Italian researchers studied 60 samples of ice from domestic, restaurant or industrial producers. They found 52 different strains of bacteria, including Pseudomonas, Staphylococcus, Bacillus and Acinetobacter, across the 60 samples of ice, some of which were 'agents of human infection' indicating environmental contamination.
In the case of each drink, they found that the population of bacteria in the sample was reduced and cited the levels of alcohol, the drink’s pH and the amount of carbon dioxide in each serve as reasons for the reduction.
However, their results also showed that the ice sample served with whisky saw the greatest reduction in bacteria – none of the bacterial strains on the ice cubes survived after they were added to the whisky. The researchers noted that this was likely to be because whisky is somewhat more acidic than vodka. They speculated that the more acidic a drink is, the less likely bacteria are able to survive.
The question remains however why in the world would you add ice to your whisky or any other alcoholic drink.
 Settanni en al: Presence of pathogenic bacteria in ice cubes and evaluation of their survival in different systems in Annals of Microbiology - 2017