Which of the following are two major groups of mollicutes?

The two major groups of mollicutes are indeed spiroplasma and phytoplasma. Mollicutes are a class of bacteria characterized by their lack of a cell wall and unique shape, and they are often associated with plant diseases.

Spiroplasma are helical, motile bacteria that can be found in a variety of hosts, including plants and insects. They are known for being involved in plant pathogenesis, particularly affecting crops. Phytoplasma, on the other hand, are a special type of mollicute that lack cell walls and are also important plant pathogens, causing diseases like yellows and witches' broom in various agricultural crops.

The other choices do not represent the groups of mollicutes. Actinobacteria and Firmicutes refer to larger classifications of bacteria, while Escherichia and Salmonella are genera that belong to the Enterobacteriaceae family, which are not part of mollicutes. Streptomyces and Clostridia also belong to different taxonomic classes and do not include the mollicutes either. Thus, spiroplasma and phytoplasma are correctly recognized as the two major groups within the mollicutes classification.