He performed extensive studies of Shigella dysenteriae during an epidemic of dysentery in the Ruhr area of Germany.
Kruse documented his findings in a 1900 treatise titled Über die Ruhr als Volkskrankheit und ihren Erreger.
In 1914 he demonstrated that the common cold could be transmitted to healthy individuals via nasal secretions that were free of bacteria.
The results of these experiments were published in a treatise called Die Erreger von Husten und Schnupfen (1914).
A specialized tool used to spread material over the surface of a culture medium is called "Kruse's brush".