For a wit, I would guess that they use only one strain of yeast. Using a separate yeast strain is more common from the "monk" breweries, or for some other styles, which they yeast can't handle the higher alcohol.
If your takin' your girlfriend out tonight
You better park the car well out of sight
'Cos if they catch you in the back seat trying to pick her locks
They're gonna send you back to mother in a cardboard box !