The virulence plasmids of bacteria of the genus Cronobacter

Title: Domestication of mobile DNA – the case study of the virulence plasmids of bacteria of the genus Cronobacter

Title in Polish: Udomowianie mobilnego DNA – studium przypadku plazmidów wirulencji bakterii z rodzaju Cronobacter

Principal investigator: prof. dr hab. Dariusz Bartosik

Grant number: 2018/29/B/NZ1/00140

Funded by: National Science Centre

Project description:

Many bacteria possess multipartite genomes consisting of numerous extrachromosomal replicons (ECRs) with diverse structures and properties. In addition to a chromosome, they contain chromids –essential ECRs that carry housekeeping genes – and dispensable plasmids, which play a crucial role in the dissemination of genetic information within bacterial populations. A characteristic feature of plasmids is their high variability, primarily due to the absence of housekeeping genes. Consequently, plasmids are considered “training sites” where diverse genetic variants are generated and “tested,” contributing to the structural diversity of these replicons.

While analyzing bacterial ECRs, we focused on the virulence plasmids of Cronobacter spp. (pVirCro), which exhibit certain unique features distinguishing them from other known plasmid groups. Although they lack housekeeping genes, they share some similarities with chromids. These replicons are likely to have been associated with the Cronobacter genus since its early evolutionary history, accompanying the genetic diversification of individual strains and their subsequent speciation. This raises important questions regarding the molecular and physiological causes and consequences of such a long-term, close association.

The pVirCro replicons thus provide an excellent model for studying the domestication of exogenous replicons in bacteria – a process that leads to the transformation of a mobile genetic element into an integral component of the bacterial genome. This domestication may result in coordinated genetic regulation of both the plasmid and chromosomal elements during the bacterial cell cycle. Our preliminary observations strongly suggest that these plasmids play a significant role in the biology of Cronobacter spp., potentially contributing to biofilm formation, exopolysaccharide production, the generation of persister cell subpopulations (which exhibit antimicrobial tolerance through dormancy), and responses to various environmental stressors such as low water activity, pH fluctuations, and temperature changes. These features may substantially enhance the adaptability and pathogenic potential of Cronobacter spp. strains.

To gain deeper insights into the role of pVirCro plasmids, we plan to conduct comprehensive analyses to investigate: (i) the potential regulatory coupling between plasmid and chromosomal replication and segregation (including the role of Dam methyltransferase in plasmid replication initiation, possible interactions between plasmid Rep proteins and chromosomal DNA, and the involvement of SeqA in plasmid origin sequestration during cell division), (ii) the host range of these plasmids, (iii) their impact on bacterial fitness, and (iv) their role in determining environmentally and clinically relevant phenotypes.

Additionally, this research will yield complete DNA sequences of numerous novel pVirCro plasmid variants from environmental and clinical Cronobacter spp. isolates. Comprehensive phylogenetic and comparative analyses of these sequences will facilitate the identification of the core genome of these replicons as well as species- and strain-specific genes, aiding in the reconstruction of the evolutionary history of this unique group of ECRs.