Observations on a Novel Bacterial Pathogen of Root-Knot Nematodes (Meloidogyne spp.).

Aurelio Ciancio

doi:10.3390/pathogens10101226

Abstract

A novel Gram-negative pathogenic bacterium (BN) was discovered in second-stage juveniles (J2) of root-knot nematodes (RKN, Meloidogyne spp.). Mature bacteria showed a peculiar rod morphology characterized by four cells sequentially joined at septa. Mature rods measured 4–5 × 0.5–0.6 μm and were characterized by the emptying and tapering of both apical cells. The data showed an electron-dense external matrix forming a coating capsule involved in host attachment. The rods were not motile and packed in parallel inside the J2 body. After J2 penetration by adhering, germinating cells, the bacterium proliferated until the host body content was completely digested, producing a lethal disease. Parasitized hosts were recognized using light microscopy by a pale creamy-brown color assumed at parasitism completion. At death, the whole nematode body was filled with cells and only a few sclerotized esophageal structures (i.e., stylet, median bulb) remained visible. The BN cells were quickly released at host body rupture, suggesting that J2 infection occurs through passive adhesion of cells dispersed in soil. The bacterium appeared fastidious, as attempts to obtain pure cultures on common nutritive media failed.

Highlights

Nematodes and bacteria have a long history of coevolutionary links, as shown by the number of highly specialized species reported in strict association with nematodes from soil or marine environments [1,2,3]
Nematodes showing symptoms of BN-induced infection were found among the second-stage juveniles (J2) of four root-knot nematode populations, with the highest prevalence levels observed by late summer
Light microscopy (LM) examinations showed that the pathology was always associated to a color switch of parasitized hosts, which turned from pale brown to creamy as the disease progressed

Summary

Introduction

Nematodes and bacteria have a long history of coevolutionary links, as shown by the number of highly specialized species reported in strict association with nematodes from soil or marine environments [1,2,3]. Progress in genome and metagenome studies showed an increased number of associations based on a range of trophic relationships varying from parasitism to different types of symbiosis [2,3]. Such a broad diversity of taxa is expected to increase further as far as studies on the nematode-bacteria associations proceed considering the diversification of the phylum Nematoda, the richness of environmental niches colonized, and the specialized metabolic interactions established [1,2,3,4]. Several Bacillus and Pseudomonas spp. and/or strains have been described or reported as biocontrol agents of plant parasitic nematodes [10,11,12,13,14] or free-living species [15]

Methods

Results

Conclusion