![](media/image1.png){width="6.498611111111111in" height="1.0in"}

**DATE:** February 27, 2007

**Subject:** Microbiological Contamination of Corn Cob Bedding

At the 2006 National AALAS meeting Dr. Julius Thigpen (NIEHS) reported
that corncob bedding carries a higher microbiological load than other
types of lab animal bedding and recommended that it should be autoclaved
before using. To confirm Dr. Thigpen's observations I analyzed various
types of lab animal bedding, used at the NIH in Bethesda, for
microbiological concentrations.

I compared the microbiological load from seven types of lab animal
bedding that was not autoclaved. The results confirmed that the
microbiological load for corncob bedding was greater than for other
types of bedding (Table 1). It was also positive for Listeria.

**Table 1**

+------------+---------+------------+-------+-------------+----------+
| **Fresh    | **Coli  | **Aerobic  | **    | **Mold      | **Yeast  |
| Bedding**  | forms** | Plate**    | Liste | Count**     | Count**  |
|            |         |            | ria** |             |          |
|            | **(M    | **Count**  |       | **(cfu/g)** | **(      |
|            | PN/g)** |            |       |             | cfu/g)** |
|            |         | *          |       |             |          |
|            |         | *(cfu/g)** |       |             |          |
+------------+---------+------------+-------+-------------+----------+
| Corn Cob   | \<3     | 80,000     | Pos   | 1,000       | 3,000    |
| 1/8        |         |            | itive |             |          |
+------------+---------+------------+-------+-------------+----------+
| Corn Cob   | 3.6     | 2,600,000  | Pos   | 260,000     | 200,000  |
| 1/4        |         |            | itive |             |          |
+------------+---------+------------+-------+-------------+----------+
| Hardwood   | \<3     | 300        | Neg   | 33,000      | \<10     |
|            |         |            | ative |             |          |
+------------+---------+------------+-------+-------------+----------+
| Pine       | \<3     | 400        | Neg   | 30,000      | \<10     |
| Shavings   |         |            | ative |             |          |
+------------+---------+------------+-------+-------------+----------+
| Tekchip    | \<3     | 2,000      | Neg   | 10          | 600      |
|            |         |            | ative |             |          |
+------------+---------+------------+-------+-------------+----------+
| Paperchip  | \<3     | 7,000      | Neg   | 50          | 70       |
|            |         |            | ative |             |          |
+------------+---------+------------+-------+-------------+----------+
| Tekfresh   | \<3     | 980        | Neg   | 600         | \<10     |
|            |         |            | ative |             |          |
+------------+---------+------------+-------+-------------+----------+

To determine if autoclaving corn cob bedding has an affect on the
microbiological load of the bedding after exposure to animal
waste,Number range CHAPTER samples of bedding were either autoclaved or
not and then placed in microisolator cages and 20 ml of a slurry made
from dirty bedding was applied to each cage of bedding . The
contaminated bedding was kept in the microisolator cages for two weeks
before analyses. The results show that the autoclaved corncob bedding
samples had lower bacteria and fungi concentrations and were negative
for Listeria compared to corncob bedding that was not autoclaved (Table
2).

**Table 2**

+------------------+--------+----------+-------+------------+---------+
| **Used Bedding** | *      | *        | **    | **Mold     | **Yeast |
|                  | *Colif | *Aerobic | Liste | Count**    | Count** |
|                  | orms** | Plate**  | ria** |            |         |
|                  |        |          |       | *          | **(c    |
|                  | **(MP  | *        |       | *(cfu/g)** | fu/g)** |
|                  | N/g)** | *Count** |       |            |         |
|                  |        |          |       |            |         |
|                  |        | **(      |       |            |         |
|                  |        | cfu/g)** |       |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob 1/8     | \<3    | 7300     | Neg   | 10         | \<10    |
| autoclaved-1     |        |          | ative |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob ¼       | \<3    | 5100     | Neg   | \<10       | \<10    |
| autoclaved-2     |        |          | ative |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob 1/8     | 43     | 2        | Neg   | \<10       | 2,900   |
| autoclaved-3     |        | ,300,000 | ative |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob 1/8     | 460    | 110,000  | Pos   | 30,000     | 20,000  |
|                  |        |          | itive |            |         |
| not autoclaved-1 |        |          |       |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob 1/4     | \      | 1        | Pos   | 200,000    | 70,000  |
|                  | >1,100 | ,800,000 | itive |            |         |
| not autoclaved-  |        |          |       |            |         |
| 2                |        |          |       |            |         |
+------------------+--------+----------+-------+------------+---------+
| Corn Cob 1/8     | 460    | 270,000  | Pos   | 40,000     | 20,000  |
|                  |        |          | itive |            |         |
| not autoclaved-  |        |          |       |            |         |
| 3                |        |          |       |            |         |
+------------------+--------+----------+-------+------------+---------+

Bedding is an excellent growth media for bacteria, yeasts, and fungi.
Noxious gases such as ammonia and carbon dioxide are products of animal
waste and microbiological metabolism. The effects of elevated carbon
dioxide levels on murine physiological responses are unknown. However,
elevated ammonia levels cause histological changes in respiratory mucosa
and increase susceptibility to murine respiratory mycoplasmosis. They
also reduce the drug-metabolizing activity of hepatic microsomal enzymes
and thus potentially alter the responsiveness of animals to
pharmacological studies (Weisbroth, 1979). Listeria can be pathogenic
causing septicemia, meningitis, encephalitis, pneumonia, intrauterine or
cervical infections resulting in abortions or still births. Listeria
contamination is an indicator of inadequate quality assurance post
heat-processing at the production plant. Some yeasts and molds can be
opportunistic pathogens causing infection in immunocompromised animals.
Molds can produce mycotoxins that are harmful. Mold spores can be
allergenic. Aerobic plate count (APC) is intended to indicate the level
of microorganisms in a product. APC can be used to indicate post
heat-processing contamination. These surveys appear to confirm Dr.
Thigpen's results and suggest that corncob bedding should be autoclaved
prior to using in lab animal cages.

Weisbroth, S. H. (1979) Chemical Contamination of Beddings: Problems and
Recommendations. Lab Anim 8, 24-34.

Dennis Barnard

Lab Animal Nutritionist, MSc

DVR/ORS/NIH