Linking Host Prokaryotic Physiology to Viral Lifestyle Dynamics in a Temperate Freshwater Lake (Lake Pavin,France)

In aquatic ecosystems, fluctuations in environmental conditions and prokaryotic host physiological states can strongly affect the dynamics of viral life strategies. The influence of prokaryote physiology and environmental factors on viral replication cycles (lytic and lysogeny) was investigated from April to September 2011 at three different strata (epi, meta, and hypolimnion) in the mixolimnion of deep volcanic temperate freshwater Lake Pavin (France). Overall, the euphotic region (epi and metalimnion) was more dynamic and showed significant variation in microbial standing stocks, prokaryotic physiological state, and viral life strategies compared to the aphotic hypolimnion which was stable within sampled months. The prokaryotic host physiology as inferred from the nucleic acid content of prokaryotic cells (high or low nucleic acid) was strongly regulated by the chlorophyll concentration. The predominance of the high nucleic acid (HNA) prokaryotes (cells) over low nucleic acid (LNA) prokaryotes (cells) in the spring (HNA/LNA?=?1.2) and vice versa in the summer period (HNA/LNA?=?0.4) suggest that the natural prokaryotic communities underwent major shifts in their physiological states during investigated time period. The increase in the percentage of inducible lysogenic prokaryotes in the summer period was associated with the switch in the dominance of LNA over HNA cells, which coincided with the periods of strong resource (nutrient) limitation. This supports the idea that lysogeny represents a maintenance strategy for viruses in unproductive or harsh nutrient/host conditions. A negative correlation of percentage of lysogenic prokaryotes with HNA cell abundance and chlorophyll suggest that lysogenic cycle is closely related to prokaryotic cells which are stressed or starved due to unavailability of resources for its growth and activity. Our results provide support to previous findings that changes in prokaryote physiology are critical for the promotion and establishment of lysogeny in aquatic ecosystems, which are prone to constant environmental fluctuations.     

The diversity of bacteria,eukaryotic cells and viruses in an oligotrophic lake

An in situ transmission electron microscopic study of biomass samples concentrated from oligotrophic lake water revealed a variety of virus-infected microbial cells and many free viruses and virus-like particles. The most abundant group of microorganisms in screened and filtered water-column samples were 2 μm or less in diameter, and included representatives of several oligotrophic genera, Prosthecomicrobium, Ancyclobacter, Caulobacter and Hyphomicrobium. Among the prokaryotic host cells, which included both heterotrophs and autotrophs, on the basis of electron microscope observations, approximately 17% were infected with bacteriophage or bore adherent phage particles on their surfaces. Several bacterial morphotypes were observed among the prokaryotic hosts. Water samples passed through a 20-μm Nitex screen allowed us to concentrate and examine the larger host cells as well, including several species of single-celled algae and two amoeba species. The infected algal cells included those Chlorella-like in appearance, photosynthetic flagellates and others that could not be positively identified. About one-third of the eukaryotic cells were infected by viruses that were larger (150–200 nm) and structurally more complex than bacteriophages (50–60 nm). None of the viruses have been isolated, but when 0.2 μm filtrate from a biomass sample was spotted onto lawns of four representative heterotrophs and a Chlorella, the clearing observed was taken as evidence of lysis. Cyanobacterial lawns showed no plaques. Thin sections of two amoeba showed food vacuoles containing what appeared to be virus particles of a type seen in certain prokaryotic and eukaryotic cells in the biomass. Received: 26 January 1996 / Received revision: 10 July 1996 / Accepted: 5 August 1996     

Does the High Nucleic Acid Content of Individual Bacterial Cells Allow Us To Discriminate between Active Cells and Inactive Cells in Aquatic Systems?

The nucleic acid contents of individual bacterial cells as determined with three different nucleic acid-specific fluorescent dyes (SYBR I, SYBR II, and SYTO 13) and flow cytometry were compared for different seawater samples. Similar fluorescence patterns were observed, and bacteria with high apparent nucleic acid contents (HNA) could be discriminated from bacteria with low nucleic acid contents (LNA). The best discrimination between HNA and LNA cells was found when cells were stained with SYBR II. Bacteria in different water samples collected from seven freshwater, brackish water, and seawater ecosystems were prelabeled with tritiated leucine and then stained with SYBR II. After labeling and staining, HNA, LNA, and total cells were sorted by flow cytometry, and the specific activity of each cellular category was determined from leucine incorporation rates. The HNA cells were responsible for most of the total bacterial production, and the specific activities of cells in the HNA population varied between samples by a factor of seven. We suggest that nucleic acid content alone can be a better indicator of the fraction of growing cells than total counts and that this approach should be combined with other fluorescent physiological probes to improve detection of the most active cells in aquatic systems.     

Chemical mapping of DNA and counter-ion content inside phage by energy-filtered TEM

Double-stranded DNA in many bacterial viruses (phage) is strongly confined, which results in internal genome pressures of tens of atmospheres. This pressure is strongly dependent on local ion concentration and distribution within the viral capsid. Here, we have used electron energy loss spectroscopy (EELS), energy-filtered TEM (EFTEM) and X-ray energy dispersive spectroscopy to provide such chemical information from the capsid and the phage tail through which DNA is injected into the cell. To achieve this, we have developed a method to prepare thin monolayers of self-supporting virus/buffer films, suitable for EELS and EFTEM analysis. The method is based on entrapment of virus particles at air–liquid interfaces; thus, the commonly used method of staining by heavy metal salts can be avoided, eliminating the risk for chemical artifacts. We found that Mg^2 + concentration was approximately 2–4 times higher in the DNA-filled capsid than in the surrounding TM buffer (containing 10 mM Mg^2 +). Furthermore, we also analyzed the DNA content inside the phage tail by mapping phosphorus and magnesium.     

Does the high nucleic acid content of individual bacterial cells allow us to discriminate between active cells and inactive cells in aquatic systems?

The nucleic acid contents of individual bacterial cells as determined with three different nucleic acid-specific fluorescent dyes (SYBR I, SYBR II, and SYTO 13) and flow cytometry were compared for different seawater samples. Similar fluorescence patterns were observed, and bacteria with high apparent nucleic acid contents (HNA) could be discriminated from bacteria with low nucleic acid contents (LNA). The best discrimination between HNA and LNA cells was found when cells were stained with SYBR II. Bacteria in different water samples collected from seven freshwater, brackish water, and seawater ecosystems were prelabeled with tritiated leucine and then stained with SYBR II. After labeling and staining, HNA, LNA, and total cells were sorted by flow cytometry, and the specific activity of each cellular category was determined from leucine incorporation rates. The HNA cells were responsible for most of the total bacterial production, and the specific activities of cells in the HNA population varied between samples by a factor of seven. We suggest that nucleic acid content alone can be a better indicator of the fraction of growing cells than total counts and that this approach should be combined with other fluorescent physiological probes to improve detection of the most active cells in aquatic systems.     

The nucleocapsid of bacteriophage phi 6 penetrates the host cytoplasmic membrane.

Bacteriophage phi 6 infects its host, the Gram-negative bacterium Pseudomonas syringae, by a protein-targeted fusion of the virus envelope with the host outer membrane. In this investigation we present results suggesting that the phage nucleocapsid penetrates the host cytoplasmic membrane via a membrane invagination and an intracellular vesicle. This indicates that the prokaryotic plasma membrane might be more dynamic and have more common features with eukaryotic membrane systems than previously expected. Most of the nucleocapsid surface lattice protein is degraded in the cell, and the nucleocapsid core particle containing the viral dsRNA segments and the proteins necessary for the viral RNA polymerase activity can be isolated from the infected cells. The penetration is dependent on the energized state of the host cytoplasmic membrane. About 25% of the entering core particles are re-used in the progeny viruses.     

Antarctic marine bacterioplankton subpopulations discriminated by their apparent content of nucleic acids differ in their response to ecological factors

Bacterial abundances determined in Drake Passage and Bransfield and Gerlache Straits (Antarctica) in the Austral summer ranged from 0.78 to 9.4×10⁵ cells ml⁻¹, and were positively correlated with standing stocks of Chl a. Two bacterial subpopulations were discriminated based in their different levels of green fluorescence and wide angle light scatter (SSC) per cell after SYTO-13 staining for the first time in Antarctic waters. High nucleic acid (HNA) and low nucleic acid (LNA) subpopulations differed considerably in their response to changes in environmental variables. The apparent content of nucleic acids per cell for the HNA subpopulation (FL1-HNA) showed vertical profiles similar to those of Chl a, including the presence of a maximum at the subsurface chlorophyll maximum. FL1-HNA was positively correlated with Chl a. No similar trends were observed for the LNA fraction. HNA and LNA subpopulations differed in the response of the wide angle light scatter signal to environmental factors as well. SSC-HNA decreased strongly with depth and was positively correlated with Chl a. Again, no similar trends were observed for the LNA subpopulation. The percentage of HNA cells (%HNA) ranged between 35.0 and 76.7% and showed a general tendency to increase with depth. This increase seemed to be larger when the stratification of the water column was higher. Differences in grazing pressure could be responsible of the unexpected vertical distribution of HNA cells. Our results shows that in situ LNA and HNA bacterioplankton subpopulations are under different ecological controls and likely to play different trophodynamic roles in Antarctic waters.     

Resources drive trade‐off between viral lifestyles in the plankton: evidence from freshwater microbial microcosms

Viruses have evolved different life strategies for coping with environmental challenges and this is a key explanation for their omnipresence in aquatic systems. However, factors that determine the balance between lytic versus lysogenic decision within natural virioplankton are poorly documented, primarily in freshwaters. This study was designed to investigate the experimental short‐term (24 h incubation) effects of added organic and inorganic nutrients on the two viral lifestyles in nutrient‐depleted freshwater microbial (i.e. < 0.8 μm fraction) microcosms, using mitomycin C as prophage inductor agent. In the absence of mitomycin, viral lytic production increased as a functional response to the strong stimulation of bacterial growth rates (0.7–0.8 day^?1) by the added nutrients, primarily the organic nutrients which appeared scarcer than inorganic nutrients and was related to the sampling period and the geomorphological peculiarities of Lake Pavin. In the presence of mitomycin, temperate phage production (frequency of lysogenically infected bacterial cells, FLC = 17–19% of total cells) significantly exceeded lytic production (frequency of lytically infected bacterial cells, FIC = 9–11%) in natural samples (i.e. without nutrient additions) as a result of enhanced prophage induction, which relatively increased with the decreasing contact probability between viruses and their potential hosts. In contrast, addition of nutrients drastically reduced FLC (< 4%) and increased FIC (> 22%). Both variables were antagonistically correlated as was the correlation between FLC and bacterial growth rates, supporting the idea that lysogeny may represent a maintenance strategy for viruses in harsh nutrient/host conditions which appeared as major instigators of the trade‐off between the two viral lifestyles. Overall, at the community level, we reject the hypothesis that nutrients but mitomicyn C stimulate temperate phage induction, and retained the hypotheses that nutrients rather (i) stimulate lytic viruses via enhanced host growth and (ii) when limiting, promote lysogenic conversion in natural waters.     

Potential Effect of Freshwater Virus on the Structure and Activity of Bacterial Communities in the Marennes-Oléron Bay (France)

Batch culture experiments using viral enrichment were conducted to test the response of a coastal bacterial community to autochthonous (i.e., co-existing) or allochthonous riverine viruses. The effects of viral infections on bacterial dynamics and activity were assessed by epifluorescence microscopy and thymidine incorporation, respectively, whereas the effect of viral infection on bacterial community composition was examined by polymerase chain reaction-single strand conformation polymorphism 16S ribosomal RNA fingerprinting. The percentages of high nucleic acid-containing cells, evaluated by flow cytometry, were significantly correlated (r ² = 0.91, n = 12, p < 0.0001) to bacterial production, making this value a good predictor of active cell dynamics along the study. While confinement and temperature were the two principal experimental factors affecting bacterial community composition and dynamics, respectively, additions of freshwater viruses had significant effects on coastal bacterial communities. Thus, foreign viruses significantly reduced net bacterial population increase as compared to the enrichment treated with inactivated virus. Moreover, freshwater viruses recurrently and specifically affected bacterial community composition, as compared to addition of autochthonous viruses. In most cases, the combined treatment viruses and freshwater dissolved organic matter helped to maintain or even enhance species richness in coastal bacterial communities in agreement to the ‘killing the winner’ hypothesis. Thus, riverine virus input could potentially influence bacterial community composition of the coastal bay albeit with modest modification of bulk bacterial growth. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Phylogenetic characterisation of picoplanktonic populations with high and low nucleic acid content in the North Atlantic Ocean

In flow cytometric analyses of marine prokaryotic picoplankton often two populations with distinct differences in their apparent nucleic acid content are discernable, one with a high and one with a low nucleic acid content (HNA and LNA, respectively). In this study we determined the phylogenetic composition of flow cytometrically sorted HNA and LNA populations, collected at six stations along a transect across three oceanic provinces from Iceland to the Azores. Catalysed reporter deposition fluorescence in situ hybridisation (CARD-FISH) analysis of sorted cells revealed distinct differences in phylogenetic composition between the LNA and HNA populations with only little overlap. At all stations the LNA population was dominated by the alphaproteobacterial clade SAR11 (45–74%). Also, Betaproteobacteria were always present at 2–4%. While the LNA composition was rather stable, the HNA populations were composed of distinct phylogenetic clades in the different oceanic provinces of Arctic and Tropics. For example Cyanobacteria dominated the North Atlantic Gyre HNA population (29–44%) with Prochlorococcus as the major clade (34–44%), but were low in Arctic and Polar waters (1% and 5%, respectively). In contrast, Bacteroidetes accounted for the majority of HNA cells in the Polar and Arctic province (26% and 32%, respectively), but were low in the Gyre region (3–10%). The DNA content of the HNA population was about 3.5 times higher than that of the LNA populations. This reflects differences in the genome sizes of closely related cultured representatives of HNA clades (3–6 Mbp) and LNA clades (1.3–1.5 Mbp).     

Seasonal Depth-Related Gradients in Virioplankton: Standing Stock and Relationships with Microbial Communities in Lake Pavin (France)

This study presents a depth-related survey of virioplankton abundance in Lake Pavin (Massif Central, France), in relation to the abundances of heterotrophic prokaryotes, picocyanobacteria (Pcy), autotrophic picoeukaryotes (Peu), and of autotrophic (ANF) and heterotrophic (HNF) nanoflagellates. The sampling strategy was designed to be representative of the physico-chemical gradients of the whole water column of the lake, and the seasonal variability as well. In mixolimnic surface waters, all communities were present and viral abundance peaked in summer and autumn. Viral abundance was significantly correlated (p < 0.001) with Pcy, Peu, and ANF, indicating that cyanophages and perhaps other phytoplankton viruses represent a significant pool of viral standing stocks in the mixolimnion of Lake Pavin. Microautotrophs were absent in the deep monimolimnic water masses, where viruses and heterotrophic prokaryotes exhibited highest seasonal abundances in summer and/or autumn and were significantly correlated (p < 0.001) to each other. This indicates that the anoxic monimolimnion of Lake Pavin is an exclusive habitat for viruses and heterotrophic prokaryotes. We conclude that in this habitat, host availability is prevalent over other factors (temperature, oxygen, nutrients, grazers) in favoring viral proliferation.     

Intra- and extracellular concentrations of glutamate, lactate and acetate during growth of Corynebacterium glutamicum on different media

Corynebacterium glutamicum ATCC 17965 was cultivated in a 4-L batch aerated fermentor with glucose, fructose and mixtures of these two sugars in various proportions as carbon sources and with different concentrations of minerals and vitamins. A multilayer centrifugation technique was devised to obtain cell extracts in order to assess intracellular production of glutamate and partitioning between intracellular and extracellular spaces for lactate and acetate, the main by-products produced during the growth phase. Glutamate production increased with the proportion of glucose in the carbon source. The average value for the intracellular concentration of glutamate obtained with basic glucose medium was increased three-fold when initial concentrations of vitamins and minerals were increased four-fold. In this case, overall production of glutamate (16.3 mM) reached the highest value obtained. Production of acetate was weak on all media types (< 1.6 mm). it was the same for lactate synthesis in media where glucose remained the major carbon source (< 2.3 mm). production of lactate was significantly higher on media where fructose was the main carbon source (> 10 mM to 60 mM). The increase in lactate production and the decrease in glutamate production were correlated to a modification of carbon flux distribution between the metabolic pathways as the fructose proportion was increased. An increase in the concentration of minerals favoured production of glutamate during growth. This was correlated with an increase in the NADPH,H⁺ production rate. Received 16 January 1996/ Accepted in revised form 14 January 1997     

Activity and Phylogenetic Diversity of Bacterial Cells with High and Low Nucleic Acid Content and Electron Transport System Activity in an Upwelling Ecosystem

We evaluated whether bacteria with higher cell-specific nucleic acid content (HNA) or an active electron transport system, i.e., positive for reduction of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), were responsible for the bulk of bacterioplankton metabolic activity. We also examined whether the phylogenetic diversity of HNA and CTC-positive cells differed from the diversity of Bacteria with low nucleic acid content (LNA). Bacterial assemblages were sampled both in eutrophic shelf waters and in mesotrophic offshore waters in the Oregon coastal upwelling region. Cytometrically sorted HNA, LNA, and CTC-positive cells were assayed for their cell-specific [³H]leucine incorporation rates. Phylogenetic diversity in sorted non-radioactively labeled samples was assayed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Cell-specific rates of leucine incorporation of HNA and CTC-positive cells were on average only slightly greater than the cell-specific rates of LNA cells. HNA cells accounted for most bacterioplankton substrate incorporation due to high abundances, while the low abundances of CTC-positive cells resulted in only a small contribution by these cells to total bacterial activity. The proportion of the total bacterial leucine incorporation attributable to LNA cells was higher in offshore regions than in shelf waters. Sequence data obtained from DGGE bands showed broadly similar phylogenetic diversity across HNA, LNA, and CTC-positive cells, with between-sample and between-region variability in the distribution of phylotypes. Our results suggest that LNA bacteria are not substantially different from HNA bacteria in either cell-specific rates of substrate incorporation or phylogenetic composition and that they can be significant contributors to bacterial metabolism in the sea.     

Determination of Trace Elements in Anti-influenza Virus Mushrooms

We have determined the trace element composition of anti-influenza virus mushrooms using atomic absorption spectrophotometer. The elements present in greater concentration in Ganoderma lucidum samples are selenium, iron, and zinc, with selenium being the element with the highest concentration of all, at 416 ± 38.5 mg/kg; in Cordyceps militaris samples are iron, selenium, and zinc, with iron being the element with the highest concentration of all, at 291 ± 20.9 mg/kg; in Kuehneromyces mutabilis samples are selenium, iron, and manganese, with selenium being the element with the highest concentration of all, at 203 ± 9.8 mg/kg; in Inonotus hispidus samples are zinc, selenium, and iron, with zinc being the element with the highest concentration of all, at 194 ± 16.9mg/kg; in the Collybia maculata samples are iron, selenium, and zinc, with iron being the element with the highest concentration of all, at 274 ± 22.2 mg/kg, respectively. The average metal concentrations in mushrooms decreases in the order: selenium > iron > zinc > chromium > manganese > copper > magnesium > lead. After the mice were administered (orally) with mushroom extracts for 8 weeks and inoculated intranasally with viral suspension, element levels in serum were also measured. Highly significantly increased values of Se, Zn, and Mg in the serum of mice supplemented with anti-influenza virus mushrooms were a characteristic finding. Se, Zn, and Mg present in mushrooms may play a direct or indirect role in their anti-influenza virus nature. They may provide prophylactic protection against influenza infection via stimulation of host innate immune response.     

Single‐cell and population level viral infection dynamics revealed by phageFISH,a method to visualize intracellular and free viruses

Microbes drive the biogeochemical cycles that fuel planet Earth, and their viruses (phages) alter microbial population structure, genome repertoire, and metabolic capacity. However, our ability to understand and quantify phage–host interactions is technique‐limited. Here, we introduce phageFISH – a markedly improved geneFISH protocol that increases gene detection efficiency from 40% to > 92% and is optimized for detection and visualization of intra‐ and extracellular phage DNA. The application of phageFISH to characterize infection dynamics in a marine podovirus–gammaproteobacterial host model system corroborated classical metrics (qPCR, plaque assay, FVIC, DAPI) and outperformed most of them to reveal new biology. PhageFISH detected both replicating and encapsidated (intracellular and extracellular) phage DNA, while simultaneously identifying and quantifying host cells during all stages of infection. Additionally, phageFISH allowed per‐cell relative measurements of phage DNA, enabling single‐cell documentation of infection status (e.g. early vs late stage infections). Further, it discriminated between two waves of infection, which no other measurement could due to population‐averaged signals. Together, these findings richly characterize the infection dynamics of a novel model phage–host system, and debut phageFISH as a much‐needed tool for studying phage–host interactions in the laboratory, with great promise for environmental surveys and lineage‐specific population ecology of free phages.     

深圳近海表层浮游细菌分布特征及其环境影响因素

于2015年3月、5月、8月和10月在深圳市近岸海域(珠江口、深圳湾和大亚湾)采集表层水样,利用流式细胞仪测定总浮游细菌、高DNA含量亚群细菌(HNA)、低DNA含量亚群细菌(LNA)的丰度,分析它们的时空分布特点,阐释环境因子对浮游细菌时空分布格局的影响。结果表明,珠江口、深圳湾和大亚湾海域表层浮游细菌的平均丰度依次降低,分别为3.82×10~6个/mL、7.67×10~6个/mL和3.38×10~6个/mL。珠江口海域浮游细菌丰度由远岸到近岸递增,深圳湾海域湾内各站位浮游细菌丰度差异较小,大亚湾海域浮游细菌丰度空间差异不显著(P0.05)。浮游细菌丰度时间差异主要受温度影响,空间差异主要受营养盐和叶绿素a影响。HNA亚群丰度时空差异性比LNA亚群的大,HNA亚群受温度影响显著(P0.01),而LNA亚群与温度相关性不显著(P0.05)。环境对HNA和LNA亚群丰度的影响有许多相似之处,但两者对某些环境因子有着不同的响应,说明它们在近海表层生态系统中可能扮演着部分重叠但略有不同的角色。     

Bacterial Growth Rate and Marine Virus–Host Dynamics

Abstract The dynamics of a marine virus–host system were investigated at different steady state growth rates in chemostat cultures and the data were analyzed using a simple model. The virus–host interactions showed strong dependence on host cell growth rate. The duration of the infection cycle and the virus burst size were found to depend on bacterial growth rate, and the rate of cell lysis and virus production were positively correlated with steady state growth rate in the cultures (r ² > 0.96, p < 0.05). At bacterial growth rates of 0.02 to 0.10 h⁻¹ in the chemostats the virus burst size increased from 12 ± 4 to 56 ± 4, and the latent period decreased from 2.0 to 1.7 h. Resistant clones of the host strain were present in the cultures from the beginning of the experiment and replaced the sensitive host cells following viral lysis in the cultures. Regrowth of resistant cells correlated significantly (r ²= 1.000, p < 0.02) with the lysis rate of sensitive cells, indicating that release of viral lysates stimulated growth of the non-infected, resistant cells. The constructed model was suitable for simulating the observed dynamics of the sensitive host cells, viruses and resistant clones in the cultures. The model was therefore used in an attempt to predict the dynamics of this virus–host interaction in a natural marine environment during a certain set of growth conditions. The simulation indicated that a steady state relationship between the specific viruses and sensitive and resistant bacterial clones may occur at densities that are reasonable to assume for natural environments. The study demonstrates that basic characterization and modeling of specific virus–host interactions may improve our understanding of the behavior of bacteria and viruses in natural systems. Received: 12 November 1999; Accepted: 2 May 2000; Online Publication: 11 August 2000     

A comparative study of the cytometric characteristics of high and low nucleic-acid bacterioplankton cells from different aquatic ecosystems

Flow cytometry has revealed the existence of two distinct fractions of bacterioplankton cells, characterized by high and low nucleic acid contents (HNA and LNA cells). Although these fractions seem ubiquitous in aquatic systems, little is known concerning the variation in the cytometric parameters used to characterize them. We have performed cytometric analyses of samples from a wide range of aquatic systems to determine the magnitude and variability in the cytometric characteristics of HNA/LNA. We show that neither group is associated to a fixed level of fluorescence and of light scatter. Rather, the relative position of HNA and LNA in the fluorescence versus side scatter cytograms varies greatly, both within and among ecosystems. Although the cytometric parameters of both groups tend to covary, there is often uncoupling between the two, particularly in light scatter. Our results show that, although the basic HNA/LNA configuration is present in most samples, its cytometric expression changes greatly in different ecosystems and along productivity gradients. The patterns in cytometric parameters do not support the simple, dichotomous view of HNA and LNA as active and inactive cells, or the notion of two distinct and independent communities, but rather suggest that there may be cells that are intrinsic to each fraction, as well as others that may exchange between fractions.     

Induction of alpha interferon by membrane interaction between viral surface and peripheral blood mononuclear cells

Cells infected with viruses and fixed when viral antigens appeared at the cell membrane induced much higher alpha interferon (IFN-alpha) levels in human peripheral blood mononuclear cells (PBMC) than free virions. Relatively few inducer cells were sufficient for triggering IFN production. Optimal IFN yields depended on inducer/producer cell ratio. The response was peculiar to PBMC as it was not found in other cells in which IFN can normally be induced by free virions. IFN inducing activity was also exerted by live virus-infected PBMC, showing that this type of induction may have physiological importance. These findings confirm that viral induction of IFN-alpha is activated by some interaction between viral components presented at the cell surface and PBMC membrane. Thus induction of IFN by circulating cells infected by viruses and presenting viral antigens at the surface may be an efficient host defense mechanism. Since IFN yields close to 10(6) international units per milliliter are obtained, this system has potential for large scale production of native IFN-alpha.