Dear Cynthia,

What kind of penicillin are you using? Penicillin G is not working well
against gram-negative  bacteria. Better to use ampicillin. We usually get
rid of contaminating bacteria by increasing the ampicillin concentration (up
to fivefold) and adding streptomycin and tetracyclin. Maybe this might help.
All the best,

Sascha.

-- 
Sascha Thewes, PhD
Institute for Biology - Microbiology
Department of Biology, Chemistry, Pharmacy
Freie Universität Berlin
Königin-Luise-Str. 12-16
14195 Berlin
Germany

Phone: +49-(0)30-838-53373
Fax: +49-(0)30-838-57773
Email: [log in to unmask]
Web: 
http://www.biologie.fu-berlin.de/arbeitsgruppen/mikrobiologie/ag_mutzel/inde
x.html


Am 08.11.2013 16:32 Uhr schrieb "Damer, Cynthia Kay" unter
<[log in to unmask]>:

> Dear Colleagues,
> 
> My lab is currently experiencing a culture contamination problem that I have
> not experienced previously.  We are culturing Dicty in HL-5 supplemented with
> penicillin and streptomycin on plastic sterile culture dishes.  The
> contaminant appears to be very small (less than a micron) and spherical and is
> not able to move.  In addition, it is very slow growing.  However, as the
> contaminant becomes more numerous, the Dicty cells become rounded and
> eventually die.  Unfortunately, even when we thaw out strains in new media,
> the contaminate appears again.  We've also tried centrifuging and washing many
> times and greatly diluting the cultures.  This seems to get rid of most of it,
> but it eventually grows up again.
> 
> Any advice would be appreciated.
> 
> Thanks,
> 
> Cynthia
> 
> Cynthia K. Damer
> Professor
> Biology Department
> Brooks Hall Room 229
> Central Michigan University
> Mount Pleasant, MI 48859
> (989) 774-3455