Gillian Young

Environmental microbiology: A drop in the ocean

long promised to elucidate the crucial roles of oceanic microbial communities in the Earth’s biogeochemical and environmental cycles. In a recent issue of the Proceedings of the National Academy of Sciences USA, DeLong, Chisholm and colleagues bring us one step closer to this aim with their description of a new method to explore oceanic diversity and elucidate taxon-specific oceanic microbial functions that combines gene-expression analyis with functional metagenomics. In this study, the authors focused on the well-characterized surfacewater microbial community of Hawaii Ocean Time Series station ALOHA in the North Pacific Subtropical Gyre. RNA-amplification methods were first adapted to enable large amounts of cDNA to be generated from small quantities of community RNA. This method involved reverse transcription of bacterial RNA to which poly-A tails had been attached, a technique that amplified the RNA signal by approximately 1,000-fold. To test the reliability of this amplification method, the authors calibrated their results against the predominant and wellcharacterized cyanobacteria genus Prochlorococcus. cDNA was then extracted from picoplankton field samples using this new amplification method, analysed by pyrosequencE N V I R O N M E N TA L M I C R O B I O LO GY

Viral pathogenesis: Measles virus in one-way crossing

highly contagious measles virus (MV) enters its host through the luminal surface of respiratory epithelial cells before spreading systemically through the lymphatic system. A new paper published in The Journal of Clinical Investigation now reveals that initial infection of the airways is not necessary for entry into the host, but that airway infection is required for MV shedding and transmission. The MV attachment protein haemagglutinin binds to signalling lymphocytic activation molecule (SLAM) on lymphatic cells. However, previous studies had suggested that SLAMdependent luminal entry does not occur immediately after infection, but rather that MV entry into the respiratory epithelium occurs basolaterally as the virus exits the host, and therefore that the virus uses an epithelial receptor (EpR) that is located on the basolateral surface of respiratory epithelial cells. To investigate the interactions between MV and this as-yet-unidentified EpR, Leonard, Cattaneo and colleagues used a structural model of MV haemagglutinin and mutational analyses to identify amino acid residues in haemagglutinin that are crucial for MV infection of epithelial cells, but are not required for infection of SLAM-expressing cells. Three residues were identified and the authors went on to generate mutant strains that remained dependent on SLAM for cell fusion, but were ‘blind’ to the EpR and were therefore unable to enter the airway epithelia. Rhesus monkeys that were inoculated with these strains intranasally developed measles symptoms, but were unable to shed the virus into the airways, which supports the model that respiratory epithelial cells are not necessarily the initial targets of MV, but that infection of these cells is required for shedding and transmission. The authors then sought to characterize this shedding-restrictive mechanism. Wild-type virus infected columnar epithelial cells that form tight junctions only when applied basolaterally and budded only from the apical side, a finding that is consistent with one-way epithelial crossing, in which epithelial cells enable shedding but are not the focus of initial infection. By contrast, the EpR-blind mutant virus did not enter epithelial cells basolaterally and did not infect epithelial cells. This confirms the hypothesis that EpR is a basolateral protein and that wild-type MV releases progeny through apical release from the epithelial barrier. This mechanism of shedding could explain why MV is transmitted through respiratory aerosols, and could also apply to other viruses that spread in a similar manner. This enlightening study could force us to rethink our view of the MV infectious cycle. The authors also point out that this new understanding could have implications for the use of MV-based vectors in oncolytic therapies. More work is needed, however, to identify the basolateral EpR.

Virus structure: One of a kind!

Despite the availability of a commercial vaccine, the highly contagious measles virus remains a substantial health risk for which there is currently no specifically targeted treatment or antiviral therapy. The structure of the measles virus haemagglutinin (MVH), which the virus uses to bind to host-cell receptors, has now been revealed in a recent study published in Nature Structural & Molecular Biology.

Symbiosis: There's more to life than bacteria

melanogaster. Image courtesy of C. Ren and P. Webster, University of Southern California, Los Angeles, USA. DOI: 10.1038/nrmicro1753 The human body is host to a range of microorganisms that participate in important mutalistic interactions. As our immune system weakens with age, the balance between microbial load and the associated innate immune response could shift in unknown, and possibly negative, ways. New research published in Cell Metabolism now indicates that bacterial load does increase with age. Intriguingly, however, there may be no associated effect on lifespan. Ren and colleagues investigated the effect of age on the bacterial load of the fruit fly Drosophila melanogaster using bacterial counts, molecular genetic assays, scanning electron microscopy (SEM) and cell staining. From bacterial counts, it was apparent that the number of bacteria present both on the surface and in the interior of D. melanogaster increased dramatically with age. In agreement, SEM and cell-staining analyses detected an increase in the numbers of spherical objects and detritus-like material on the surface of older flies, which the authors suggest corresponds to bacteria and the biofilm-like material that they excrete. As confirmed by SEM and realtime quantification of 16S ribosomal RNA (rRNA), the bacterial load could be drastically reduced using antibiotics and axenic culturing methods. As expected, the number of bacterial 16S rRNA sequences that were detected from control group flies dramatically increased from between ~500 and ~5,000 (depending on the strain tested) per young fly to ~107 per old fly (regardless of the strain). By contrast, no bacterial sequences were detected from young or old flies that had been axenically cultured. Most importantly, however, there was no difference between the average lifespan of flies that had been axenically cultured or treated with antibiotics and the average lifespan of control flies. This study demonstrates that bacterial load increases with age, but the associated increase in the immune response does not necessarily lead to a direct trade-off, whereby extra energy is expended and lifespan decreases. The authors conclude that the costs and benefits associated with such mutalistic interactions are more complicated than was previously anticipated. Gillian Young

Bacterial virulence: Return of the 'spacebugs'

10.1038/nrmicro1782 The detrimental effect on the human immune system of a low-gravity environment such as that experienced during space flight has been well established, but the effects on bacteria that infect humans has not been probed. New findings published in the Proceedings of the National Academy of Sciences USA show that the space-flight environment affects the cellular and physiological responses of Salmonella enterica serovar Typhimurium (S. typhimurium) and, surprisingly, increases the virulence of this strain. Wilson, Nickerson and colleagues tested the responses of a virulent S. typhimurium strain during a flight of the Space Shuttle Atlantis. Cultures were activated during the space mission and grown for a specified time period, and were fixed, using an RNA or protein fixative, to terminate growth. All samples were compared with identical cultures that were grown simultaneously on the ground at the Kennedy Space Center. The transcriptome and proteome were examined to evaluate how space flight affected gene and protein expression. Exposure to space-flight conditions caused the differential expression of 167 genes and 73 proteins compared with groundcontrol cultures. The RNA-binding protein Hfq was identified as a global regulator that is involved with aspects of the tested responses. Hfq has a documented role in stress responses and the virulence of many bacterial pathogens, including S. typhimurium. The virulence of space-flight or synchronous ground-control strains was assessed by infecting BALB/c mice. Space-flight cultured S. typhimurium killed more mice and had a decreased LD50 value and decreased time to death compared with control strains. These intriguing results indicate that strains that are exposed to space-flight conditions (low gravity) display increased virulence compared with strains grown under normal-gravity conditions. The authors propose that a signal is sensed during exposure to lowgravity conditions which induces global alterations that, ultimately, result in increased virulence. This fascinating study has important implications for the assessment of disease risk during space flight and the production of novel antibacterial therapeutics that target Hfq. Gillian Young