Is there genetic diversity in the ‘leucaena bug’ Synergistes jonesii which may reflect ability to degrade leucaena toxins?

Leucaena leucocephala, a nutritionally rich forage tree legume, contains a non-protein amino acid, mimosine, which is degraded by ruminal bacteria to toxic metabolites 3,4-DHP and 2,3-DHP, resulting in goitre-like symptoms in animals, severely restricting weight gain. Raymond Jones, in the early 1980s, discovered the ‘leucaena bug’ in the rumen of goats in Hawaii that degraded these toxic DHP metabolites into non-toxic compounds (Jones and Lowry 1984), which was named Synergistes jonesii (Allison et al. 1992). Subsequently, a rumen inoculum containing S. jonesii was used as an ‘oral drench’ for cattle, kept in continuous culture (Klieve et al. 2002) and supplied to farmers to dose cattle foraging on leucaena. Studies on Queensland herds that received this oral drench showed that up to 50% of 44 herds grazing on leucaena had apparent subclinical toxicity based on high 3,4-DHP and 2,3-DHP excretion in urine (Dalzell et al. 2012). In another study by Graham et al. (2013), a 16S rDNA nested PCR showed that rumen digesta from 6 of 8 farms tested had a variant DNA profile from S. jonesii ATCC 78.1 strain, which suggested a different strain of the bacterium. It was postulated that the continually cultured oral inoculum may have undergone genetic modification and/or animals could harbor other DHP-degrading bacteria or S. jonesii strains with differential DHP-degrading potential (C. McSweeney et al. unpublished). The present study looks at changes in the 16S rDNA gene at the molecular level, that may suggest divergence from the type strain S. jonesii ATCC 78.1 in Queensland cattle as well as in cattle