We’ve all heard the term “bacteria” in our daily lives, whether it is in a TV advertisement or from a doctor. According to the typical layperson’s perspective, bacteria are tiny organisms that cause diseases in human beings. Additionally, this organism can only be visualized using a microscope and lacks many organelles found in the higher organisms. However, a recent study by Volland et al. 2022 (https://pubmed.ncbi.nlm.nih.gov/35737788/) and Hocher et al. 2023 (https://www.biorxiv.org/content/10.1101/2023.01.26.525422v1) disputes this long-held notion.
In the mangroves of Guadeloupe, researchers discovered a centimeter-long bacterium that could be visualized by the naked eye. Initially, researchers believed this bacterium to be an unidentified filament-like organism. However, upon careful analysis using fluorescence, X-ray, electron microscopy, and genome sequencing, it was finally characterized as the largest bacterium discovered so far, which is almost 50 times larger than any known bacterium. This bacterium was named Candidatus (Ca.) Thiomargarita magnifica, owing to the sulfur-rich environment where it is found.
In yet another interesting study, histone proteins have been discovered in bacteria. Histones are positively charged proteins that bind to negatively charged DNA and help in maintaining chromosome structure and controlling gene expression. These proteins are found in both eukaryotes as well as archaea. Further, our tiny friends’ bacteria were deemed to lack histone proteins. Bacteria in turn had NAPs (nucleoid-associated proteins). Interestingly, the signature structure histone fold is also observed in bacteria. This recent study suggests that histone proteins are found to be essential nucleoid components in the bacteria Bdellovibrio bacteriovorus and Leptospira interrogans. These scientific findings are very significant and play an important role in changing our perception of the microbial world.
Special features of the giant bacterium
Thiomargarita magnifica thrives in a sulfur-rich environment, attached to sunken leaves of Rhizophora mangle. These bacteria appear as whitish filaments similar in size to a human eyelash. To cater to its huge size, this bacterium has a large central vacuole that enables diffusion across the cell (the only mode of intracellular transport in a bacterium). The other organelles majorly lie beneath the cell envelope in “pepins” (membranous covering). ATP synthesis or the energy requirement of the cell is met by ATP synthase distribution around pepins and throughout the membrane. In contrast, in other bacteria, ATP synthase is present only in the cell envelope. Further, this bacterium is polyploid i.e., the cells contain a large number of genome copies. The number of genes in this giant bacterium is almost threefold more than that of normal prokaryotes. Moreover, as an adaptation to the large size, there is an increase in the number of cell elongation genes but an absence of some core cell division proteins. In addition to these unique characteristics, the bacterium is believed to exhibit a dimorphic life cycle (ability to switch between two morphologies) with the detachment of the apical bud from the filament and release into the environment. This also leads to asymmetric chromosome segregation. All these traits ensure that the giant bacterium has several adaptations to ensure this huge size.
Salient Features of Histones in Bacteria
Bdellovibrio bacteriovorus is a bacterial predator with a biphasic life cycle and the degree of compaction of the nucleoid varies through its life cycle. In such a case, the role of histones in this group seems to be very relevant. The bacterial histone protein BD0055 in Bdellovibrio bacteriovorus is structurally similar to eukaryotic and archaeal histones. The characteristic “histone fold” found in eukaryotes and archaea is also observed in BD0055. The only contradiction is that in these bacteria, histones wrap the DNA whereas, in eukaryotes, DNA winds around histones Further, this protein also coats linear DNA. Moreover, deletion of this gene BD0055 is lethal for the bacterium suggesting that this protein is essential for viability.
Similarities with other higher organisms
Apart from its large size, Thiomargarita magnifica has many features that resemble higher eukaryotes. DNA in bacteria is scattered throughout the cytosol and is termed the nucleoid. Also, no internal membranes are observed in all prokaryotes, including bacteria. Contrastingly, the DNA in Thiomargarita magnifica is compartmentalized in a membrane. Further, even ribosomes in this bacterium are compartmentalized. These compartments that house the DNA and ribosomes are termed pepins.
Bacteria were believed to possess only NAP (Nucleoid Associated Proteins), but histones seemed to be absent from the bacterial world. Strikingly, bacteria like Bdellovibrio bacteriovorus and Leptospira interrogans also have histone proteins. All of this suggests a similarity with eukaryotes.
Bacteria have always been believed to be tiny, invisible, disease-causing organisms. It was always surmised that the existence of these organisms can only be explored through a microscope. However, this recent study on giant bacteria has proved us wrong. The giant bacterium has evolved in myriad ways to ensure its survival. Notably, the origin of such evolution is still a mystery. It is unclear whether this size difference is an adaptation to survive among its other little partners or whether it is the result of a long-term evolutionary process that has produced numerous enormous bacteria. The identification of histones as a crucial bacterial protein also provides insight into the prokaryotic cells’ complex genetic machinery. The role of histone proteins in bacteria needs to be investigated further. Further studies might reveal whether horizontal gene transfer facilitated the transfer of histone genes from bacteria to other eukaryotes. To be more precise, bacteria are no more “the bag of enzymes” they used to be considered earlier. Future studies will help us get a better understanding of these tiny yet complex organisms.
Author: Dr. Dipika Mishra. I hold a Ph.D. degree in Life Sciences from the National Institute of Science Education and Research (NISER) and am a SciCom enthusiast (eLife Community Ambassador for the year 2019-2020). I have composed several scientific poems, some of which have also been featured in the Consilience journal, blogs of GYBN, and the Xylom. Also, I have editorial experience as I have been a part of the ASAPbio Preprint reviewer network and also CACTUS communications.