Characterization of Serratia entomophila strains by genomic DNA fingerprints and plasmid profiles

Genome sequence of the entomopathogenic Serratia entomophila isolate 626 and characterisation of the species specific itaconate degradation pathway

BACKGROUND

Isolates of Serratia entomophila and S. proteamaculans (Yersiniaceae) cause disease specific to the endemic New Zealand pasture pest, Costelytra giveni (Coleoptera: Scarabaeidae). Previous genomic profiling has shown that S. entomophila isolates appear to have conserved genomes and, where present, conserved plasmids. In the absence of C. giveni larvae, S. entomophila prevalence reduces in the soil over time, suggesting that S. entomophila has formed a host-specific relationship with C. giveni. To help define potential genetic mechanisms driving retention of the chronic disease of S. entomophila, the genome of the isolate 626 was sequenced, enabling the identification of unique chromosomal properties, and defining the gain/loss of accessory virulence factors relevant to pathogenicity to C. giveni larvae.

RESULTS

We report the complete sequence of S. entomophila isolate 626, a causal agent of amber disease in C. giveni larvae. The genome of S. entomophila 626 is 5,046,461 bp, with 59.1% G + C content and encoding 4,695 predicted CDS. Comparative analysis with five previously sequenced Serratia species, S. proteamaculans 336X, S. marcescens Db11, S. nematodiphila DH-S01, S. grimesii BXF1, and S. ficaria NBRC 102596, revealed a core of 1,165 genes shared. Further comparisons between S. entomophila 626 and S. proteamaculans 336X revealed fewer predicted phage-like regions and genomic islands in 626, suggesting less horizontally acquired genetic material. Genomic analyses revealed the presence of a four-gene itaconate operon, sharing a similar gene order as the Yersinia pestis ripABC complex. Assessment of a constructed 626::RipC mutant revealed that the operon confer a possible metabolic advantage to S. entomophila in the initial stages of C. giveni infection.

CONCLUSIONS

Evidence is presented where, relative to S. proteamaculans 336X, S. entomophila 626 encodes fewer genomic islands and phages, alluding to limited horizontal gene transfer in S. entomophila. Bioassay assessments of a S. entomophila-mutant with a targeted mutation of the itaconate degradation region unique to this species, found the mutant to have a reduced capacity to replicate post challenge of the C. giveni larval host, implicating the itaconate operon in establishment within the host.

Transformation of 2,4,6-trinitrotoluene (TNT) by Raoultella terrigena

Manufacture of nitroorganic explosives generates toxic wastes leading to contamination of soils and waters, especially groundwater. For that reason bacteria living in environments highly contaminated with 2,4,6-trinitrotoluene (TNT) and other nitroorganic compounds were investigated for their capacity for TNT degradation. One isolate, Raoultella terrigena strain HB, removed TNT at concentrations between 10 and 100 mg l(-1) completely from culture supernatants under optimum aerobic conditions within several hours. Only low concentrations of nutrient supplements were needed for the cometabolic transformation process. Radioactivity measurements with ring-labelled (14)C-TNT detected about 10-20% of the initial radioactivity in the culture supernatant and the residual 80-90% as water-insoluble organic compounds in the cellular pellet. HPLC analysis identified aminodinitrotoluenes (2-ADNT, 4-ADNT) and diaminonitrotoluenes (2,4-DANT) as the metabolites which remained soluble in the culture medium and azoxy-dimers as the main products in the cell extracts. Hence, the new isolate could be useful for the removal of TNT from contaminated waters.

Serratia entomophila bacteriophages: host range determination and preliminary characterization

Eight bacteriophages specific to Serratia entomophila, a commercially available bacterial pathogen of the New Zealand grass grub (Costelytra zealandica), were characterized by host range determination, morphology and restriction endonuclease patterns of DNA. Phages were originally isolated from grass grub larvae and fermenter broth where phages had disrupted large-scale production of S. entomophila. Seven of the phages (CW1-CW5, BC, and BT) had heads similar in size (approximately 60 x 60 nm) and long noncontractile tails (185 x 10 nm). Phage AgRP8 (P8) had a smaller head and a short tail structure. Restriction endonuclease analysis divided the phages into four groups: CW2, CW4, CW5, BC, and BT gave identical patterns, white CW1, CW3, and P8 each gave different patterns. Six distinct phage groups were distinguished by host range determination, after screening phages against 70 bacterial isolates: CW1, CW2/CW4, CW3, CW5, BC/BT, and P8. While confirming the indicated groupings by DNA analysis, it was possible to distinguish between some of the phages in the largest group: CW2/4 could be distinguished from CW5 and BC/BT. Screening of soil bacterial isolates of S. entomophila against nondiluted phages will aid in monitoring the establishment and persistence of strains applied for biological control of the grass grub.