Re: How do I measure bacterial growth in a jar?

You have several problems.

First, it doesn't seem to me that bacterial growth is what you really set 
out to measure.  More likely, you wanted to compare numbers ("loads") of 
bacteria on the toilets.  This is usually done by spreading the bacteria on 
a surface and allowing time for each to multiply and form a "colony" that 
can be seen without magnification — maybe 1-2 days.  

When sampling a surface, one either rubs a moist swab over a standard 
amount of area and then dilutes the fluid from the swab and spreads it onto 
a growth surface or (if there aren't many bacteria) presses the growth 
surface directly onto the surface to be sampled.  The contact (press) 
sampling method is usually done with a sterile liquid medium that has had 
agar added to it so it forms a gel like tough Jell-O.  

You might be able to use the cut surface of a potato as the growth surface, 
but you wouldn't want to rub the potato onto the toilet surface.  You would 
need to cut the potato so that there were no bacteria on the cut surface 
(maybe bleach the outside of the potato and cut with a knife blade that had 
been in boiling water).  Then, rather than rub the potato on the toilet 
surface, you would want to "moosh" the potato straight down onto the 
surface to be sampled, and lift it straight up again.  That surface of the 
potato should not be touched by anything afterward — if you want to 
incubate it in a jar, you need to figure a way to get the potato in without 
the sample surface touching the jar's inside.  

The jar might have a few extra drops of boiled water in it and be tightly 
closed, but you would have to get your results within a couple of days 
anyway, for fear that mold would take over the whole surface.  If the 
surface dries out, nothing will grow, as you noted.  This probably means 
that those jars weren't tightly sealed, and the moisture evaporated.  
Nothing grows on potato chips, either, because of dryness.

The biggest problem that I see is that you will have no idea of what kind 
of bacteria you are measuring.  Surfaces everywhere have bacteria on them, 
most of which have nothing to do with sanitation or health.  Because public 
toilet seats and bowls are often sanitized daily, perhaps with a product 
that leaves an antibacterial residue on the surface, there could easily be 
fewer bacteria on the toilet than on the floor; but just maybe the ones on 
the seat got there when someone sat on it, rather than drifting down from 
the air.  And if you really did try to sample the sat-on surface of the 
seat, you are dealing with a curved surface that would be very hard to 
contact evenly with anything but a swab.  

So, what I'm saying is that everything has bacteria on it, and most of them 
just dropped out of the air and do us no harm.  If you sample toilet seats 
(you didn't say "seats," but I'm inclined to assume it), they needn't have 
more bacteria than other surfaces, though they might have bacteria that 
could cause disease — or not.  Getting a sample that accurately represents 
the number of bacteria is hard, and the cut-potato assay may not yield 
numbers.  All the same, it's not the number of bacteria but what kind they 
are that counts from a health standpoint, and you don't have the means to 
identify them.  If you rub the surface with a cut potato and put the potato 
into a moist jar, the bacteria that grow fastest will probably be those 
that like to grow on potato starch, which probably aren't disease bacteria. 
 After 1 or 2 days, the total amount of bacterial growth on the cut potato 
surface really won't be related to the number of bacteria that were on the 
toilet unless you spread them out so that they form individual colonies, or 
there were few enough that they didn't all grow together (called confluent 
growth) on the potato by the time they were large enough to see. 

I think we get too preoccupied with public toilets.  You don't have the 
means at your disposal to measure the public health risk that these might 
represent, compared, say, to the surface of your desk at school.  Before 
there were public toilets, people relieved themselves wherever they were 
when they felt the need.  With any reasonable care, public toilets are much 
better than that.