 Unlike some of the great names of the time, among them Carl Wilhelm Nägeli, Theodor Billroth (1829–1894), Ferdinand Cohn (1828–1898), and Robert Koch (1843–1910), Gruber recognized that bacteria possess a variability within limits partially determined by the culture medium.This theory was important for the differentiation of the categories of bacteria and gained significance for Gruber in his examinations of cholera vibrios, enabling him to distinguish them from other vibrios.In 1882 Gruber was habilitated as a lecturer in Vienna, and two years later he became associate professor and head of the newly established Institute for Hygiene at the University of Graz.