Biology subjects

Ledda, A.

Publications and source records attributed to Ledda, A..

8 recordsLinked to original sources

bioRxiv2026

Natural selection driven by escape from shifting antibody classes shapes SARS-CoV-2 evolution

The phenotypic fitness landscape defines the action of natural selection on pathogens, linking changes in their phenotypes to transmission and evolution. The rapidly changing nature of epidemic spread and antigenic landscapes pushes viruses to evolve on fitness seascapes. As a result, evolution of viruses such as SARS-CoV-2 proceeds in a neverending series of waves, driven by epistatic interactions and by the arms race between viral adaptation and human immunity. Phenotypic characterisation of these rapidly changing fitness seascapes is an open challenge. Using a Phenotypic Selection Inference framework that links phylogenetic estimates of mutation fitness effects with deep mutational scanning data, we traced how selective pressures on viral phenotypes have shifted throughout the COVID-19 pandemic. Natural selection has favoured enhanced ACE2 binding since the emergence of SARS-COV-2, with relatively constant selective pressure even for the most recent variants. The strength of selection for antibody escape was comparable to ACE2 binding during early evolution, but as population immunity rose, escape from class 3 and then class 2 antibodies became dominant. For variants circulating in 2024, natural selection shifted toward class 3 antibody escape, while those circulating in 2025 have experienced dynamic, rapidly changing pressures for escape from all antibody classes. These transitions reflect an ongoing arms race between viral adaptation and human immunity. Our findings reveal that SARS-CoV-2 antigenic evolution is governed by dynamic, class-specific immune pressures, and that selection for replication capacity has been continuously present during the pandemic, presumably to compensate for the effects of antigenic escape on viral replication. Our approach for the inference of phenotypic selection provides a framework to understand and anticipate the evolution of future variants.

evolutionary biology↗

bioRxiv2025

Nanopore long-read only genome assembly of clinical Enterobacterales isolates is complete and accurate

Whole bacterial genome sequence reconstruction using Oxford Nanopore Technologies ("Nanopore") long-read only sequencing may offer a lower-cost, higher-throughput alternative for pathogen surveillance to hybrid assembly with recent improvements in Nanopore sequencing accuracy. We evaluated the accuracy, including plasmid reconstruction, of Nanopore long-read only genome assemblies of Enterobacterales. We sequenced 92 genomes from clinical Enterobacterales isolates, collected in England under a national surveillance program, with long-read Nanopore (R10.4.1, Dorado v5.0.0 super-high-accuracy basecalled) and short-read Illumina (NovaSeq) sequencing approaches. Genomes were assembled using three long-read only (Flye; Hybracter long; Autocycler), and three hybrid assemblers (Hybracter hybrid; Unicycler normal; bold). Three polishing modalities (Medaka v2 with subsampled or un-subsampled long-reads; Polypolish + Pypolca with short-reads) were investigated. Autocycler circularised the most chromosomes (87/92 [95%]). Plasmid sequence reconstruction was comparable between all assemblers except Flye, all recovering 90-96% of plasmids, although the ground truth was uncertain. Flye performed worse than other assemblers on almost all metrics. Autocycler + Medaka (un-subsampled long-reads) was the most accurate long-read only assembler/polisher combination, comparable to hybrid assemblies (median 0 [IQR:0-0] SNPs and 0 [IQR:0-1] indels per genome; quality value/Q score, 100 [IQR: 64-100]), with only 4/92 genome sequences having >10 SNPs/indels. Medaka polishing with un-subsampled long-reads resulted in small improvements in indels but not SNPs for both Flye and Autocycler assemblies. Seven-locus MLST, antimicrobial resistance, virulence, and stress gene annotation was equivalent across assembler/polisher combinations. Nanopore long-read only bacterial genome assembly with Autocycler combined with Medaka polishing (using un-subsampled reads) is similarly accurate and possibly more complete than hybrid assemblies, representing a viable alternative for incorporating high-quality genomic data, including plasmids, into Enterobacterales surveillance. Data SummaryNanopore long-reads and Illumina short-reads from the 92 Enterobacterales isolates from this study have been uploaded to ENA (BioProject accession: PRJEB93885). Code for the Nextflow assembly pipeline, downstream analysis scripts, and R statistical analysis scripts are available on GitHub (https://github.com/oxfordmmm/NEKSUS_ont_hybrid_assembly_comparison). The following supplementary data tables are available on FigShare (https://figshare.com/account/home#/projects/253775): O_LIENA Sample accessions and sample metadata (accessions_and_metadata.csv) C_LIO_LISeqkit stats summaries of the Illumina and Nanopore reads (raw_qc_sup.cav) C_LIO_LISummary of assembly contig features (contigs_summary_sup_cleaned.csv) C_LIO_LIPairwise mash distances between contigs (mash_cleaned.csv) C_LIO_LIPlasmids matching across different assemblers compared to the Hybracter (hybrid) and manually-curated reference sets (plasmids_match_hybracter_mash.csv; plasmids_match_manual_mash.csv, respectively) C_LIO_LISeven-locus multi-locus sequence type annotation (mlst_cleaned.csv) C_LIO_LICheckM2 summaries of assemblies (checkm2_cleaned.csv) C_LIO_LINucleotide-level accuracy of assemblies (SNP, Indels, and Quality value compared to short-read mapping; assembly_nucleotide_accuracy_cleaned.csv) C_LIO_LIBakta annotation (bakta_by_contig_cleaned.csv) C_LIO_LIAMRFinderPlus annotations of contigs (amrfinder_plus_cleaned.csv) C_LIO_LIMOB-suite annotation summaries of contigs (mobsuite_cleaned.csv) C_LI Impact StatementNanopore long-reads have historically been too error-prone to use alone for accurate bacterial genome assembly, necessitating additional Illumina short-reads to achieve structurally complete and accurate hybrid genome assemblies for public health surveillance. This increases cost and complexity. Previous studies have shown that recent improvements in Nanopore chemistry (R10.4.1 flowcell) and basecalling (super-high accuracy) allow high-quality long-read only assemblies on a small number of laboratory reference strains. This is the first evaluation, to our knowledge, to assess Nanopore long-read only genome assembly compared with hybrid assembly on a large number of clinical isolates. In addition, this is the first large-scale evaluation of the recently released automated consensus long-read assembly tool, Autocycler. We show that Autocycler long-read only assemblies are more structurally complete for chromosomal sequences, while reconstructing a similar number of plasmids to other long-read and hybrid assemblers. Most long-read polished, Autocycler-assembled genome sequences have 0 errors (median: 0 SNPs/indels) relative to a short-read polished (hybrid) Autocycler assemblies, enabling accurate annotation of key genes.

bioinformatics↗

bioRxiv2025

Large-scale genomic characterisation of phage-plasmids in clinical Enterobacteriaceae isolates

The life cycle of certain bacteriophages involves their maintenance within the bacterial cell as extrachromosomal elements, complete with replication and partitioning systems. These "phage-plasmids" (P-Ps) are distributed widely among bacterial phyla but are not typically included during genomic surveillance studies, and previous reports do not consider the context of their host strain diversity. We recently identified a P1-like P-P carrying the blaCTX-M-15 resistance gene in Salmonella enterica serovar Typhi, prompting a subsequent investigation into the overall frequency of P-Ps among gastrointestinal bacteria under routine genomic surveillance within England. We expanded our study to include P-P groups known to be associated with Enterobacteriaceae (P1, D6, SSU5, N15) and scanned a collection of 66,856 genomes of diarrhoeagenic Escherichia spp., Shigella spp. and S. enterica. All four P-P groups were detected in our dataset, totalling 9% of E. coli and Shigella genomes and 2% of S. enterica genomes. A small subset harboured two distinct P-P groups. P1-like P-Ps, some of which carried a cytotoxic necrotising factor, were predominantly associated with Shiga toxin-producing E. coli strains of public health concern, including clonal complexes CC11 (O157:H7), CC29 (O26:H11) and CC165. In contrast, SSU5 group P-Ps were linked with multidrug-resistant lineages of both S. Typhi and Shigella sonnei. We found multiple antimicrobial resistance genes inserted into P-Ps via transposons, integrons and insertion sequences, and numerous defence/anti-defence systems. There was evidence of vertical transmission but also many links across species, time and geography, indicating that horizontal transfer is occurring regularly. Phage-plasmids are often described only as cryptic elements or not detected during genomic surveillance. We show that P-Ps are associated with clinically relevant lineages of human pathogens and can acquire accessory genes that may impact on disease severity and therefore should play a more prominent role in pathogen surveillance and epidemiology.

genomics↗

bioRxiv2024

Plasmid conjugation drives within-patient plasmid diversity

Plasmids are well known vehicles of antimicrobial resistance (AMR) genes dissemination. Through conjugation, plasmid-encoded AMR genes are spread among neighbouring bacteria, irrespective of their strain or even their species. This process is very concerning from a public health perspective, as plasmid-borne AMR gene outbreaks are often not confined to single species or strains and are therefore more difficult to be fully uncovered. At the moment, the impact of plasmid conjugation on within-patient plasmid diversity is not well understood. In this work we will tackle the role of conjugation on within-patient plasmid diversity using a dataset of carbapenemase-producing Enterobacterales (CPEs). The dataset of 256 sequences from 115 patients was sampled across England over 30 months. Each patient has more than one sequence, with at least one sequence carrying an OXA-48 gene, a well-known plasmid-borne carbapenemase-encoding gene. If more than one sequence carries the OXA-48 gene, they are carried on different bacterial hosts. Using a hybrid de novo-on-reference assembly pipeline, we were able to reconstruct the full OXA-48 plasmid from short read sequencing data for 232 of the 256 sequences. Of the 115 patients, 83 (72%) of patients had an identical OXA-48 plasmid in two or more sequences. Only 2 patients carried very different (>200 SNPs) alleles of the OXA-48 plasmid, probably from separate acquisitions. Our study shows that when more than one bacterial host carrying an OXA-48 plasmid is found in a patient, it is most likely that the same plasmid has been shared via conjugation. The event of separate acquisition of different plasmids in different bacterial hosts is highly unlikely in our dataset. Data StatementWe use data provided by Hopkins et al 2022 [16]. The data can be accessed from the National Center for Biotechnology Information (NCBI) and can be found at Bioproject Accession no. PRJNA788733. None of the data used was synthetically generated. Impact StatementConjugative plasmids are well known vessels of horizontal gene transfer, with a prominent role in the spread of antimicrobial resistance genes among different bacterial species or strains. At the epidemiological level, conjugation combined with sequencing a single colony per patient, results in plasmids outbreaks carrying antimicrobial resistance genes being found in different bacterial species and strains in different patients, potentially eluding surveillance protocols based on same bacterium/same resistance scheme. In this study we analyse within-patient plasmid diversity in a dataset with more than one sequence per patient. Only two patients show clear genomic signs of separate plasmids acquisition, while 83 patients share identical plasmids in different bacterial hosts. This points out to a very strong role of plasmid conjugation in shaping within-patient plasmid diversity.

bioinformatics↗

bioRxiv2022

The bounded coalescent model: conditioning a genealogy on a minimum root date

The coalescent model represents how individuals sampled from a population may have originated from a last common ancestor. The bounded coalescent model is obtained by conditioning the coalescent model such that the last common ancestor must have existed after a certain date. This conditioned model arises in a variety of applications, such as speciation, horizontal gene transfer or transmission analysis, and yet the bounded coalescent model has not been previously analysed in detail. Here we describe a new algorithm to simulate from this model directly, without resorting to rejection sampling. We show that this direct simulation algorithm is more computationally efficient than the rejection sampling approach. We also show how to calculate the probability of the last common ancestor occurring after a given date, which is required to compute the probability of realisations under the bounded coalescent model. Our results are applicable in both the isochronous (when all samples have the same date) and heterochronous (where samples can have different dates) settings. We explore the effect of setting a bound on the date of the last common ancestor, and show that it affects a number of properties of the resulting phylogenies. All our methods are implemented in a new R package called BoundedCoalescent which is freely available online.

evolutionary biology↗

bioRxiv2021

Bayesian inference of clonal expansions in a dated phylogeny

Microbial population genetics models often assume that all lineages are constrained by the same population size dynamics over time. However, many neutral and selective events can invalidate this assumption, and can contribute to the clonal expansion of a specific lineage relative to the rest of the population. Such differential phylodynamic properties between lineages result in asymmetries and imbalances in phylogenetic trees that are sometimes described informally but which are difficult to analyse formally. To this end, we developed a model of how clonal expansions occur and affect the branching patterns of a phylogeny. We show how the parameters of this model can be inferred from a given dated phylogeny using Bayesian statistics, which allows us to assess the probability that one or more clonal expansion events occurred. For each putative clonal expansion event we estimate their date of emergence and subsequent phylodynamic trajectories, including their long-term evolutionary potential which is important to determine how much effort should be placed on specific control measures. We demonstrate the applicability of our methodology on simulated and real datasets.

evolutionary biology↗

bioRxiv2020

Hospital outbreak of carbapenem-resistant Enterobacteriales associated with an OXA-48 plasmid carried mostly by Escherichia coli ST399

A hospital outbreak of carbapenem-resistant Enterobacteriales was detected by routine surveillance. Whole genome sequencing and subsequent analysis revealed a conserved promiscuous OXA-48 carrying plasmid as the defining factor within this outbreak. Four different species of Enterobacteriales were involved in the outbreak. Escherichia coli ST399 accounted for 20/55 of all the isolates. Comparative genomics with publicly available E. coli ST399 sequence data showed that the outbreak isolates formed a unique clade. The OXA-48 plasmid identified in the outbreak differed from other known plasmids by an estimated five homologous recombination events. We estimated a lower bound to the plasmid conjugation rate to be 0.23 conjugation events per lineage per year. Our analysis suggests co-evolution between the plasmid and its main bacterial host to be a key driver of the outbreak. This is the first study to report carbapenem-resistant E. coli ST399 carrying OXA48 as the main cause of a plasmid-borne outbreak within a hospital setting. This study supports complementary roles for both plasmid conjugation and clonal expansion in the emergence of this outbreak.

evolutionary biology↗

bioRxiv2018

Genetic distance between complex repeats

Complex nucleotide or aminoacid repeats with long units play an important role in proteins. The evolutionary analysis of these variants is challenging due to genetic diversity within repeat units as well as variability in the arrangement of different units along the repeat sequence. Here we present a new approach for the computation of genetic distances between complex repeats. This method takes into account evolutionary processes including point mutations, insertions and deletions of repeat units, as well as duplication of single units. We provide an algorithm for the computation of these distances along with the corresponding global pairwise alignment of repeats. As an example, we apply our approach to the evolution of repeat units in the highly polymorphic zinc-finger repeat domain of the PRDM9 protein across wild populations of house mice. This approach opens the way for new insights into the evolutionary history of polymorphic repeats.

bioinformatics↗