Could you provide the source of such a suggestion/conclusion? Probably you mean susceptibility among mesophilic microorganisms? If you mean the tolerance to heat treatments, such as sterilization, one should check the formation of spores by any microbe as spores are more resistant. Otherwise, thermophilic and hyperthermophilic bacteria live and divise at 90-100°C; no actinomycetes can do that.

In general, major number of bacteria can not tolerate more than 35 degree celsius while most of the actinomycetes/actinobacteria may very well tolerate above 50 degree celsius. For example, under arid environment where summer soil temperature ranges between 50-55 degree Celsius you may find much higher actinomycetes population than bacteria while under low temperature condition (25-35 degree celsius ) the bacterial population is at least 100 times more than the actinobacteria/actinomycetes population.

Actinomycetes are a specific group as bacteria. Morphologically they resemble fungi because of their elongated cells that branch into filaments or hyphae. Due to its hyphal and ascospore formation nature which is the property of fungus make it resistant to high temperature but i m agree with Dr. Vehary Sakanyan that tolerancy is not as that of thermophilic bacteria.

Actinomycetes and actinobacteria belong to bacteria. I know some streptomycetes such as S. clavuligerus are very sensitive to high temperature (won't grow at 37C).

Dr. Li, actinomycetes now called as actinobacteria and they consider as a separate group as they are different from bacteria in many ways.

I could be wrong. But according to NCBI taxonomy, the order should be : cellular organisms-Bacteria-Terrabacteria group-Actinobacteria. Thus bacteria includes actinobacteria.  

https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Tree&id=84992&lvl=3&lin=f&keep=1&srchmode=1&unlock

think Bergey;s has Domain Bacteria, Phylum Actinobacteria, Class Actinobacteria

According to my view, the factor may be GC content which differentiates their thermostability.

Dr. Sandilya, that may be one of the factor but I think lot of research is needed to know the details reasons for thermostability of microbial cells.

Dear Dr. Tarafdar Sir, I would like to inform you that I am not Dr. Sandilya, perhaps I am still pursuing my Ph. D. Anyway thanks a lot for appreciating my view and I also agree that there might be more factors responsible for thermostability of microbes.