Bacterial Adaptation to Hot and Dry Deserts

The analysis of microbial diversity highlights the idea that tolerance of desiccation is more important than tolerance of temperature as an adaptive microbial trait for desert conditions. The culture-independent analysis of this microbial diversity in deserts revealed that 70–80 % of 16S-rDNA sequences do not match with described bacterial species, suggesting the presence of a majority of ‘new’ species and genera to be described and highlighting the necessity to make this biodiversity culturable, and for that we propose the use of the grain-by-grain cultivation as an efficient technique allowing the isolation of bacteria strongly attached to a sand grain surface. In Sahara samples, most of the bacterial isolates or clone sequences belong to phyla and genera for which the mechanisms of adaptation to desiccation have been described (sporulation in Firmicutes and Actinobacteria, DNA repair in Deinococcus) or remain to be unravelled (most Proteobacteria). A novel mechanism of desiccation tolerance was found for the betaproteobacterium Ramlibacter tataouinensis and involves the ability to divide as a desiccation-tolerant cyst. Surprisingly, the very low density of bacteria in desert sand is not due to a much lower diversity but to a very low number of bacteria per taxon. The main conclusion is that numerous bacterial species have been able to adapt in response to extreme conditions prevailing in hot and dry deserts.