Bacterial enumeration from coloidal silica

Bob Presswood rpressw at ix.netcom.com
Mon Jul 14 14:25:05 EST 1997

Phil Geis <jorge2 at earthlink.net> wrote:

>GS wrote:
>> 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.
>> GS
>> microhero at compuserve.com

>Suggest you maintain high(er) pH.  Bug adapted to the silica environment
>described may not grow well in neutral pH, enriched system.

Actually, a somewhat opaque suspension would not interfere with an
estimate based upon ATP quantitation by bioluminescence of firefly
luciferase.  Dilution and culturing would not be necessary, the result
could be obtained in a few minutes, and it's enormously easier than

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