A Metabolic Pathway for Activation of Dietary Glucosinolates by a Human Gut Symbiont.

Consumption of glucosinolates, pro-drug-like metabolites abundant in Brassica vegetables, has been associated with decreased risk of certain cancers. Gut microbiota have the ability to metabolize glucosinolates, generating chemopreventive isothiocyanates. Here, we identify a genetic and biochemical basis for activation of glucosinolates to isothiocyanates by Bacteroides thetaiotaomicron, a prominent gut commensal species. Using a genome-wide transposon insertion screen, we identified an operon required for glucosinolate metabolism in B. thetaiotaomicron. Expression of BT2159-BT2156 in a non-metabolizing relative, Bacteroides fragilis, resulted in gain of glucosinolate metabolism. We show that isothiocyanate formation requires the action of BT2158 and either BT2156 or BT2157 in vitro. Monocolonization of mice with mutant BtΔ2157 showed reduced isothiocyanate production in the gastrointestinal tract. These data provide insight into the mechanisms by which a common gut bacterium processes an important dietary nutrient.

Outer membrane utilisomes mediate glycan uptake in gut Bacteroidetes.

Bacteroidetes are abundant members of the human microbiota, utilizing a myriad of diet- and host-derived glycans in the distal gut1. Glycan uptake across the bacterial outer membrane of these bacteria is mediated by SusCD protein complexes, comprising a membrane-embedded barrel and a lipoprotein lid, which is thought to open and close to facilitate substrate binding and transport. However, surface-exposed glycan-binding proteins and glycoside hydrolases also play critical roles in the capture, processing and transport of large glycan chains. The interactions between these components in the outer membrane are poorly understood, despite being crucial for nutrient acquisition by our colonic microbiota. Here we show that for both the levan and dextran utilization systems of Bacteroides thetaiotaomicron, the additional outer membrane components assemble on the core SusCD transporter, forming stable glycan-utilizing machines that we term utilisomes. Single-particle cryogenic electron microscopy structures in the absence and presence of substrate reveal concerted conformational changes that demonstrate the mechanism of substrate capture, and rationalize the role of each component in the utilisome.

Human gut bacteria tailor extracellular vesicle cargo for the breakdown of diet- and host-derived glycans.

Extracellular vesicles are produced in all three domains of life, and their biogenesis has common ancient origins in eukaryotes and archaea. Although bacterial vesicles were discovered several decades ago and multiple roles have been attributed to them, no mechanism has been established for vesicles biogenesis in bacteria. For this reason, there is a significant level of skepticism about the biological relevance of bacterial vesicles. Bacteroides thetaiotaomicron (Bt), a prominent member of the human intestinal microbiota, produces significant amounts of outer membrane vesicles (OMVs) which have been proposed to play key physiological roles. Here, we employed a dual marker system, consisting of outer membrane- and OMV-specific markers fused to fluorescent proteins to visualize OMV biogenesis by time-lapse microscopy. Furthermore, we performed comparative proteomic analyses to show that, in Bt, the OMV cargo is adapted for the optimal utilization of different polysaccharides. We also show that a negatively charged N-terminal motif acts as a signal for protein sorting into OMVs irrespective of the nutrient availability. Our results demonstrate that OMV production is the result of a highly regulated process in Bt.

Mapping human microbiome drug metabolism by gut bacteria and their genes.

Individuals vary widely in their responses to medicinal drugs, which can be dangerous and expensive owing to treatment delays and adverse effects. Although increasing evidence implicates the gut microbiome in this variability, the molecular mechanisms involved remain largely unknown. Here we show, by measuring the ability of 76 human gut bacteria from diverse clades to metabolize 271 orally administered drugs, that many drugs are chemically modified by microorganisms. We combined high-throughput genetic analyses with mass spectrometry to systematically identify microbial gene products that metabolize drugs. These microbiome-encoded enzymes can directly and substantially affect intestinal and systemic drug metabolism in mice, and can explain the drug-metabolizing activities of human gut bacteria and communities on the basis of their genomic contents. These causal links between the gene content and metabolic activities of the microbiota connect interpersonal variability in microbiomes to interpersonal differences in drug metabolism, which has implications for medical therapy and drug development across multiple disease indications.

Multiple TonB homologs are important for carbohydrate utilization by Bacteroides thetaiotaomicron.

IMPORTANCE: The human gut microbiota, including Bacteroides, is required for the degradation of otherwise undigestible polysaccharides. The gut microbiota uses polysaccharides as an energy source, and fermentation products such as short-chain fatty acids are beneficial to the human host. This use of polysaccharides is dependent on the proper pairing of a TonB protein with polysaccharide-specific TonB-dependent transporters; however, the formation of these protein complexes is poorly understood. In this study, we examine the role of 11 predicted TonB homologs in polysaccharide uptake. We show that two proteins, TonB4 and TonB6, may be functionally redundant. This may allow for the development of drugs targeting Bacteroides species containing only a TonB4 homolog with limited impact on species encoding the redundant TonB6.

Comparative genomics provides structural and functional insights into Bacteroides RNA biology.

Bacteria employ noncoding RNA molecules for a wide range of biological processes, including scaffolding large molecular complexes, catalyzing chemical reactions, defending against phages, and controlling gene expression. Secondary structures, binding partners, and molecular mechanisms have been determined for numerous small noncoding RNAs (sRNAs) in model aerobic bacteria. However, technical hurdles have largely prevented analogous analyses in the anaerobic gut microbiota. While experimental techniques are being developed to investigate the sRNAs of gut commensals, computational tools and comparative genomics can provide immediate functional insight. Here, using Bacteroides thetaiotaomicron as a representative microbiota member, we illustrate how comparative genomics improves our understanding of RNA biology in an understudied gut bacterium. We investigate putative RNA-binding proteins and predict a Bacteroides cold-shock protein homolog to have an RNA-related function. We apply an in silico protocol incorporating both sequence and structural analysis to determine the consensus structures and conservation of nine Bacteroides noncoding RNA families. Using structure probing, we validate and refine these predictions and deposit them in the Rfam database. Through synteny analyses, we illustrate how genomic coconservation can serve as a predictor of sRNA function. Altogether, this work showcases the power of RNA informatics for investigating the RNA biology of anaerobic microbiota members.

A Possible Aquatic Origin of the Thiaminase TenA of the Human Gut Symbiont Bacteroides thetaiotaomicron.

TenA thiamin-degrading enzymes are commonly found in prokaryotes, plants, fungi and algae and are involved in the thiamin salvage pathway. The gut symbiont Bacteroides thetaiotaomicron (Bt) produces a TenA protein (BtTenA) which is packaged into its extracellular vesicles. An alignment of BtTenA protein sequence with proteins from different databases using the basic local alignment search tool (BLAST) and the generation of a phylogenetic tree revealed that BtTenA is related to TenA-like proteins not only found in a small number of intestinal bacterial species but also in some aquatic bacteria, aquatic invertebrates, and freshwater fish. This is, to our knowledge, the first report describing the presence of TenA-encoding genes in the genome of members of the animal kingdom. By searching metagenomic databases of diverse host-associated microbial communities, we found that BtTenA homologues were mostly represented in biofilms present on the surface of macroalgae found in Australian coral reefs. We also confirmed the ability of a recombinant BtTenA to degrade thiamin. Our study shows that BttenA-like genes which encode a novel sub-class of TenA proteins are sparingly distributed across two kingdoms of life, a feature of accessory genes known for their ability to spread between species through horizontal gene transfer.

Complex pectin metabolism by gut bacteria reveals novel catalytic functions.

The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiota. It is unclear, however, whether bacterial consortia or single organisms are required to depolymerize highly complex glycans. Here we show that the gut bacterium Bacteroides thetaiotaomicron uses the most structurally complex glycan known: the plant pectic polysaccharide rhamnogalacturonan-II, cleaving all but 1 of its 21 distinct glycosidic linkages. The deconstruction of rhamnogalacturonan-II side chains and backbone are coordinated to overcome steric constraints, and the degradation involves previously undiscovered enzyme families and catalytic activities. The degradation system informs revision of the current structural model of rhamnogalacturonan-II and highlights how individual gut bacteria orchestrate manifold enzymes to metabolize the most challenging glycan in the human diet.

The intracellular proteome of the gut bacterium Bacteroides thetaiotaomicron is widely unaffected by a switch from glucose to sucrose as main carbohydrate source.

Bacteroides thetaiotaomicron is a gram negative bacterium within the human gut microbiome that metabolizes a wide range of dietary and mucosal polysaccharides. Here, we analyze the proteome response of B. thetaiotaomicron cultivated on two different carbon sources, glucose and sucrose. Two quantitative LC-MS based proteomics approaches, encompassing label free quantification and isobaric labeling by tandem mass tags were applied. The results obtained by both workflows were compared with respect to the number of identified and quantified proteins, peptides supporting identification and quantification, sequence coverage, and reproducibility. A total of 1719 and 1696 proteins, respectively, were quantified, covering 35 % of the predicted B. thetaiotaomicron proteome. The data show that B. thetaiotaomicron widely maintains its intracellular proteome upon change of the carbohydrates and that major changes are observed solely in the machinery necessary to make use of the carbon sources provided. With respect to the central role of carbohydrates on gut health these data contribute to the understanding of how different carbohydrates contribute to shape bacterial community in the gut microbiome. All proteomics raw data have been uploaded to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD033704.

Identification and characterization of 3-ketosphinganine reductase activity encoded at the BT_0972 locus in Bacteroides thetaiotaomicron.

Bacterial sphingolipid synthesis is important for the fitness of gut commensal bacteria with an implied potential for regulating mammalian host physiology. Multiple steps in bacterial sphingolipid synthesis pathways have been characterized previously, with the first step of de novo sphingolipid synthesis being well conserved between bacteria and eukaryotes. In mammals, the subsequent step of de novo sphingolipid synthesis is catalyzed by 3-ketosphinganine reductase, but the protein responsible for this activity in bacteria has remained elusive. In this study, we analyzed the 3-ketosphinganine reductase activity of several candidate proteins in Bacteroides thetaiotaomicron chosen based on sequence similarity to the yeast 3-ketosphinganine reductase gene. We further developed a metabolomics-based 3-ketosphinganine reductase activity assay, which revealed that a gene at the locus BT_0972 encodes a protein capable of converting 3-ketosphinganine to sphinganine. Taken together, these results provide greater insight into pathways for bacterial sphingolipid synthesis that can aid in future efforts to understand how microbial sphingolipid synthesis modulates host-microbe interactions.

Distinct protein architectures mediate species-specific beta-glucan binding and metabolism in the human gut microbiota.

Complex glycans that evade our digestive system are major nutrients that feed the human gut microbiota (HGM). The prevalence of Bacteroidetes in the HGM of populations worldwide is engendered by the evolution of polysaccharide utilization loci (PULs), which encode concerted protein systems to utilize the myriad complex glycans in our diets. Despite their crucial roles in glycan recognition and transport, cell-surface glycan-binding proteins (SGBPs) remained understudied cogs in the PUL machinery. Here, we report the structural and biochemical characterization of a suite of SGBP-A and SGBP-B structures from three syntenic ß(1,3)-glucan utilization loci (1,3GULs) from Bacteroides thetaiotaomicron (Bt), Bacteroides uniformis (Bu), and B. fluxus (Bf), which have varying specificities for distinct ß-glucans. Ligand complexes provide definitive insight into ß(1,3)-glucan selectivity in the HGM, including structural features enabling dual ß(1,3)-glucan/mixed-linkage ß(1,3)/ß(1,4)-glucan-binding capability in some orthologs. The tertiary structural conservation of SusD-like SGBPs-A is juxtaposed with the diverse architectures and binding modes of the SGBPs-B. Specifically, the structures of the trimodular BtSGBP-B and BuSGBP-B revealed a tandem repeat of carbohydrate-binding module-like domains connected by long linkers. In contrast, BfSGBP-B comprises a bimodular architecture with a distinct ß-barrel domain at the C terminus that bears a shallow binding canyon. The molecular insights obtained here contribute to our fundamental understanding of HGM function, which in turn may inform tailored microbial intervention therapies.

Ruminococcus bromii enables the growth of proximal Bacteroides thetaiotaomicron by releasing glucose during starch degradation.

Complex carbohydrates shape the gut microbiota, and the collective fermentation of resistant starch by gut microbes positively affects human health through enhanced butyrate production. The keystone species Ruminococcus bromii (Rb) is a specialist in degrading resistant starch; its degradation products are used by other bacteria including Bacteroides thetaiotaomicron (Bt). We analysed the metabolic and spatial relationships between Rb and Bt during potato starch degradation and found that Bt utilizes glucose that is released from Rb upon degradation of resistant potato starch and soluble potato amylopectin. Additionally, we found that Rb produces a halo of glucose around it when grown on solid media containing potato amylopectin and that Bt cells deficient for growth on potato amylopectin (∆sus Bt) can grow within the halo. Furthermore, when these ∆sus Bt cells grow within this glucose halo, they have an elongated cell morphology. This long-cell phenotype depends on the glucose concentration in the solid media: longer Bt cells are formed at higher glucose concentrations. Together, our results indicate that starch degradation by Rb cross-feeds other bacteria in the surrounding region by releasing glucose. Our results also elucidate the adaptive morphology of Bt cells under different nutrient and physiological conditions.

The aerobic electron flux is deficient in fumarate respiration of a strict anaerobe Bacteroides thetaiotaomicron.

Why oxygen ceases the growth of strictly anaerobic bacteria is a longstanding question, yet the answer remains unclear. Studies have confirmed that the dehydratase-fumarase containing an iron-sulfur cluster ([4Fe-4S]) is inactivated upon exposure to oxygen in the intestinal obligate anaerobe, Bacteroides thetaiotaomicron (B. thetaiotaomicron); this blocks fumarate respiration, which is the essential energy-producing pathway in anaerobes. Here, we substituted the [4Fe-4S]-dependent fumarase in B. thetaiotaomicron with an iron-free isozyme from E. coli (Ec-FumC). Results show that Ec-FumC successfully performed the catalytic function of fumarase in B. thetaiotaomicron, as the fum-mutant strain that expressed Ec-FumC exhibited succinate-producing ability under anaerobic growth conditions. Ec-FumC is oxygen-resistant and remains active to produce fumarate upon aeration; however, B. thetaiotaomicron mutant that expressed Ec-FumC did not convert fumarate to succinate during air exposure. Biochemical assays of inverted membrane vesicles from wild-type B. thetaiotaomicron confirmed that the electron flux from NADH to fumarate was less efficient in the presence of air as compared to that without oxygen. Our findings suggest that the anaerobic fumarate respiration might be paralyzed due to electron dissipations upon aeration of the obligate anaerobe.

Polysaccharide Capsules Equip the Human Symbiont Bacteroides thetaiotaomicron to Modulate Immune Responses to a Dominant Antigen in the Intestine.

Bacteria express multiple diverse capsular polysaccharides (CPSs) for protection against environmental and host factors, including the host immune system. Using a mouse TCR transgenic CD4+ T cell, BθOM, that is specific for B. thetaiotaomicron and a complete set of single CPS-expressing B. thetaiotaomicron strains, we ask whether CPSs can modify the immune responses to specific bacterial Ags. Acapsular B. thetaiotaomicron, which lacks all B. thetaiotaomicron CPSs, stimulated BθOM T cells more strongly than wild-type B. thetaiotaomicron Despite similar levels of BθOM Ag expression, many single CPS-expressing B. thetaiotaomicron strains were antistimulatory and weakly activated BθOM T cells, but a few strains were prostimulatory and strongly activated BθOM T cells just as well or better than an acapsular strain. B. thetaiotaomicron strains that expressed an antistimulatory CPS blocked Ag delivery to the immune system, which could be rescued by Fc receptor-dependent Ab opsonization. All single CPS-expressing B. thetaiotaomicron strains stimulated the innate immune system to skew toward M1 macrophages and release inflammatory cytokines in an MyD88-dependent manner, with antistimulatory CPS activating the innate immune system in a weaker manner than prostimulatory CPS. The expression of antistimulatory versus prostimulatory CPSs on outer membrane vesicles also regulated immune responses. Moreover, antistimulatory and prostimulatory single CPS-expressing B. thetaiotaomicron strains regulated the activation of Ag-specific and polyclonal T cells as well as clearance of dominant Ag in vivo. These studies establish that the immune responses to specific bacterial Ags can be modulated by a diverse set of CPSs.

A Novel Family of RNA-Binding Proteins Regulate Polysaccharide Metabolism in Bacteroides thetaiotaomicron.

Human gut microbiome composition is constantly changing, and diet is a major driver of these changes. Gut microbial species that persist in mammalian hosts for long periods of time must possess mechanisms for sensing and adapting to nutrient shifts to avoid being outcompeted. Global regulatory mechanisms mediated by RNA-binding proteins (RBPs) that govern responses to nutrient shifts have been characterized in Proteobacteria and Firmicutes but remain undiscovered in the Bacteroidetes. Here, we report the identification of RBPs that are broadly distributed across the Bacteroidetes, with many genomes encoding multiple copies. Genes encoding these RBPs are highly expressed in many Bacteroides species. A purified RBP, RbpB, from Bacteroides thetaiotaomicron binds to single-stranded RNA in vitro with an affinity similar to other characterized regulatory RBPs. B. thetaiotaomicron mutants lacking RBPs show dramatic shifts in expression of polysaccharide utilization and capsular polysaccharide loci, suggesting that these RBPs may act as global regulators of polysaccharide metabolism. A B. thetaiotaomicron ΔrbpB mutant shows a growth defect on dietary sugars belonging to the raffinose family of oligosaccharides (RFOs). The ΔrbpB mutant had reduced expression of BT1871, encoding a predicted RFO-degrading melibiase, compared to the wild-type strain. Mutation of BT1871 confirmed that the enzyme it encodes is essential for growth on melibiose and promotes growth on the RFOs raffinose and stachyose. Our data reveal that RbpB is required for optimal expression of BT1871 and other polysaccharide-related genes, suggesting that we have identified an important new family of global regulatory proteins in the Bacteroidetes. IMPORTANCE The human colon houses hundreds of bacterial species, including many belonging to the genus Bacteroides, that aid in breaking down our food to keep us healthy. Bacteroides have many genes responsible for breaking down different dietary carbohydrates, and complex regulatory mechanisms ensure that specific genes are only expressed when the right carbohydrates are available. In this study, we discovered that Bacteroides use a family of RNA-binding proteins as global regulators to coordinate expression of carbohydrate utilization genes. The ability to turn different carbohydrate utilization genes on and off in response to changing nutrient conditions is critical for Bacteroides to live successfully in the gut, and thus the new regulators we have identified may be important for life in the host.

Do reactive oxygen species or does oxygen itself confer obligate anaerobiosis? The case of Bacteroides thetaiotaomicron.

Bacteroides thetaiotaomicron was examined to determine whether its obligate anaerobiosis is imposed by endogenous reactive oxygen species or by molecular oxygen itself. Previous analyses established that aerated B. thetaiotaomicron loses some enzyme activities due to a high rate of endogenous superoxide formation. However, the present study establishes that another key step in central metabolism is poisoned by molecular oxygen itself. Pyruvate dissimilation was shown to depend upon two enzymes, pyruvate:formate lyase (PFL) and pyruvate:ferredoxin oxidoreductase (PFOR), that lose activity upon aeration. PFL is a glycyl-radical enzyme whose vulnerability to oxygen is already understood. The rate of PFOR damage was unaffected by the level of superoxide or peroxide, showing that molecular oxygen itself is the culprit. The cell cannot repair PFOR, which amplifies the impact of damage. The rates of PFOR and fumarase inactivation are similar, suggesting that superoxide dismutase is calibrated so the oxygen- and superoxide-sensitive enzymes are equally sensitive to aeration. The physiological purpose of PFL and PFOR is to degrade pyruvate without disrupting the redox balance, and they do so using catalytic mechanisms that are intrinsically vulnerable to oxygen. In this way, the anaerobic excellence and oxygen sensitivity of B. thetaiotaomicron are two sides of the same coin.

Phase-variable bacteria simultaneously express multiple capsules.

Capsular polysaccharides (CPSs) protect bacteria from host and environmental factors. Many bacteria can express different CPSs and these CPSs are phase variable. For example, Bacteroides thetaiotaomicron (B. theta) is a prominent member of the human gut microbiome and expresses eight different capsular polysaccharides. Bacteria, including B. theta, have been shown to change their CPSs to adapt to various niches such as immune, bacteriophage, and antibiotic perturbations. However, there are limited tools to study CPSs and fundamental questions regarding phase variance, including if gut bacteria can express more than one capsule at the same time, remain unanswered. To better understand the roles of different CPSs, we generated a B. theta CPS1-specific antibody and a flow cytometry assay to detect CPS expression in individual bacteria in the gut microbiota. Using these novel tools, we report for the first time that bacteria can simultaneously express multiple CPSs. We also observed that nutrients such as glucose and salts had no effect on CPS expression. The ability to express multiple CPSs at the same time may provide bacteria with an adaptive advantage to thrive amid changing host and environmental conditions, especially in the intestine.

Endogenous superoxide is a key effector of the oxygen sensitivity of a model obligate anaerobe.

It has been unclear whether superoxide and/or hydrogen peroxide play important roles in the phenomenon of obligate anaerobiosis. This question was explored using Bacteroides thetaiotaomicron, a major fermentative bacterium in the human gastrointestinal tract. Aeration inactivated two enzyme families-[4Fe-4S] dehydratases and nonredox mononuclear iron enzymes-whose homologs, in contrast, remain active in aerobic Escherichia coli Inactivation-rate measurements of one such enzyme, B. thetaiotaomicron fumarase, showed that it is no more intrinsically sensitive to oxidants than is an E. coli fumarase. Indeed, when the E. coli enzymes were expressed in B. thetaiotaomicron, they no longer could tolerate aeration; conversely, the B. thetaiotaomicron enzymes maintained full activity when expressed in aerobic E. coli Thus, the aerobic inactivation of the B. thetaiotaomicron enzymes is a feature of their intracellular environment rather than of the enzymes themselves. B. thetaiotaomicron possesses superoxide dismutase and peroxidases, and it can repair damaged enzymes. However, measurements confirmed that the rate of reactive oxygen species production inside aerated B. thetaiotaomicron is far higher than in E. coli Analysis of the damaged enzymes recovered from aerated B. thetaiotaomicron suggested that they had been inactivated by superoxide rather than by hydrogen peroxide. Accordingly, overproduction of superoxide dismutase substantially protected the enzymes from aeration. We conclude that when this anaerobe encounters oxygen, its internal superoxide levels rise high enough to inactivate key catabolic and biosynthetic enzymes. Superoxide thus comprises a major element of the oxygen sensitivity of this anaerobe. The extent to which molecular oxygen exerts additional direct effects remains to be determined.

Structure of a gut microbial diltiazem-metabolizing enzyme suggests possible substrate binding mode.

When administrated orally, the vasodilating drug diltiazem can be metabolized into diacetyl diltiazem in the presence of Bacteroides thetaiotaomicron, a human gut microbe. The removal of acetyl group from the parent drug is carried out by the GDSL/SGNH-family hydrolase BT4096. Here the crystal structure of the enzyme was solved by mercury soaking and single-wavelength anomalous diffraction. The protein folds into two parts. The N-terminal part comprises the catalytic domain which is similar to other GDSL/SGNH hydrolases. The flanking C-terminal part is made up of a ß-barrel subdomain and an α-helical subdomain. Structural comparison shows that the catalytic domain is most akin to acetyl-xylooligosaccharide esterase and allows a plausible binding mode of diltiazem to be proposed. The ß-barrel subdomain is similar in topology to the immunoglobulin-like domains, including some carbohydrate-binding modules, of various bacterial glycoside hydrolases. Consequently, BT4096 might originally function as an oligosaccharide deacetylase with additional subdomains that could enhance substrate binding, and it acts on diltiazem just by accident.

Nutrient-dependent morphological variability of Bacteroides thetaiotaomicron.

Unique morphologies can enable bacteria to survive in their native environment. Furthermore, many bacteria change their cell shape to adapt to different environmental conditions. For instance, some bacteria increase their surface area under carbon or nitrogen starvation. Bacteriodes thetaiotaomicron is an abundant human gut species; it efficiently degrades a number of carbohydrates and also supports the growth of other bacteria by breaking down complex polysaccharides. The gut provides a variable environment as nutrient availability is subject to the diet and health of the host, yet how gut bacteria adapt and change their morphologies under different nutrient conditions has not been studied. Here, for the first time, we report an elongated B. thetaiotaomicron morphology under sugar-limited conditions using live-cell imaging; this elongated morphology is enhanced in the presence of sodium bicarbonate. Similarly, we also observed that sodium bicarbonate produces an elongated-length phenotype in another Gram-negative gut bacterium, Escherichia coli. The increase in cell length might provide an adaptive advantage for cells to survive under nutrient-limited conditions.