Calicotome villosa is a shrubby leguminous tree. Also known as spiny broom, its vivid yellow flowers and hairy pods can be spotted all around the Mediterranean climate regions. First, a large-scale quantitative field survey in Israel determined that the shrub, while quite common in general, was entirely absent in a few large regions, despite being well within its species range in terms of precipitation and soil type. Then from root nodules, the authors identified the nitrogen-fixing bacterial mutualist, in this case a specific Bradyrhizobium strain. They also excluded nutrient deficiency as a major cause for the differences in shrub abundance.

Through inoculation experiments in different types of soil, the researchers realized that only the presence of the same Bradyrhizobium strain can rescue nodulation and restore subsequent growth of the shrub when grown in the soil of regions where it is naturally absent. Finally, and although it is difficult to unequivocally prove an absence, microbial composition analyses in soil samples could not find Bradyrhizobium genus bacteria in regions where the shrub didn’t grow.

Calicotome villosa是一种灌木状豆科乔木。也被称为刺扫帚，其鲜艳的黄色花朵和毛茸茸的豆荚在地中海气候地区随处可见。首先，在以色列进行的一项大规模定量实地调查确定，这种灌木虽然总体上相当常见，但在一些大的地区完全不存在，尽管在降水量和土壤类型方面完全属于其物种范围。然后，作者从根瘤中鉴定出了固氮细菌互利共生体，在本例中是一种特定的慢生根瘤菌菌株。他们还排除了营养缺乏是灌木丰度差异的主要原因。

通过在不同类型土壤中的接种实验，研究人员意识到，只有相同的慢生根瘤菌菌株的存在才能挽救结瘤并恢复灌木在自然不存在根瘤菌的地区的土壤中生长的后续生长。最后，尽管很难明确证明不存在，但土壤样本中的微生物组成分析未能在灌木不生长的地区发现慢生根瘤菌属细菌。