“You are an island.” This poetical sentence is originally intended to highlight the character of a lone warrior. However, who claimed that this island is uninhabited? Just think of the large amount of microbial symbionts you are hosting on and in your body. You are a sort of island for them. You are the world for them. So even the loneliest warrior is not lonely at all. In fact, you will never be completely alone. This knowledge about the colonies of microorganisms for example on our skin or in our gut seduces us to think that this is sort of a rule for any larger organism. So it doesn’t make sense to study any animal alone, but you need to understand the animal as “holobiont”, as host plus its microbiome, because just the interplay between the commensal, pathogenic, and mutualistic microorganisms and their host make the host to what it seems to be… a rabbit, a dog, a human,… . The usage of antibiotics, which damage our gut microbiome, shows perfectly how much our health is connected to these little gut inhabitants. Many organisms show developmental disorders if there is something wrong with their gut microbiome. Many organisms… not all! Caterpillars doesn’t have a gut microbiome. This is the message of the paper from Tobin J. Hammer et al. (2017). Gut microbiome analyzation of 124 different species (15 different families) of wild leaf-feeding caterpillars in the United States and Costa Rica showed that Lepidoptera (butterflies) had gut bacterial densities multiple orders of magnitude lower than the microbiomes of other insects. Moreover, the majority of the small amount of gut bacteria which could be found in the guts of caterpillars, were leaf-associated bacteria. They didn’t live in the gut but were more transient visitors because their original home was eaten. So caterpillars seem not to depend on a gut microbiome which help them to digest the plant parts. Indeed, treatment with antibiotics didn’t affected the growth and development of the caterpillar. In contrast, in the laboratory, it actually increases growth because it kills pathogenic microorganisms. So we may should correct our idea that all organisms are like us: large hosts which life and health are connected to millions of small symbionts. Walking sticks, sawfly larvae and certain ants as well as a herbivorous goose (Branta bernicla) and a insectivorous bat (Myotis lucifugus) all these species are known for showing low fecal bacterial loads comparable to the caterpillars. They all seem to resign the additional digest help of gut bacteria in order to get more energy out of their food and instead focus on a sort of “more is more” strategy. They all have short guts with rapid digestive transit (so high feeding rate). So even if they don’t get much out of a single portion of food, they compensate this by eating a lot. This has the benefit that they can keep all nutrients for themselves, as the don’t have to “pay” the gut bacteria for their work, and they are maybe more flexibel what they can eat, as they just have to satisfy their own taste and not the taste of one million gut inhabitants. "Caterpillars lack a resident gut microbiome."
Tobin J. Hammer, et al. (2017) PNAS, doi:10.1073/pnas.1707186114
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Maybe it is not clear how I choose my random paper of the day. However, I am quite sure that everybody will understand why I chose the paper of W. Michael Dunne, Jr. (2002). How could I not choose a paper with such a great title: “Bacterial Adhesion: Seen Any Good Biofilms Lately?” I have to admit, I haven’t seen any biofilms lately. Or more correct: I was not aware of any biofilms around me lately, but I am sure there have been some as they can be found on a variety of “wet” surfaces like “contact lenses, ship hulls, dairy and petroleum pipelines, rocks in streams, and all varieties of biomedical implants and transcutaneous devices”. Biofilms are the results of a combination of aquatic bacterial populations, a surface and glycocalyx. The latter is the “slime” surrounding the bacteria and is mainly made of polysaccharides (sugars). The biofilm structure differs between the bacteria species. However, it seems true for all species that bacteria in biofilms are harder to destroy with antibiotics as their planktonic (solved) form. The “slime” of the biofilm helps to gather essential minerals and nutrients from the surrounding and protects the bacteria from enemies like antibiotics, bacteriophages and predators. Not all bacteria can bind on all surfaces. If a bacteria is near a surface, the net sum of different forces like e.g. electrostatic and hydrophobic interactions decides if the bacteria can dock on or not. These forces differ between different bacteria species. For example, in contrast to the most bacteria, which are negatively charged, Stenotrophomonas maltophilia is posetively charged at physiological pH and can therefore bind on negatively charged surfaces like Teflon where the other bacteria are repelled. The adhesion process is strengthened by production of adhesin and/or receptor-specific ligands on the surface or on structures extending from the cell surface, such as pili, fimbriae, and fibrillae. The method can differ in the same bacteria depending on the surface: “In the case of Vibrio cholerae El Tor, a toxin-coregulated pilus is used […] during the process of human infection, whereas […] adhesin [is] used to anchor to abiotic surfaces in an aquatic environment.“ Adhesion of a bacteria species can enhance the adhesion of another bacteria species by forming a heterogeneous biofilm. This makes it hard/impossible to prevent biofilms on certain surfaces even though they are known to cause infections, biofouling and corrosion. However, biofilms can also be beneficial for nitrogen fixation and bioremediation of wastewater. Summing up: Biofilms are universal and as Dunne wrote: “One has only to experience the process of cleaning a J trap in a clogged sink drain to fully appreciate the potential magnitude of bacterial biofilms and the process of biofouling on a small scale.” "Bacterial adhesion: seen any good biofilms lately?"
W. Michael Dunne Jr. Clinical microbiology reviews 15.2 (2002): 155-166. Which factors determine micro-arthropods (like mites and springtails) abundance and diversity in the soil? Stef Bokhorst et al. addressed this question in a paper which was published in 2014. They analyzed the effect of plant removals in pine forest sectors in Sweden which differ in their age (time scince last forest fire) on the micro-arthropod community living in the soil. With increasing forest age, the soil fertility and amount of fast growing plant species decreases while the humus layer and amount of slow growing plant species increases. The forest sectors in this experiments were between 44 and 364 years old. In each sector, Bokhorst et al. did the same two experiments: I) removal of feather mosses and dwarf shrubs as the two main understory functional groups (understory = plant life on the ground of the forest) and II) removal of specific dwarf shrub species. While feather mosses are thought to have a large effect on the soil ecosystem because of its temperature and humidity regulation effects, dwarf shrubs (something between herb and bush) influence the soil with their litter production. However, as leaves are also produced by trees, it doesn’t wonder that the removal of moss had larger influence on the abundance of micro-arthropods as the removal of dwarf shrubs (in experiment I). All in all, the dwarf shrubs species had just minor effects on the micro-arthropods abundance and diversity, independent of their litter quality (experiment II). So mites and springtails care mainly about moss. But what about the age of the forest which affects the properties of the soil? Interestingly, only a few groups of micro-arthropods were affected by the forrest age. The others don’t care. And this is already the summary of this article: Plant removal, forest age? If you don’t touch the moss, the micro-arthropods don’t care so much what you are doing there on the surface. "Impact of understory mosses and dwarf shrubs on soil micro-arthropods in a boreal forest chronosequence."
Stef Bokhorst, et al. Plant and soil 379.1-2 (2014): 121-133. When you look at a leaf you may think about photosynthesis, seasons, plant diseases,…, but have you ever thought about what is living on and in the leaf? The term phyllosphere describes leaves as habitat for different microorganisms. Fungi, yeasts, bacteria and bacteriophages can be found in and on leaves. Their interplay with the plant can be beneficial or pathological, dependend on plant and bacterial genetics and physical aspects like the weather. Leaves as habitat are no difficult habitat if you compare the temperature maximum and minimum or radiation with other environments at which you find microorganisms (e.g. hot springs). The difficulties of this habitat are the (rapid) day-night changes of temperature, radiation, rain and wind, and the short existence of the habitat of several weeks in annual plants because of seasons. Nevertheless, many different bacteria can be found on leaves and the dominant species may change with plant age or with environmental conditions. One example of leaf-living bacteria is Pseudomonas syringae and it has a real paradoxial way of rewarding its own success. In 2000, Susan S. Hirano and Christen D. Upper published a review about P.syringae with the focus on its role in the leaf ecosystem. P.syringae is a known pathogen which creates lesions. Besides the lesions, P. syringae is known for its ability to nucleate supercooled water to form ice which damages the plant. The probabilty of both, the lesion formation and ice-nucleation, increases with increasing population size. That means that if the leaf offers good conditions for successful reproduction for P.syringae, it destroys its own habitat by lesion formation and ice-nucleation. That is paradoxial! Therefore, the paper claims that these overpopulations are just accidents. “The real function of P.syringae is to live on healthy leaves.” So maybe there is just a sensitive interplay between the host plant and the bacteria which normally restricts the population size. “Only when conditions become unusually favorable and population sizes of the bacteria become too large does the entire system crash, to the detriment of both host and bacteria.” The question is why this happens. Why the plant can't prohibit this process and creates a favorable habitat for its enemy. And why has P.syringae no feedback mechanism, which prevents the destruction of its own habitat? Maybe P.syringae just doesn’t care as it immigrates by the plant seeds and by the wind… Bacteria in the Leaf Ecosystem with Emphasis onPseudomonas syringae—a Pathogen, Ice Nucleus, and Epiphyte.
Susan S. Hirano, and Christen D. Upper Microbiology and molecular biology reviews 64.3 (2000): 624-653. |
IdeaI love to increase my general science knowledge by reading papers from different fields of science. Here I share some of them. Archiv
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