Bacterial enumeration from coloidal silica

Phil Geis jorge2 at earthlink.net
Mon Jul 14 00:00:25 EST 1997

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.

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