The atmosphere of Venus is composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases, most notably sulfur dioxide.
The main cloud deck is located in the 48-70 km altitude range and is composed mainly (75-96%) of sulfuric acid droplets, with a temperature of 27$⁰$C and a pressure of 0.53 atm at 55 km.
But to put the acidity into perspective, the mass loading of the droplets is 10 mg/m$³$, while the density of CO$_2$ at that altitude is about 950 g/m$³$.
Nostoc commune for instance,is a colonial species of cyanobacterium which forms a gelatinous mass for protection and can withstand acidic exposure and other extreme conditions like desiccation and high and low temperatures.
In addition the cells contain pigments that absorb ultraviolet radiation, which enables it to survive high levels of it, and in some cells nitrogen-fixing occurs.
There are also bacteria, like Enterobacter aerogenis, which produce hydrogen, and when it could live inside the gelatinous mass it could provide the necessary buoyancy to float within the clouds of Venus.
Assuming the biofilms could be provided with the necessary trace elements, could they be airtight to some degree, to even float in the air with gas vesicles for buoyancy ?