What did a rotten apple in Paris, France and a rotten fig in Bangalore, India have in common?
They harbored common microscopic saprophytic worms that were host to the most remarkable worm-digesting bacteria.
Biological control has been endorsed by many as one of the ways to overcome resistance to anthelmintic drugs in livestock nematodes. Extensive research has been done with the microfungus Duddingtonia flagrans, which traps larvae in the pasture, and the bacterium Pasteuria penetrans, which has been used to control the root-knot nematodes of certain plants. The most recent event in this subfield of parasite control has been the isolation of a new bacteria called the golden death bacillus by a team of researchers based in Glasgow, Scotland. (Read the paper here).
Two isolates of the Golden death bacteria were isolated from worms found in a rotten apple obtained from Paris, France, and a rotten fig from Bangalore, India. The bacteria formed golden, mucoid colonies on agar and had a pungent odor. Microbiological testing and phylogenetic analysis of the whole genomes of the isolated bacteria revealed that they were very closely related but still distinct, and the name Chryseobacterium nematophagum was conferred on them, to describe both their color and their activity.
The biological activity of Chryseobacterium nematophagum was experimentally studied in the model non-parasitic nematode C. elegans intially. C. elegans is a fantastic model for this study because it is a bacterivorous nematode that can be grown on bacterial lawns in the lab. Compared to control cultures of E. coli and a closely related isolate of Chryseobacterium gallinarum from a chicken, Chryseobacterium nematophagum was able to completely kill all L1 larvae placed on them within 7 hrs. As few as 200 bacteria were able to kill larvae in 24 hrs. C. elegans larvae were attracted to the bacteria in cultures, which is remarkable as they are repelled by other pathogens such as Serratia marcescens and Pseudomonas fluorescens. The larvae could not be killed by other related bacteria of the same family (Flavobacteriaceae).
The authors observed that Chryseobacterium nematophagum multiplies in the pharynx of the C. elegans larvae and digested them from the inside out, which was demonstrated by the degradation of collagen and chitosan, but not beta-actin. The authors hypothesized that the effect of bacteria was due to chitinases and collagenases, and by comparative genomics of related bacteria were able to show that Chryseobacterium nematophagum indeed had copies of several nematode killing genes that encoded collagenases, chinases, cytolysins, hemolysins and others. Unique to Chryseobacterium nematophagum are slo- genes that code for a thiol activated cytolysins. Pertussis toxin S1 type secretion systems associated with motility and virulence were also found.
Chryseobacterium nematophagum was also able to kill bacterivorous, free-living larval stages of the important ruminant nematodes of the genera Haemonchus, Ostertagia, Cooperia and Trichoctrongylus, equine cyathostomins, and canine hookworm Ancylostoma among others. The bacteria, however, was not able to kill the potato nematode Globodera pallida or mosquito larvae.
Concerns on the safety of the bacteria to humans and animals have to be addressed before the bacteria can be deployed in full force in the field. We do not yet know if there are wild type Chryseobacterium nematophagum or similar bacteria already circulating in the pastures of the world, keeping nematode numbers in check. However, it is undeniable that the discovery of the golden death bacillus - Chryseobacterium nematophagum is a remarkable achievement.
References:
Page, Antony P., et al. "The golden death bacillus Chryseobacterium nematophagum is a novel matrix digesting pathogen of nematodes." BMC biology 17.1 (2019): 10.
Stiling, Peter, and Tatiana Cornelissen. "What makes a successful biocontrol agent? A meta-analysis of biological control agent performance." Biological control 34.3 (2005): 236-246.
