Joanne M. Santini

Klebsiella pneumoniae species complex (KPSC) is a leading cause of bloodstream infections (BSIs), with capsule diversity complicating vaccine development. Certain capsule types are disproportionately associated with disease, including the under-studied KL16. Here, we isolated and characterised four novel podophages that, despite encoding multiple depolymerases, exhibit strict K16 tropism. We show that selected phage M6 encodes two active depolymerases (K16-Dep46 and K16-Dep47), the first identified for this capsule type and the first example of a phage carrying dual enzymes with conserved substrate specificity. The phages infected all K16 strains tested, including clinical and environmental isolates from Malawi, the UK and Thailand, with diagnostic precision. Purified K16-Dep47 expanded the host range of other phages, enabling M6 to act in concert with capsule-independent phages, as well as with ceftazidime to improve bacterial clearance. These findings define the molecular basis of K16-phage specificity and highlight targeted phage isolation for therapeutic and diagnostic applications.

Copper bioleaching is a green technology for the recovery of copper from chalcopyrite (CuFeS ₂ ) and chalcocite (Cu ₂ S) ores. Much remains to be learned about how mineral type and surface chemistry influence microbial community composition. Here, we established a microbial consortium from a copper bioleaching column in Cyprus on chalcopyrite and then sub‐cultured it to chalcocite to investigate how the community composition shifts due to changes in mineral structure and the absence of mineral‐derived Fe. The solution chemistry was determined and microbial communities characterised by genome‐resolved metagenomics after 4 and 8 weeks of cultivation. Acidithiobacillus species and strains, a Rhodospirilales , Leptospirillum ferrodiazotrophum and Thermoplasmatales archaea dominated all enrichments, and trends in abundance patterns were observed with mineralogy and surface‐attached versus planktonic conditions. Many bacteria had associated plasmids, some of which encoded metal resistance pathways, sulphur metabolic capacities and CRISPR‐Cas loci. CRISPR spacers on an Acidithiobacillus plasmid targeted plasmid‐borne conjugal transfer genes found in the same genus, likely belonging to another plasmid, evidence of intra‐plasmid competition. We conclude that the structure and composition of metal sulphide minerals select for distinct consortia and associated mobile elements, some of which have the potential to impact microbial activity during sulphide ore dissolution.

This article summarises the activities of the International Committee on Taxonomy of Viruses Bacterial Viruses Subcommittee, detailing developments in the classification of bacterial viruses. We provide here an overview of all new, abolished, moved and renamed taxa proposed in 2024, approved by the Executive Committee, and ratified by membership vote in 2025. Through the collective efforts of 74 international contributors of taxonomy proposals in this round, 43 ratified proposals have led to the creation of one new phylum, one class, four orders, 33 families, 14 subfamilies, 194 genera and 995 species. These proposals mark significant progress in refining the taxonomy of bacterial viruses. Key updates include the creation of new orders and families that include existing taxa to better reflect genomic and evolutionary relationships. As sequencing and bioinformatics approaches continue to advance, further expansion and refinements in viral taxonomy can be anticipated in the coming years.

Background/Objectives: The significant outbreak of multidrug-resistant Klebsiella pneumoniae has emerged as a primary global concern associated with high morbidity and mortality rates. Certain strains of K. pneumoniae are highly resistant to most antibiotics available in clinical practice, exacerbating the challenge of bacterial infections. Methods: Phage vB_KpnP_PW7 (vKPPW7) was isolated and characterized. Its morphology, stability, adsorption rate, one-step growth curve, lytic activity, whole-genome sequence analysis, and antibacterial and antibiofilm activities were evaluated. Results: The virulent phage has a 73,658 bp linear dsDNA genome and was classified as a new species of the genus Kaypoctavirus, subfamily Enquatrovirinae, and family Schitoviridae. Phage vKPPW7 has a high adsorption rate, a short latent period, and a large burst size. The phage showed activity against 18 K. pneumoniae isolates with the K24 capsular type but was unable to lyse K. pneumoniae isolates whose capsular type was not classified as K24. Additionally, phage vKPPW7 demonstrated strong stability across various temperatures and pH values. The phage exhibited antibacterial activity, and scanning electron microscopy (SEM) confirmed its ability to lyse MDR K. pneumoniae with the K24 capsular type. Furthermore, phage vKPPW7 effectively removed preformed biofilm and prevented biofilm formation, resulting in reduced biofilm biomass and biofilm viability compared to controls. The architecture of phage-treated biofilms was confirmed under SEM. Conclusions: These findings suggest that phage vKPPW7 holds promise for development as a therapeutic or biocontrol agent.

The discovery of viruses that can devour bacteria, bacteriophages (phages), was in 1915. Phages are ubiquitous, outnumbering the organisms they devour, and genomically, morphologically, and ecologically diverse. They were critical in our development of molecular biology and biotechnology tools and have been used as therapeutics for over 100 years, primarily in Eastern Europe with thousands of patients from all over the world treated in Georgia. The rise of antimicrobial resistance and the lack of new antimicrobials, has brought them back into the spotlight dawning the New Age of the Phage. This special issue will provide further insight to phage diversity across ecosystems, including humans, animals, and plants, i.e. the basis of a One Health approach, and the requirements for turning phages into viable medicines for the many and not just for the few.

Klebsiella pneumoniae is an opportunistic pathogen with significant clinical relevance. K. pneumoniae-targeting bacteriophages encode specific polysaccharide depolymerases with the ability to selectively degrade the highly varied protective capsules, allowing for access to the bacterial cell wall. Bacteriophage depolymerases have been proposed as novel antimicrobials to combat the rise of multidrug-resistant K. pneumoniae strains. These enzymes display extraordinary diversity, and are key determinants of phage host range, however with limited data available our current knowledge of their mechanisms and ability to predict their efficacy is limited. Insight into the resolved structures of Klebsiella-specific capsule depolymerases reveals varied catalytic mechanisms, with the intra-chain cleavage mechanism providing opportunities for recombinant protein engineering. A detailed comparison of the 58 characterised depolymerases hints at structural and mechanistic patterns, such as the conservation of key domains for substrate recognition and phage tethering, as well as diversity within groups of depolymerases that target the same substrate. Another way to understand depolymerase specificity is by analyzing the targeted capsule structures, as these may share similarities recognizable by bacteriophage depolymerases, leading to broader substrate specificities. Although we have only begun to explore the complexity of Klebsiella capsule depolymerases, further research is essential to thoroughly characterise these enzymes. This will be crucial for understanding their mechanisms, predicting their efficacy, and engineering optimized enzymes for therapeutic applications.

High-throughput sequencing for uncultivated viruses has accelerated the understanding of global viral diversity and uncovered viral genomes substantially larger than any that have so far been cultured. Notably, the Lak phages are an enigmatic group of viruses that present some of the largest known phage genomes identified in human and animal microbiomes, and are dissimilar to any cultivated viruses. Despite the wealth of viral diversity that exists within sequencing datasets, uncultivated viruses have rarely been used for taxonomic classification. We investigated the evolutionary relationships of 23 Lak phages and propose a taxonomy for their classification. Predicted protein analysis revealed the Lak phages formed a deeply branching monophyletic clade within the class Caudoviricetes which contained no other phage genomes. One of the interesting features of this clade is that all current members are characterised by an alternative genetic code. We propose the Lak phages belong to a new order, the ‘Grandevirales’. Protein and nucleotide-based analyses support the creation of two families, three sub-families, and four genera within the order ‘Grandevirales’. We anticipate that the proposed taxonomy of Lak megaphages will simplify the future classification of related viral genomes as they are uncovered. Continued efforts to classify divergent viruses are crucial to aid common analyses of viral genomes and metagenomes.

High-throughput sequencing for uncultivated viruses has accelerated the understanding of global viral diversity and uncovered viral genomes substantially larger than any that have so far been cultured. Notably, the Lak phages are an enigmatic group of viruses that present some of the largest known phage genomes identified in human and animal microbiomes, and are dissimilar to any cultivated viruses. Despite the wealth of viral diversity that exists within sequencing datasets, uncultivated viruses have rarely been used for taxonomic classification. We investigated the evolutionary relationships of 23 Lak phages and propose a taxonomy for their classification. Predicted protein analysis revealed the Lak phages formed a deeply branching monophyletic clade within the class Caudoviricetes which contained no other phage genomes. One of the interesting features of this clade is that all current members are characterised by an alternative genetic code. We propose the Lak phages belong to a new order, the “Grandevirales”. Protein and nucleotide-based analyses support the creation of two families, three sub-families, and four genera within the order “Grandevirales”. We anticipate that the proposed taxonomy of Lak megaphages will simplify the future classification of related viral genomes as they are uncovered. Continued efforts to classify divergent viruses are crucial to aid common analyses of viral genomes and metagenomes.

Bioleaching offers a low-input method of extracting valuable metals from sulfide minerals, which works by exploiting the sulfur and iron metabolisms of microorganisms to break down the ore. Bioleaching microbes generate energy by oxidising iron and/or sulfur, consequently generating oxidants that attack sulfide mineral surfaces, releasing target metals. As sulfuric acid is generated during the process, bioleaching organisms are typically acidophiles, and indeed the technique is based on natural processes that occur at acid mine drainage sites. While the overall concept of bioleaching appears straightforward, a series of enzymes is required to mediate the complex sulfur oxidation process. This review explores the mechanisms underlying bioleaching, summarising current knowledge on the enzymes driving microbial sulfur and iron oxidation in acidophiles. Up-to-date models are provided of the two mineral-defined pathways of sulfide mineral bioleaching: the thiosulfate and the polysulfide pathway.