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What's Hot in the Microbial Sciences

How Does Pseudomonas aeruginosa Deal With Competitors?

3D illustration of lung infection caused by bacteria Pseudomonas aeruginosa.
Source: istock.com. The answer is poison. A , reported that airborne hydrogen cyanide (HCN) produced by the pathogenic bacterium Pseudomonas aeruginosa inhibits Staphylococcus aureus growth in biofilm and in vivo lung environments. The observation sheds light on competitive dynamics of polymicrobial communities that cause chronic infections in cystic fibrosis (CF) patients.

CF is a genetic disease in which bodily secretions, such as mucus and digestive juices, become thick and sticky, providing an ideal environment for pathogens to thrive. As a result, the lungs and airways of CF patients are often colonized by multiple bacterial pathogens, including P. aeruginosa and S. aureus. Although S. aureus usually colonizes the lungs first during CF infection, when P. aeruginosa arrives on the scene, it outcompetes and replaces S. aureus.

Scientists have identified several extracellular factors that may contribute to the colonization shift; however, less was known about how airborne HCN (which is produced by P. aeruginosa metabolism and can rapidly diffuse into the environment) factors into the equation. HCN is a known respiratory chain inhibitor and is toxic to a wide range of eukaryotes. Scientists, therefore, hypothesized that it might also poison HCN-sensitive bacteria in a variety of niches.

The study revealed that HCN limits growth of a wide array of S. aureus strains and can do so from a distance. Low-oxygen environments were found to enhance P. aeruginosa production as well as S. aureus sensitivity to the compound. These results were demonstrated in microaerobic in vitro biofilms as well as in vitro CF lung sputum medium. Furthermore, consistent inhibition of S. aureus growth was reported in mouse model airways that were coinfected with both bacterial pathogens.

The study provides new context for the management and monitoring of P. aeruginosa lung infections and interactions with other HCN-sensitive microbes that may contribute to polymicrobial disease.

Can Mosquitos 'Smell' Dengue and Zika Viruses?

Mosquitos are attracted to high levels of the volatile organic compound acetophone produced by commensal skin microbiota.
Source: istock.com. during the summer of 2022 demonstrated that mosquitos are not only more attracted to hosts infected with certain arboviruses (i.e., dengue and Zika), but also that the attraction is driven by a volatile organic compound (VOC) that is overproduced by the host's skin microbiota.

Scientists separated mice into 2 cohorts—those that were infected with dengue or Zika virus, and those that were not. Then they blew air over each cohort and observed that mosquitos preferentially flocked to the infected test subjects over controls. Upon collection and measurement of the volatiles emitted from the mice, researchers discovered that acetophone (C₈H₈O), an aromatic ketone that has natural roles as a photosensitizing agent, metabolite and xenobiotic, was significantly enriched in the infected cohort.

How did these arboviruses manipulate skin microbes to produce a scent that stimulates mosquito olfaction? It turns out, flavivirus infection suppresses expression of REMα, an essential antimicrobial that occurs on host skin. In the absence of this natural antimicrobial, commensal bacteria that produce acetophone as a metabolite are able to thrive. High acetophone levels attract more mosquitoes to the host, where feeding and spread of disease may be propagated.

A simple vitamin A derivative, isotretinoin, which can be given, in measured doses, as a dietary supplement, was shown to induce REMα production, decrease acetohpone concentration and reduce feeding of mosquitos on the treated hosts—perhaps illuminating a unique strategy for arboviral control.

Why Don't Bats Get Sick From Diseases That are Deadly to Humans?

Close-up image of Cynopterus brachyotis (Lesser short-nosed fruit bat).
Source: Wikimedia. New suggests that selective pressure on a ubiquitous mammalian antiviral protein may assist bats in their unique ability to asymptomatically house viruses that are deadly to other mammals—including humans. Tetherin is a restriction enzyme that prevents particles of enveloped viruses (e.g., retroviruses, coronaviruses, filoviruses and paramyxoviruses) from escaping host cells. As a secondary function, tetherin triggers the immune signaling pathway NF-κB and stimulates antiviral interferon responses. The protein is common to all mammals; however, data indicate that bat tetherin genes have undergone expansion and diversification relative to other mammals, which may enhance their ability to withstand viruses, such as Ebola, Marburg, SARS-CoV-1, Hendra and Nipah, without showing clinical signs of disease.

In this study, researchers mined databases of publicly available sequence reads of bat tetherin homologs. Using an isoform from the fruit bat (Pteropus alceto) as their query sequence, they reported that the structure, number and expression of tetherin genes varied amongst the 27 bat species that were studied. For example, while the fruit bathas 3 isoforms of a single tetherin gene, 1 species of vesper bat (Myotis marcopus) was found to express 5 distinct tetherin genes and another, Myotis lucifugus, may have up to 7 more than any other mammal reported to date. (Most mammals carry only a single tetherin gene, and humans express the enzyme in 2 alternative isoforms). Notably, some of the variants discovered across bat species were structurally unique and had activity against different viral particles.

Scientists also examined tetherin protein expression, paying particular attention to the tissues of fruit bats. They found that the antiviral protein is expressed widely and variably throughout fruit bat tissues, with the highest levels reported in the thymus and lungs. These observations may point to the defensive role of tetherin against respiratory pathogens. Finally, when stimulated with Toll-like receptor agonists (e.g., lipopolysaccharide), increased tetherin expression was detected in the spleen. The importance of this observation remains unknown. What is clear is that bats have evolved forms of tetherin that are unique from those observed in other mammalian species. Given bats are known reservoirs of viruses that cause severe disease in other mammals—often without development of any observable symptoms—unique evolution and diversification of antiviral genes, like tetherin, warrant further investigation.

Can Fermented Food Flavor Be Reproduced Without Geographical Limitations?

Baijiu, a Chinese liquor, is fermented with the help of fungus-bacterium cooperative metabolisms.
Source: Tagosaku/Flickr. Fermentation is in human history. Lauded for its practicality and distinct flavor profiles, food fermentation has become something of both art and science. Replicating the flavors of fermented foods is often thought to be geography-dependent, but in a , researchers attempted to replicate the complex processes of Chinese liquor (baijiu) fermentation in the lab.

The team collected 403 baijiu samples, belonging to 3 aroma types, from 9 different locations in China, across a latitude range of 27°N to 37°N, then applied culture-independent (metagenomics, metabolomics and metatranscriptomics) as well as culture-dependent tools to the samples. The analysis identified 735 bacterial genera and 290 fungal genera across the baijiu microbiome and revealed that fungus-bacterium cooperative metabolism had a greater impact on geography-dependent flavor than on either microbial counterpart alone. Furthermore, geographical characteristics could be attributed to enrichment of species from various microbial pools, which were largely governed by pH, precipitation and nutrition. As a result, researchers suggested adjusting the composition and distribution of species as an option for flavor regulation, stating that their findings provided rationale for developing a microbiome design to achieve intended flavor goals.

Can Microbes Make Rocket Fuel?

Space shuttle Atlantis, attached to its bright orange external fuel tank and twin solid-rocket boosters.
Source: U.S. Government (Public Domain). Converting petroleum to fuel contributes to polluting emissions, including the environmentally costly greenhouse gas carbon dioxide (CO2). Yet, energy-demanding applications, such as rocketry, aviation and shipping, require combustion of an energy-dense fuel source. Fuels that are rich in cyclic molecules with strained angles (i.e., cyclopropanes) can store the most energy. Cyclopropanes are hard to organically synthesize, which has propagated global dependence on petroleum. However, , scientists used bacteria to synthesize a molecule that has a higher energy density than any petroleum product on the planet.

The group explored thousands of bacterial genomes in search of naturally synthesized cyclopropanated molecules and identified a set of iterative polyketide synthases (iPKSs) that were predicted to produce polycyclopropanated fatty acids (POP-FAs). Next, they engineered a heterologous system to express iPKSs in Streptomyces coelicolor. The engineered bacterial host produced polycyclopropanated fatty acid methyl ester (POP-FAME), which can have energy densities greater than 50 MJ/L (higher than the most widely used rocket and aviation fuels on the market). Calculation of enthalpy of combustion, energy density (as net heating values) and vapor pressure lead scientist to conclude that POP-FAMEs are viable replacements for petroleum-based fuel sources in energy-demanding applications.

The study acknowledged that the next hurdle will be scaling up this system to be commercially useful. Once the structure of the molecules was determined, scientists were able to use this engineered biosynthetic pathoway to increase POP-FAME production by 22-fold.

Interested in learning more about the intersections between space travel and microbiology? Join us at ASM Microbe 2023 and hear from Andy Weir, author of New York Times Bestsellers, The Martian, Artemis and Project Hail Mary, as the inaugural Science and Society Keynote Lecturer.

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