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<jats:title>Photons not welcome</jats:title> <jats:p> Two identical fermions cannot occupy the same quantum state, or so says the Pauli principle. For a cold gas of fermionic atoms, this means that all states up to the Fermi energy will be occupied, with only the atoms with the highest energy able to change their state. Such conditions have long been predicted to suppress light scattering off gases because the atoms receiving a kick from collisions with photons would have no state to move to. Deb <jats:italic>et al</jats:italic> ., Margalit <jats:italic>et al</jats:italic> ., and Sanner <jats:italic>et al</jats:italic> . now describe this so-called Pauli blocking of light scattering. —JS </jats:p>

<jats:title>Photons not welcome</jats:title> <jats:p> Two identical fermions cannot occupy the same quantum state, or so says the Pauli principle. For a cold gas of fermionic atoms, this means that all states up to the Fermi energy will be occupied, with only the atoms with the highest energy able to change their state. Such conditions have long been predicted to suppress light scattering off gases because the atoms receiving a kick from collisions with photons would have no state to move to. Deb <jats:italic>et al</jats:italic> ., Margalit <jats:italic>et al</jats:italic> ., and Sanner <jats:italic>et al</jats:italic> . now describe this so-called Pauli blocking of light scattering. —JS </jats:p>

<jats:title>Photons not welcome</jats:title> <jats:p> Two identical fermions cannot occupy the same quantum state, or so says the Pauli principle. For a cold gas of fermionic atoms, this means that all states up to the Fermi energy will be occupied, with only the atoms with the highest energy able to change their state. Such conditions have long been predicted to suppress light scattering off gases because the atoms receiving a kick from collisions with photons would have no state to move to. Deb <jats:italic>et al</jats:italic> ., Margalit <jats:italic>et al</jats:italic> ., and Sanner <jats:italic>et al</jats:italic> . now describe this so-called Pauli blocking of light scattering. —JS </jats:p>

<jats:title>Strength by cyclic torsion</jats:title> <jats:p> For most alloys, conventional or high entropy, increasing strength comes at the cost of poor ductility. Although there are many strategies to break this inverse relationship, Pan <jats:italic>et al</jats:italic> . now show that cyclic torsion on a high-entropy alloy enhances strength without degrading ductility (see the Perspective by Yeh). Cyclic torsion creates a gradient of dislocations and low-angle grain boundaries from the surface to the interior that organize into tiny stacking faults and twin when straining begins. These structures allow for the good ductility while simultaneously helping to work harden the alloy. —BG </jats:p>

<jats:title>SARS-CoV-2: To have or to have not</jats:title> <jats:p> In June 2021, official records in Kenya showed fewer than 4000 confirmed deaths and 180,000 confirmed cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These data tend to reflect the economically advantaged strata of society who can afford smartphones and have access to medical attention and tests. Brand <jats:italic>et al</jats:italic> . developed an epidemiological model to estimate the impact of the pandemic in Kenya, the population of which was split into two socioeconomic strata. The authors predicted that 75% of the Kenyan population (about 39 million people) had been exposed to the virus by June 2021. If a fourth wave of infection is observed in the future, it would likely be driven by a variant with enhanced transmissibility or natural immune escape. —CA </jats:p>

<jats:title>Deadly surge in Delhi</jats:title> <jats:p> In the spring of 2021, Delhi, India experienced a wave of coronavirus cases that overwhelmed healthcare services despite the population showing a high level of immune positivity. Dhar <jats:italic>et al</jats:italic> . collated a mixture of serosurveillance, quantitative polymerase chain reaction, and genomic data, finding that waves of variants had passed through the Delhi population during 2020 and 2021. The alpha (B.1.1.7) variant dominated in March 2021 and was rapidly replaced by the delta (B.1.617.2) variant in April and May 2021. The delta variant outcompeted its predecessors by mutations that enhanced replication, immune evasion, and host receptor avidity, thus increasing transmissibility, reinfection, and vaccination breakthrough. —CA </jats:p>

<jats:title>Variations in disease enhancement</jats:title> <jats:p> Secondary Dengue virus (DENV) infections can be dangerous if levels of antibodies from prior infection are inadequate to clear the virus. This RNA flavivirus exploits the presence of lower levels of heterotypic antibodies to infect immunoglobulin Fcγ receptor–bearing cells. Many RNA viruses also exhibit antigenic variation, which classically allows evasion of immune responses. Katzelnick <jats:italic>et al</jats:italic> . investigated whether antigenic variation in DENV has a biological function in a virus that courts immune responses to enhance replication (see the Perspective by Rohani and Drake). Using antigenic cartography on a panel of more than 400 DENV1-4 subtype samples isolated in Bangkok, Thailand, the authors found that antigenic variation in virus populations oscillated between similarity and dissimilarity across subtypes over time, with outbreaks correlating with periods of antigenic dissimilarity within serotypes. This pattern may be at least in part a result of the conflicting evolutionary pressures of immune evasion and immune enhancement. —CA </jats:p>

<jats:title>A copper-containing antibiotic</jats:title> <jats:p> Bacteria require transition metal ions for biological processes and must also protect themselves against excess metal, which is toxic. Patteson <jats:italic>et al</jats:italic> . explored how the environmental bacterium <jats:italic>Pseudomonas aeruginosa</jats:italic> uses a five-enzyme pathway to synthesize a small-molecule complex, fluopsin C, which is built from cysteine and contains a copper ion. The biosynthesis involves unusual enzymatic transformations that convert cysteine to a thiohydroximate, two of which chelate a copper ion in the final natural product. Fluopsin C protects <jats:italic>P. aeruginosa</jats:italic> from excess copper and also acts as a broad-spectrum antibiotic against other bacteria. —VV </jats:p>

<jats:title>How the body regulates fear</jats:title> <jats:p> Although fear is important for survival, it is maladaptive if it is either too strong, as in anxiety disorders, or too weak, as in exaggerated risk taking. Working in mice, Klein <jats:italic>et al</jats:italic> . observed that the insular cortex has an unparalleled dual role in either enhancing or weakening the extinction of fear, depending on the internal fear state of the animal (see the Perspective by Christianson). This insula function helps to maintain fear within a homeostatic range and depends on bodily feedback signals: Fear-induced freezing behavior is associated with a slowed heart rate, which in turn dampens fear-evoked activity of the insular cortex. Two opposite signals, prediction of threat by fear-associated cues and negative feedback signals from the body, are thus integrated within the insular cortex. —PRS </jats:p>