This episode of This Week in Microbiology (TWiM 331) delves into two fascinating microbial studies. First, the hosts discuss a research snippet describing the isolation and characterization of a new bacterial species, Niellia tanggongensis, discovered on the China Space Station. The conversation explores its potential origin, unique metabolic capabilities (like gelatinase activity), and possible adaptations to the space environment, including considerations of humidity and radiation shielding. Second, the main segment focuses on a paper published in Cell detailing a remarkable "chemical radar" system in the plant pathogenic bacterium Pseudomonas syringae. This system allows P. syringae to detect the presence of predatory amoeba (Polyphondilium pallidum) through a specific molecule (a processed derivative of bacterial syringactin) produced by the amoeba. This signal activates a gene cluster in the bacterium, triggering the production of a potent toxin (pyroactin) that kills the amoeba, effectively turning a prey-predator interaction into a counter-attack mechanism likened to a microbial tennis match involving a pro-drug intermediate.
Exploring Microbes in Space and a Bacterial Chemical Radar
0:24 Podcast Introduction: This Week in Microbiology (TWiM) episode 331, hosted by Vincent Racaniello with guests Petra Levin, Michael Schmidt, and Mark Martin, discusses two topics: a bacterium from the China Space Station and a bacterial "chemical radar."
3:01 China Space Station Snippet: Mark Martin presents a paper on Niellia tanggongensis, a bacterium isolated from the China Space Station (Tiangong, meaning "Heavenly Palace"). The station, operational since 2021, includes experiments with plants.
4:22 Space Environment Considerations: The space station environment differs from Earth (microgravity, potentially different atmosphere/humidity, cosmic rays). Cleaning efforts are made before launch, but human occupants bring their own microbes.
10:26 Sampling Method: Sampling involved wiping predefined locations on orbit using a specialized kit. Controls included waving wipes in the air and leaving wipes unopened. Samples were stored at 4Β°C and processed back on Earth using sterile water, vortexing, sonication (possibly enriching for spores), and culturing on tryptic soy agar (TSA).
13:55 Identification and Relation: An isolate was obtained and identified using full 16S rRNA sequencing and Average Nucleotide Identity (ANI). It was found to be closely related to Cytobacillus but distinct enough to be a new species, Niellia tanggongensis.
17:07 Unique Gelatinase Activity: Genome sequencing revealed typical bacterial genes but also potentially unique ones, including gelatinase activity, which appears unique among its closest relatives examined. This could allow it to utilize collagen found in skin flakes (a potential nutrient source in the space station).
18:50 Potential Food Source: Human skin flakes, especially if moist or mobilized by systems like the stool extraction device, could serve as a nutrient source in microgravity. The lack of showers also contributes to shedding skin microbes.
20:59 Salt Tolerance: The bacterium tolerates but doesn't thrive in high sodium chloride concentrations, unlike many skin-associated microbes, suggesting it might be adapted to a less saline environment within the station.
21:11 Gelatinase and Biofilms: Gelatinase is a known factor in biofilm formation. Biofilms are likely beneficial in microgravity. Humidity levels on the station (unknown) could influence microbial survival and spore formation.
22:56 Isolate vs. Survey: The isolate likely represents what was culturable and perhaps selected for by the processing method (sonication), possibly favoring spores. A broader survey is needed to understand the full microbial diversity and how it changes over time in space.
25:13 Mutation Rate in Space: Future studies could investigate the mutation rate of bacteria in space, potentially influenced by increased cosmic radiation compared to sea level.
28:01 Chemical Radar Paper: Petra Levin presents a paper from Cell titled "A chemical radar allows bacteria to detect and kill predators," focusing on the interaction between Pseudomonas syringae (a plant pathogen) and predatory amoeba (Polyphondilium pallidum).
30:49 Edible vs. Inedible Strains: The study compared two closely related P. syringae strains: SZ47 (inedible by amoeba) and SZ57 (edible). Plaque assays demonstrated amoeba clearing lawns of the edible strain but not the inedible one.
32:54 Syringactin as a Pro-Drug: Both strains produce syringactin, a surfactant that aids bacterial swarming. When amoeba consume syringactin, they partially digest it into peptide derivatives.
34:55 The Killing Mechanism: The edible strain (SZ57) does little with the processed peptide. The inedible strain (SZ47) converts the peptide into pyroactin, which is toxic to the amoeba, killing them. This is described as a pro-drug mechanism.
35:26 Microbial Tennis Match: The interaction is likened to a tennis match: Bacteria serve syringactin (pro-drug), amoeba return a processed peptide (intermediate), bacteria return a lethal molecule (pyroactin) that "spikes" and kills the amoeba.
37:28 Identifying the Genes: A specific gene cluster (CRA: chemical radar) in SZ47 is responsible. CRA A and CRA C are the synthetic enzymes, while CRA R is a regulatory gene. Knocking out CRA A/C abolishes toxicity; introducing them into E. coli allows it to make pyroactin.
38:54 The Sensor/Regulator (CRA R): CRA A and CRA C are conserved in both strains, but the regulator CRA R is present only in the inedible strain SZ47. CRA R senses the processed peptide signal from the amoeba and turns on the CRA A/C genes to produce pyroactin, ensuring the toxin is only made when the predator is detected.
40:56 Ecological Significance: This chemical radar allows the plant pathogen P. syringae SZ47 to evade predation by amoeba, potentially enhancing its ability to infect and colonize plants.
42:40 Open Access: The paper is open access in Cell Press, making it freely available.
43:09 Syringactin Function: Syringactin facilitates bacterial swarming/motility (makes them "slippery"). Amoeba may deacylate it to stop swarming, making the bacteria easier to phagocytose, inadvertently creating the signal molecule for the bacteria's counter-attack.
44:49 Predator-Prey Dynamics: This study provides a novel example of complex chemical interactions and counter-adaptations in microbial predator-prey relationships.