Morphological, physiological, and biochemical characteristics and the DNA base compositions of 133 thermophilic strains were determined. A total of 54 of these strains were received as identified species (mainly , and ) from international culture collections, and 79 newly isolated strains, which were isolated mainly from sugar diffusion juices of Italian plants, were also examined. Numerical taxonomy techniques (simple matching coefficient and unweight pair grouping using the mathematical average) and DNA G+C values showed that the strains aggregated into nine clusters. Both and were well separated from the other organisms. and were confirmed as separate clusters and exhibited greater heterogeneity than previously shown. The strains clustered into four groups, three of which have been recognized previously by other authors; the members of the fourth group had distinctive characteristics, including considerable biochemical inertness, an inability to grow at temperatures greater than 60°C, and a high G+C content. Within the cluster the strains with characteristics very similar to those of the new species clustered together. However, the remaining strains were still clearly separated into two groups; one of these groups was considered sensu stricto, and the other was distinguished by morphological and biochemical criteria, such as spores which do not swell the sporangia, utilization of citrate, a higher proteolytic activity, and acidification of some carbohydrates. Our results were confirmed by comparing them with distinctive characteristics of recently described thermophilic species.


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