1. A method of surveillance for the presence of a biological agent, which method comprises: assaying a sample derived from street debris materials collected from a sample domain for the presence of a biological agent comprising the steps of: (a) treating the sample to reduce the concentration of bacteria that are preferentially bioencapsulated by Tetrahymena pyriformis relative to the bioencapsulation of Bacillus anthracis, (b) introducing Tetrahymena pyriformis to the sample, and (c) assaying for Bacillus anthracis; wherein the sample domain is a route undertaken by a street sweeper machine through a city street and comprises at least one collection point from which the materials are collected from a city street in a pre-existing operation, otherwise unrelated to surveillance.