This episode of "This Week in Microbiology" (TWIM) discusses two distinct topics. The first segment, presented as a snippet, explores the isolation and characterization of a novel bacterium, Niellia tanggongensis, from the Chinese space station. The hosts discuss the environment of the space station, methods used for microbial sampling and isolation (including old-school techniques), and the genetic analysis of the isolated bacterium, noting potential adaptations related to stress tolerance and potential sources within the human environment. The second segment dives into a Cell paper detailing a "chemical radar" mechanism in the plant pathogen Pseudomonas syringae that allows it to detect and kill predatory amoebae (Polysphondylium pallidum). This involves the amoeba partially digesting a bacterial molecule (syringactin), which the bacterium then converts into a toxin (pyroactin). The mechanism acts as a defense against predation and enhances the bacterium's ability to infect plants.
TWIM Episode 331: Space Station Microbes and Bacterial Chemical Radar
0:00 Introduction: The episode begins with introductions of the hosts, Vincent Racaniello, Petra Levin, and Michael Schmidt, and guest Mark Martin, noting current weather in their locations.
2:00 Snippet: Microbes on the China Space Station: Mark Martin presents a paper describing Niellia tanggongensis, a new bacterium isolated from the Chinese Space Station (Heavenly Palace).
3:51 China Space Station Details: The space station, launched in 2021, is continuously occupied, features 340 sq meters of space (about the size of a large house), and hosts various experiments, including plant growth. It is currently occupied only by Chinese nationals.
5:24 ISS De-orbit: Discussion touches on the International Space Station (ISS) nearing the end of its life and planning a controlled de-orbit.
9:34 Human Habitation and Microbes in Space: The hosts discuss how human presence inevitably brings microbes to space environments, questioning the effectiveness of pre-flight cleaning protocols for human-rated missions.
10:51 Microbial Sampling Methods: The paper used standard, "old school" techniques for sampling (wiping surfaces) and culturing from the space station, including negative controls. Sonication was used to extract microbes from wipes.
12:50 Isolated Organism: The isolated bacterium was a relative of Bacillus, specifically closer to Cytobacillus. It produced white colonies and endospores. The sonication step likely favored spore recovery.
15:35 Taxonomic Analysis: Full 16S sequencing, Average Nucleotide Identity (ANI), and digital DNA-DNA hybridization placed the isolate in the genus Niellia.
17:43 Genetic Insights: Genome sequencing revealed genes potentially related to stress tolerance, such as a bacillithiol biosynthesis deacetylase (combating oxidative stress) and a transcriptional regulator for spore photoproduct lyase (dealing with UV damage).
18:58 Metabolic Uniqueness: The isolated strain utilizes gelatin, which appears unique among its close relatives, potentially suggesting an adaptation to utilizing resources like skin flakes (collagen-rich) found in the space environment.
21:14 Biofilm Formation: Gelatinase is noted as a potential virulence factor facilitating biofilm formation, which could be advantageous in microgravity.
22:00 Humidity Considerations: The relative humidity on the space station is discussed as a key environmental factor influencing microbial survival and spore formation on surfaces.
23:00 Takeaway on Isolate: The primary takeaway is that the paper successfully isolated one specific organism from the space station, confirming human-associated microbes can establish there, but it doesn't provide a broad survey of the entire microbial community or definitive evidence of space-induced evolution.
24:24 Broader Questions: The discussion broadens to question how much is truly learned about space adaptation from a single isolate and the potential impact of increased cosmic radiation on mutation rates.
26:38 Radiation Shielding: Michael Schmidt explains the use of Whipple shields (multi-layer non-metallic shielding) on the ISS to protect the crew from radiation, suggesting similar shielding is likely used on the Chinese station.
28:01 Paper: Bacterial Chemical Radar: Petra Levin presents a Cell paper titled "A chemical radar allows bacteria to detect and kill predators," focusing on the interaction between Pseudomonas syringae (plant pathogen) and Polysphondylium pallidum (amoeba).
29:56 Predator-Prey Model: The study uses P. syringae and a predatory amoeba that eats bacteria as a model to understand molecular interplay. The amoeba's predation protects plants.
30:46 Identifying Strains: Two P. syringae strains from the same environment were compared: one "edible" (eaten by amoeba) and one "inedible" (not eaten). Plaque assays visualized amoeba predation.
32:50 Chemical Communication/Weaponry: Mass spectrometry and NMR revealed that both strains produce syringactin. When the inedible strain is co-cultured with amoeba, two derivative peptides appear. The amoeba partially digests syringactin, producing these peptides.
34:54 Pro-drug Activation: Feeding the amoeba-modified peptide to the inedible P. syringae strain converts it into pyroactin. Feeding pyroactin to amoebae kills them.
35:26 Microbial Tennis Match: The interaction is likened to a tennis match where the bacterium sends a molecule (syringactin), the amoeba returns a modified version (peptide), and the bacterium converts it into a lethal "spiked" ball (pyroactin) to kill the amoeba.
36:36 Analogy to Macrophages: The interaction is compared to bacterial defenses against macrophages, as amoebae are phagocytic predators.
37:33 Identifying the Mechanism: The researchers identified a gene cluster, cRA (chemical radar), responsible for this defense. cRA R is a regulatory gene, while cRA A and cRA C perform modifications.
38:17cRA Genes:cRA A and cRA C are conserved in both edible and inedible strains, but cRA R (the 'on switch') is only in the inedible strain. Introducing cRA R into the edible strain makes it inedible.
39:28 Horizontal Transfer: The regulator gene cRA R appears to have been acquired by the inedible strain, possibly via horizontal gene transfer or recombination.
40:41 Ecological Advantage: The ability to kill predatory amoebae via this "chemical radar" enhances the inedible P. syringae strain's ability to infect plants, linking predation defense to pathogenicity.
41:46 Sensor Mechanism:cRA R acts as the sensor, binding the amoeba-digested peptide (deacylated syringactin) and activating the cRA genes to produce pyroactin only when predators are present.
43:03 Open Access: The paper is published in Cell and is open access.
43:06 Syringactin Function: Syringactin is a surfactant that helps Pseudomonas swarm (motility). The amoeba deacylating it may prevent swarming, but this action also triggers the bacterium's counter-attack.
45:08 Digestible Figures: The paper is praised for having clear, easy-to-understand figures.
45:36 Conclusion: The hosts thank the guests and contributors, providing contact and support information.