"The Stolen Bacillus" belongs to the genre of science fiction and, to understand why, it is useful to look at a relevant definition. According to the University of California, for example, a science fiction story is one which presents an "alternative world" (see the reference link provided). In "The Stolen Bacillus," the setting is London but it is a very different London to the one which Wells' readers were accustomed to. In Wells' London, for instance, the city is at risk from anarchists who use deception to wage biological warfare by poisoning the water supply.
In addition, science fiction stories have very distinctive character types, like "the scientist" and "the action hero" (see the reference link provided) and "The Stolen Bacillus" is no exception. Wells conforms to these character types by creating "the Anarchist" and "the Bacteriologist," two typical yet contrasting character types who also create the story's central conflict.