences in their capacities to inhibit the growth metabolism of S. aureus were observed. By analyzing the power-time curves, crucial parameters such as the rate constant of bacterial growth (k), inhibitory rate (I), and generation time (tG) were determined. The growth rate constant (k) of S. aureus (in the log phase) in the presence of the drugs decreased with increasing concentrations of the drugs regularly. The relationship of k and c is nearly linear for diselenide-bridged bis(porphyrin) 2. The sequence of the antibacterial activities of these selenium compounds tested was 2 > 1 > Na2SeO3.

The Structure and Binding Behavior of the Bacterial Cell Surface Layer Protein SbsC

Surface layers (S-layers) comprise the outermost cell envelope component of most archaea and many bacteria. Here we present the structure of the bacterial S-layer protein SbsC from Geobacillus stearothermophilus, showing a very elongated and flexible molecule, with strong and specific binding to the secondary cell wall polymer (SCWP). The crystal structure of rSbsC((31-844)) revealed a novel fold, consisting of six separate domains, which are connected by short flexible linkers. The N-terminal domain exhibits positively charged residues regularly spaced along the putative ligand binding site matching the distance of the negative charges on the extended SCWP. Upon SCWP binding, a considerable stabilization of the N-terminal domain occurs. These findings provide insight into the processes of S-layer attachment to the underlying cell wall and self-assembly, and also accommodate the observed mechanical strength, the polarity of the S-layer, and the pronounced requirement for surface flexibility inherent to cell growth and division.

Spatial variability of phytoplankton, bacteria and viruses in the mesotidal salt wedge Douro Estuary (Portugal)

In order to assess the variability of phytoplankton, bacteria and viruses, a box model was applied to the mesotidal salt wedge Douro Estuary (NW Portugal). The estuary was divided into boxes with two vertical layers and was driven by salinity and freshwater discharge data acquired during cruises in the spring, summer and fall of 1995. Model runs yielded estimates of the vertical non-advective exchange coefficients for each box, which were then used to establish balance equations for the microbial variables. Such equations allowed calculation of horizontal and vertical fluxes of phytoplankton, bacteria and viruses as well as their respective rates of production or consumption. Bacterial numbers peaked at intermediate salinities (12–18) whereas freshwater phytoplankton biomass decreased linearly with salinity, and estuarine communities increased at intermediate salinities. Virus-like particles had a similar behavior. The lower layer in the Douro Estuary was always a sink for phytoplankton, bacteria and viruses, giving way to negative or close to zero microbial fluxes. In the upper layer of the lower estuary, close to the coastal end where nutrients are abundant, water transparency was high, bacterial generation times were shorter and microbial fluxes were always positive, i.e. net growth occurred. The freshwater end of the estuary was a source of phytoplankton, irrespective of season, whereas it was a source of bacteria and viruses only in summer and spring, respectively. It is suggested that salinity acts as a boundary between different microbial communities, especially on a vertical scale.

Monitoring and modeling horizontal gene transfer.

Monitoring efforts have failed to identify horizontal gene transfer (HGT) events occurring from transgenic plants into bacterial communities in soil or intestinal environments. The lack of such observations is frequently cited in biosafety literature and by regulatory risk assessment. Our analysis of the sensitivity of current monitoring efforts shows that studies to date have examined potential HGT events occurring in less than 2 g of sample material, when combined. Moreover, a population genetic model predicts that rare bacterial transformants acquiring transgenes require years of growth to out-compete wild-type bacteria. Time of sampling is there-fore crucial to the useful implementation of monitoring. A population genetic approach is advocated for elucidating the necessary sample sizes and times of sampling for monitoring HGT into large bacterial populations. Major changes in current monitoring approaches are needed, including explicit consideration of the population size of exposed bacteria, the bacterial generation time, the strength of selection acting on the transgene-carrying bacteria, and the sample size necessary to verify or falsify the HGT hypotheses tested.

Effect of dechlorination point location and residual chlorine on biofouling in a seawater reverse osmosis plant

Experiments were carried out to test the effect of dechlorination dosing point location and the concentration of residual chlorine on bacterial growth and biofouling in a seawater reverse osmosis (SWRO) plant. The plant is located in Al-Birk on the Red Sea coast, southern Saudi Arabia. The plant was routinely operated with injection of chlorine as a biocide in the intake chamber in the open sea and with the injection of sodium metabisulfite (SBS) to remove chlorine after the dual media filter (DMF). During the experiments, the SBS dosing point was shifted to two locations after the micron cartridge filter (MCF): first, to a point 13 m ahead of the high pressure pump (HPP) and second, to a point just ahead (<1 m) of the HPP. Due to fluctuations in residual chlorine resulting from certain operational and physical circumstances, it was possible to assess biofouling potential when free chlorine is regarded as high (≥0.5 mg/l) or low (<0.5 mg/l) within a maximum of 1 mg/l. Bacterial generation (doubling) time was used to evaluate biofouling. Generation time was higher (lower multiplication capacity) when the SBS dosing point was before the MCF (after DMF). It decreased significantly, reflecting higher multiplication capacity and higher biofouling potential, when the SBS dosing point was moved to after the MCF. Generation times in high-pressure RO feed water were similar when the SBS dosing point was moved to two locations after the MCF. This indicated minimal contribution of the low-pressure pipe between the MCF and the HPP to biofouling. In general, biofouling increased as the SBS dosing point was moved forward along the pretreatment line, closer to the RO membranes. Generation times were similar when residual chlorine was less or more than 0.5 mg/l. Bacteria were capable of biofilm formation in the chlorinated section of the plant.

Viral and bacterial production in the North Water: in situ measurements, batch-culture experiments and characterization and distribution of a virus–host system

Abstract Growth and viral lysis of bacterioplankton at subzero temperatures were measured in the North Water polynya in July 1998. In situ measurements of bacterial carbon consumption in surface waters ranged from 15 to 63 μg C l−1 d−1 in the eastern and 6 to 7 μg C l−1 d−1 in the northern part of the polynya. Both bacterial abundance and activity appeared to increase in response to the decay of the phytoplankton bloom that developed in the North Water. Organic carbon was the limiting substrate for bacteria in the polynya since addition of glucose, but not inorganic nutrients, to batch cultures increased both the carrying capacity of the substrate and the growth rate of the bacteria. Bacterial growth rates ranged from 0.11 to 0.40 d−1, corresponding to bacterial generation times of 1.7–6.3 d. The in situ viral production rate was estimated both from the frequency of visibly infected cells and from the rate of viral production in batch cultures; it ranged from 0.04 to 0.52 d−1 and from 0.25 to 0.47 d−1, respectively. From 6% to 28% of bacterial production was found to be lost due to viral lysis. The average virus–bacteria ratio was 5.1±3.1, with the abundance of viruses being correlated positively with bacterial production. A Pseudoalteromonas sp. bacterial host and an infective virus were isolated from the polynya; characteristics and distribution of the virus–host system were examined. The Pseudoalteromonas sp. showed psychrotolerant growth and sustained significant production of viruses at 0°C. The virus–host system was found throughout the polynya. Overall the results suggested that a large amount of organic carbon released during the development and breakdown of the spring phytoplankton bloom was consumed by planktonic bacteria and that the microbial food web was an important and dynamic component of the planktonic food web in the North Water.

Preliminary characterization of fast growing rhizobial strains isolated from soyabean nodules in Brazil

A survey of soyabean rhizobia was carried out with six Asian and one modern soyabean genotypes as trap hosts. Soyabean seedlings were inoculated with soils from 22 Brazilian field sites, including undisturbed areas or areas traditionally cropped with this legume. A total of 30 fast growing strains, able to establish an effective symbiosis with both types of genotypes, were isolated from 12 of the 22 soils, representing 17% of the population in cropped areas and 24% in undisturbed soils. The bacterial mean generation time varied from 85 to 225 min and after 4 days of growth in YM medium the final pH ranged from 3.7 to 6.9. Although isolated from acid soils, only 37% of the strains were able to grow in TY or YM media at pH 4.0, while 60% were alkaline tolerant (pH 9.5). Most strains produced abundant extracellular polysaccharides (73%), were tolerant to 0.5 M NaCl (60%) and a temperature of 40°C (77%), grew in LB medium (67%) and synthesized melanin (53%). The strains differed in the use of C compounds supplied as sole C sources. The majority of the strains showed an intrinsic resistance to the antibiotics (μg ml −1) chloramphenicol (10), erythromycin (50), gentamicin (20), kanamicin (30), rifampicin (20) and tetracycline (10) and to the heavy metals cobalt chloride (0.5 mM) and potassium chromate (0.25 mM). A cluster analysis with 81 morphological and physiological parameters placed the strains from undisturbed soils in the central part of the dendrogram, even when isolated from distant areas, indicating that they had a common background. The relatedness decreased with the use of the soils for agriculture. Most of the strains isolated from cropped soils under conventional tillage were characterised by a high tolerance to stressful conditions and the ability to grow with several C sources, contrary to most of the strains isolated from soils under no-tillage management system. The strains differed from S. fredii in several characteristics and the protein and lipopolysaccharide profiles showed that each strain was unique. Therefore, although soyabean is an exotic plant in Brazil, several indigenous rhizobial strains may also establish an effective symbiosis with this legume.