"IN SOME CASES, BACTERIAL INFECTION CAN BE ADVANTAGEOUS FOR AN INSECT BECAUSE IT SUPPLIES NUTRIENTS IT NEEDS"
Feb 18, 2021
There is a constant interaction between bacteria and their hosts, that is, we ourselves for example, which can come to host up to two kilograms of bacteria, above all in our intestines. Human intestinal microbiota and its effects on various diseases is one of her areas of research. In her scientific career, the study of aphids led her to research on the bacteria that have lived symbiotically with these insects for the last 200 million years. We talk with professor of Genetics Amparo Latorre who will participate in the scientific education series "Una Comunitat amb ciencia" to share her research on genetic sequencing and comparative genetics in a scientific field that turns out to be really fascinating. She has participated in the discovery of cases like that of the cedar aphid that hosts the endosymbiont with the smallest genome known up until then. Or one of the cockroaches who have a rich intestinal microbiota similar in complexity to mammals' intestines, which resulted in a new line of research: knowing if both systems (endosymbiont and intestinal microbiota) "talk" to each other in some way.
What do Lynn Margulis and her theory of symbiosis mean for cellular evolution?
There is no doubt that the endosymbiotic theory of Lynn Margulis, whom I had the pleasure to know and share some wonderful moments, meant a radical change in our understanding of the origin of the mitochondria and chloroplasts that took place in the formation of the eukaryotic cell. She proposed her theory at a time when it was difficult to corroborate. Perhaps because of that (and possibly for being a woman as well), she had serious difficulties getting them to accept it for publication. But she was indefatigable and devoted herself to the subject of symbiosis until the end. Currently, it has been demonstrated that the origin of mitochondria came from an alphaproteobacterium and plastos from a cyanobacterium.
What kinds of symbiosis exist in insects? How are they researched?
In regard to the benefit or harm that symbionts can cause to the host, symbiosis is defined as mutualism when both the host and the symbiont benefit, parasitism when one benefits at the cost of the other, and commensalism when one benefits and the other is unharmed. In addition, they can be classified according to their location within the insect, their degree of dependence, etc. Our research started with the study of the aphid/endosymbiont system. Later we wanted to know if the case was similar in other models, and we studied carpenter ants, the white fly, cochineals and cockroaches. We carried out this last study precisely because we did not understand how some insects who live on a complex diet (they're omnivores) carried an endosymbiont, whose role is nutritional, providing the insect with the nutrients they cannot get from their diet. The discovery that cockroaches, in addition to an endosymbiont, have a rich intestinal microbiota similar in complexity to what happens in the intestine of mammals, has led to a new line of research in which we are trying to understand if both systems (endosymbiont and intestinal microbiota) "talk" to each other in some way. As far as we know, it is the only case of an insect where the two kinds of symbionts coexist. Ants, in comparison with cockroaches, have lost their endosymbiont and have only a very specialised intestinal microbiota. The research is based mainly on the sequencing of the genome and comparative genetics, but we also use other methodologies, like microscopy, hybridisation with probes, "omic" technologies (metagenomics, metatranscriptomics, metaproteomics and metabolomics). In addition, we treat the insects with antibiotics or diets poor in nitrogen, and measure biological efficiency parameters.
You say that the relationship can be so close that the bacteria are found "abducted" into the host's specialized cells, the bacteriomes. What happens?
The insects that have a relationship of enforced mutualism with one (or a few) bacterial endosymbionts stand out because they have developed some specialised cells, which are thus eukaryotic, where they host the bacteria. This is a process that happens in development in which, on one side, the insect's genes have been specialised to develop these cells and, on the other, a few bacteria infect these cells with eggs or embryos (in the case of parthenogenetic insects) in the females in order to be passed on to the following generation. It is, thus, a maternal inheritance similar to what happens in mitochondria. It is still not clear how the immune system of the insect fails to recognise these bacteria at the moment of infection, when they are freely found in the cytoplasm. The data, we have, seem to indicate that the process is not universal and that each lineage has developed different mechanisms to ensure the proper transmission.
Some of the cases you analyse take place in aphids. What examples or models are you investigating?
Like the previous case, some studies have led to others. The problem with studying endosymbionts is that they cannot be cultured, which means that the research cannot be carried out in the traditional way that bacteria have been studied in the field of microbiology. As a result, at the beginning of the research we resort to genetics in order to learn their genetic content and derive their functions in that way. We cooperate with entomologists to collect aphids of different lineages, with different lifecycles, etc. Thus, we began sequencing the genome of the endosymbiont of an aphid from a distant family from the one studied up until then, and started the field of comparative genetics. Later we studied representatives from other families, and it was then we found ourselves with the case of a cedar aphid that hosted the endosymbiont with the smallest genome known up to then. This marked a milestone in the field, and we are still devoted to studying other members of the family, and we have found some fascinating cases. In addition, the group has participated in the sequencing of the genome of some aphids.
Last year they discovered a new gene family of antimicrobial proteins in the German cockroach which would explain these insects' adaptation to unhealthy environments. What does the work entail?
The genes that encode the antimicrobial peptides, one of the defence mechanisms against microbial infections, are small and difficult to detect in the genomes. In this work the genome of the Blattella germanica has been used and a transcriptome of adult females to define the genetic repertory. It has been shown that the species has 39 genes that encode five kinds of antimicrobial peptides, a number considerably greater than in the majority of insect species. A new kind of genes has been detected that is called blattellicins. These genes are an evolutionary innovation that has emerged in the Blattella lineage, and the proteins derived contain, in addition to a domain with an antimicrobial function (atacine domain), a long strand of amino acids made up of glutaminic acids and glutamines, whose function has not been described yet. The genes are only expressed in adults.
Why is there a bacterium that can become a parasite, but instead, in other symbioses can have positive effects for the host insect?
That is a widely debated and still unresolved issue. If anything, the opposite could happen. As I have commented earlier, all we eukaryotes have defence systems against micro-organisms. When these don't work, the bacteria are pathogens. In some cases, bacterial infection can turn out to be advantageous for the insect because it supplies nutrients it needs. In that case, then, a clear case of co-evolution is produced in which the host and the symbiont need each other, continuing to evolve to the mutualist symbiosis we see currently. It is clear that over the course of evolution there must have been many attempts on the part of ancestral insects to colonise new niches, but that, if they hadn't had the contribution of nutrients from some bacterium, they would not have succeeded. There are examples of bacteria of the same genus that are pathenogenic for some insects, but mutualistic for others, or even for other organisms.
Your research revolves around the study of human intestinal microbiota and its impact on various diseases. To what degree is our health affected by these micro-organisms?
Our knowledge in the field of symbiosis led us to collaborate in studies of human intestinal microbiota and its relationship with different illnesses. This is a field that has evolved a lot in a few years, again thanks to the "omic" and bioinformatic technologies in dealing with the large quantity and complexity of the data gathered. We know that the intestinal microbiota is beneficial and needed for health, and we know, in addition, that its composition can vary according to various parameters such as age, diet, the environment, individual genetic makeup, etc. When there is an imbalance in the composition required as the result of disruption, for example, when taking antibiotics, this composition changes and can affect our health. A specific case on which we have worked is the bacterium Clostidium difficile, which, although present in very small amounts in the intestine, has its activity controlled by other bacteria. If this one disappears or is reduced, illness results.