PROGRAMS AND PLANS--(1) Disposal of petri dishes containing bacteria media, and (2) Aspergillus 

In Reply Refer To:                                    April 19, 1993
WGS-Mail Stop 412


Subject: PROGRAMS AND PLANS--(1) Disposal of petri dishes containing
                             bacteria media, and (2) Aspergillus 

The purpose of this memorandum is to provide guidelines for
sterilization and disposal of petri dishes that contain cultures of
bacteria or media for culturing bacteria, and to present information
on Aspergillus, a fungal pathogen.


Cultured petri dishes produced by Water Resources Division (WRD)
activities are classified as infectious waste and should be handled
with care.  Petri dishes containing bacteria media must be routinely
sterilized prior to disposal, regardless of whether or not they have
been used to culture bacteria.  Cultured plates should be sterilized
as soon as practicable after colonies are counted.

Autoclaving is the only method endorsed by WRD for sterilizing used
petri dishes.

Autoclave all dishes at 121 degrees C at 1.05 kg/cm2 (15 psi) for 30
minutes before discarding (Britton and Greeson, 1987, pages 39 and

After counting, petri dishes should be secured in a plastic autoclave
bag and autoclaved prior to disposal.  During multi-day field trips,
place used petri dishes in plastic autoclave bags, and close the bags
for transport to the office.  Then, following autoclaving, place the
closed bags of petri dishes in dumpsters or other containers that will
be emptied mechanically, to prevent accidental contact with people or
animals.  Never place petri dishes in motel or office wastebaskets.
Suitable autoclaves are available for less than $500.00 (less than
$200.00 with external heat source).


The fungus Aspergillus is common in the environment, particularly on
rotting wood, leaves and wood products.  Aspergillus colonies are
white to beige or light brown, becoming darker with age (24-48 hours).
The colonies are round, fuzzy, mold-like in appearance, and have
radiating grooves.  They are not easily discerned from many other
fungi and molds that grow on culture media.  Most of the media used in
WRD are selective and reduce or eliminate the growth of fungi like
Aspergillus.  However, nonselective, enriched nutrient agars, such as
the tryptone glucose extract agar used for the Standard Plate Count
method (Britton and Greeson, 1987, page 7), sometimes grow fungi and
yeast that may be pathogenic.  These agars are often used when
sampling drinking water, well water, and soils.

Aspergillus can cause disease in immune-suppressed individuals.
People with intact immune systems are not normally affected.  People
whose immune systems are compromised should not be put in situations
where they will be occupationally exposed to this pathogen.  When
working with bacterial plates, it is good microbiological practice to
assume that all bacterial samples are potentially hazardous and to use
normal sterile technique and safety practices.  For safety reasons,
and to avoid contamination of plates, care must be taken to avoid
physical contact.

If cursory examination of plates reveals fungi which fit the
Aspergillus description, extra precautions can be taken.  A dust mask
can be worn to reduce inhalation of spores, and a laminar hood can be
used to reduce exposure to pathogens that might become airborne during
counting.  Even if these precautions are employed, exposure will not
be eliminated, because the spores are common in human environments.
Working with plates with their covers on is not recommended because it
can reduce the ability to see colonies.


Britton, L.J., and Greeson, P.E., 1987, Methods for the collection and
     analysis of aquatic biological and microbiological samples:
     Techniques of Water-Resources Investigations of the United
     States Geological Survey, Book 5, Chapter A4, pgs. 7, 39, and 53.

                                                          David A. Rickert

This memorandum does not supersede any previous Office of Water
Quality Technical Memorandum.

Key Words: aspergillus, bacteria, disposal, fungus, microbiology,
           petri dish, safety, sterilization, water quality

Distribution:  A, B, S, FO, PO