Structural insight of the role of the Hahella chejuensis HapK protein in prodigiosin biosynthesis

The ICU environment contributes to the endemicity of the "Serratia marcescens complex" in the hospital setting

Serratia marcescens is an opportunistic pathogen historically associated with sudden outbreaks in intensive care units (ICUs) and the spread of carbapenem-resistant genes. However, the ecology of S. marcescens populations in the hospital ecosystem remains largely unknown. We combined epidemiological information of 1,432 Serratia spp. isolates collected from sinks of a large ICU that underwent demographic and operational changes (2019-2021) and 99 non-redundant outbreak/non-outbreak isolates from the same hospital (2003-2019) with 165 genomic data. These genomes were grouped into clades (1-4) and subclades (A and B) associated with distinct species: Serratia nematodiphila (1A), S. marcescens (1B), Serratia bockelmannii (2A), Serratia ureilytica (2B), S. marcescens/Serratia nevei (3), and S. nevei (4A and 4B). They may be classified into an S. marcescens complex (SMC) due to the similarity between/within subclades (average nucleotide identity >95%-98%), with clades 3 and 4 predominating in our study and publicly available databases. Chromosomal AmpC β-lactamase with unusual basal-like expression and prodigiosin-lacking species contrasted classical features of Serratia. We found persistent and coexisting clones in sinks of subclades 4A (ST92 and ST490) and 4B (ST424), clonally related to outbreak isolates carrying blaVIM-1 or blaOXA-48 on prevalent IncL/pB77-CPsm plasmids from our hospital since 2017. The distribution of SMC populations in ICU sinks and patients reflects how Serratia species acquire, maintain, and enable plasmid evolution in both "source" (permanent, sinks) and "sink" (transient, patients) hospital patches. The results contribute to understanding how water sinks serve as reservoirs of Enterobacterales clones and plasmids that enable the persistence of carbapenemase genes in healthcare settings, potentially leading to outbreaks and/or hospital-acquired infections.IMPORTANCEThe "hospital environment," including sinks and surfaces, is increasingly recognized as a reservoir for bacterial species, clones, and plasmids of high epidemiological concern. Available studies on Serratia epidemiology have focused mainly on outbreaks of multidrug-resistant species, overlooking local longitudinal analyses necessary for understanding the dynamics of opportunistic pathogens and antibiotic-resistant genes within the hospital setting. This long-term genomic comparative analysis of Serratia isolated from the ICU environment with isolates causing nosocomial infections and/or outbreaks within the same hospital revealed the coexistence and persistence of Serratia populations in water reservoirs. Moreover, predominant sink strains may acquire highly conserved and widely distributed plasmids carrying carbapenemase genes, such as the prevalent IncL-pB77-CPsm (pOXA48), persisting in ICU sinks for years. The work highlights the relevance of ICU environmental reservoirs in the endemicity of certain opportunistic pathogens and resistance mechanisms mainly confined to hospitals.

Structure-based Mechanistic Insights into Terminal Amide Synthase in Nosiheptide-Represented Thiopeptides Biosynthesis

Nosiheptide is a parent compound of thiopeptide family that exhibit potent activities against various bacterial pathogens. Its C-terminal amide formation is catalyzed by NosA, which is an unusual strategy for maturating certain thiopeptides by processing their precursor peptides featuring a serine extension. We here report the crystal structure of truncated NosA1-111 variant, revealing three key elements, including basic lysine 49 (K49), acidic glutamic acid 101 (E101) and flexible C-terminal loop NosA112-151, are crucial to the catalytic terminal amide formation in nosiheptide biosynthesis. The side-chain of residue K49 and the C-terminal loop fasten the substrate through hydrogen bonds and hydrophobic interactions. The side-chain of residue E101 enhances nucleophilic attack of H2O to the methyl imine intermediate, leading to Cα-N bond cleavage and nosiheptide maturation. The sequence alignment of NosA and its homologs NocA, PbtH, TpdK and BerI, and the enzymatic assay suggest that the mechanistic studies on NosA present an intriguing paradigm about how NosA family members function during thiopeptide biosynthesis.

Expression, crystallization and preliminary crystallographic data analysis of PigI, a putative L-prolyl-AMP ligase from the prodigiosin synthetic pathway in Serratia

Prodigiosin, a member of the prodiginines, is a tripyrrole red pigment synthesized by Serratia and some other microbes. A bifurcated biosynthesis pathway of prodigiosin has been proposed in Serratia in which MBC (4-methoxy-2,2'-bipyrrole-5-carbaldehyde) and MAP (2-methyl-3-N-amyl-pyrrole) are synthesized separately and then condensed by PigC to form prodigiosin. The first step for the synthesis of MBC is the activation of L-proline by PigI, but its catalytic mechanism has remained elusive. To elucidate its mechanism, recombinant PigI was purified and crystallized. Crystals obtained by the sitting-drop method belonged to space group P1 and diffracted to 2.0 Å resolution, with unit-cell parameters a = 51.2, b = 62.8, c = 91.3 Å, α = 105.1, β = 90.1, γ = 92.2°. Matthews coefficient analysis suggested two molecules in the asymmetric unit, with a VM of 2.6 Å(3) Da(-1) and a solvent content of 52.69%.

Crystal structure of dihydrodipicolinate synthase from Hahella chejuensis at 1.5 A resolution

Lysine biosynthesis has been of interest in plant research, because lysine is the most limiting amino acid in crop protein production. Dihydrodipicolinate synthase (DHDPS) catalyzes the branch point reaction leading to meso-diaminopimelate (DAP) and (S)-lysine in lysine biosynthesis. In this report, we present the crystal structure of DHDPS from the marine bacterium Hahella chejuensis (HcDHDPS) at 1.5 A resolution. The four subunits of the asymmetric unit assemble to form a tetramer with an approximate 222 symmetry. At the active site of HcDHDPS, three residues Tyr132, Thr43 and Tyr106 are observed to constitute a catalytic triad and are located at similar positions of the corresponding residues of Escherichia coli DHDPS. The structural similarities in the overall fold and the active site environment between these two enzymes imply that HcDHDPS functions by a mechanism similar to E. coli DHDPS. However, unlike E. coli DHDPS, HcDHDPS has a unique extensive dimer-dimer interface that is mediated by not only strong hydrophobic interactions but also a hydrogen bond network.