I'm not surprised the finings weren't clean. It was common to use sour, returned beer as a solvent for the finings. Sounds like a good way of spreading an infection to every single barrel you brew.
"The Franco-Prussian war of 1870-1871 and the subsequent defeat and occupation of parts of France actually provided the stimulus for Pasteur's temporary obsession with beer. Being highly patriotic, he thought that by improving the quality of French he could 'get back at the Germans' by producing superior prioducts which would gradually replace the hitherto more consistent and popular German beers from their European markets. French beer production at that time was wholly unscientific and it was pure chance that a wholesome batch would be produced. In 1871, Pasteur visited the laboratory of Prof. Emile Duclaux at Clermont-Ferrand and soon became associated with a small brewery at nearby Chamalieres. There he devised a new method of brewing (which he patented on 28 June 1871). The whole ethos of the new method was to avoid contact between beer and the atmosphere as far as possible, and hence reduce the likelihood of contamination. According to Pasteur, beers brewed bt this new method should be called 'Bières de la Revanche Nationale', or 'Revenge Beers'. The brewery at Chamalieres was very small and Pasteur felt the need to work at a far larger consern. Having no desire to visit a German brewery, he turned to England where he arrived in September 1871 with a small entourage. From his base at the Grosvenor Hotel in Victoria he visited a number of London breweries during his fortnight stay. The only surviving record of his visit is of 9 September 1871 when he toured the Whitbread Brewery in Chiswell Street. Although some major British breweries employed microscopes at this time, Whitbread & Co. did not, and during his investigations (he carried his own microscope) Pasteur found serious contaminations in the porter an ale yeast. The beer-finings were also found to be contaminated. As a result of a management meeting, fresh yeast was obtained from a nearby brewery and it was agreed that many of Pasteur's brewing tenetsshould be instigated. On revisiting the Chiswell Street brewery about one week later he found that a microscope had been purchased and new yeast management procedures adopted. Thus, the huge Whitbread tradition of scientific laboratory control of the brewing process emanates from Pasteur's visit."
"Brewing" by Ian Spencer Hornsey, 1999, pages 7-8.
I found a brewing log from just a couple of weeks after Pasteur's visit. You can see it above. Sadly, it has no mention of the yeast's origin, at least not as far as I can see.