Bacterial enumeration from coloidal silica

GS microHero at compuserve.com
Thu Jul 10 23:50:54 EST 1997

Rocky Baker wrote:
> Can anyone suggest a technique to enumerate bacteria in a colloidal silica
> solution.  The solution is composed of colloidal silica (15 nm particle
> size) in water containing an amine.  The pH is 10 and the solution has a
> susdsy appearance when shaken.  The concentration of silica is unknown to
> me but the solution is slightly opaque.  We are interested in techniques
> to enumerate any bacteria present in the slurry using epifluorescence or
> culturing methods. The solution will rapidly clog a 0.2 u membrane filter.
> Thanks.
> Rocky Baker

How many bacteria do you expect? How fast do you need results? If it's
more than around 100000 per mL you'll need to use a dilution method, and
you'll have to bring the pH down anyway. Either way, spread-inoculating
a known volume on to a solid medium should work. It depends on the
amount of silica - "slightly opaque" is a little imprecise. If you're
looking at counting relatively small numbers of bacteria, it's more
difficult. One way would be using one of the methods used for testing
bacterial count in reservoir or tap water - you inoculate relatively
large volumes into a number of bottles of liquid media, and count the
number which show growth. If you only want to determine order of
magnitude, do a set of 10-fold dilutions, inoculate a known volume of
each dilution into medium and count the number showing growth - you
assume that the highest positive dilution came from at least 1 and not
more than 9 bugs. Since you have such a high pH most of the bacteria
present will be pretty sick, so you'll need a rich medium, I guess, and
allow a longer than usual incubation time.

microhero at compuserve.com

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