microorganism especially the actinomycetes, produce a variety of antibiotics.

As a result, they have been studied very extensively, especially by industrial research laboratories. (A Course in Basic Scientific English, P107)

微生物，特别是放线菌，产生多种抗生素。 因而受到广泛的研究，尤其是在工业实验室中。

显而易见，上句的微生物和放线菌，都是泛指，那为什么微生物没有the: 用了 microorganism， 而放线菌却用了the: the actinomycetes?

the是不是误用，应去掉呢？