Environmental Microbiology: Bacteria & Fungi on the Foods We Eat

Abstract

The near daily news reports on food-borne diseases caused by contaminated produce, dairy, or meats suggests to the public that the safety of the U.S. food supply is in jeopardy. These reports, as well as a general distrust in federal agencies due in part to mad cow disease and toxigenic forms of E. coli in ground beef, have resulted in an increasing demand for "organic" foods and an interest in sustainable agriculture (Macilwain, 2004). It is, therefore, timely and appropriate to provide classroom lectures and laboratory experiences to show that microbes are ubiquitous in our environment, including in the foods that we eat (Scholthof, 2001; Scholthof, 2003). In presenting such material, it is important that we distinguish between common microbes and potentially harmful pathogens. In particular, it is imperative to place an emphasis on the fact that many bacteria and fungi are both beneficial and necessary for crop production and robust ecosystems. This hands-on laboratory experience has the added benefit of reinforcing safe food storage and preparation. The lab described in this article was specifically adopted for an undergraduate course "Pathogens, the Environment, and Society" for the Bioenvironmental Sciences undergraduate degree program at Texas AM Scholthof, 2003). Materials & Methods The goal of this student laboratory experience is to use basic microbiological techniques to examine common bacteria and fungi on fruits and vegetables. Inexpensive conventionally grown produce (see Table 1) that is available year-round was selected from a supermarket. We also purchased organically grown carrots, apples, and tomatoes from the supermarket to determine if there were differences in the types of bacteria and fungi when compared to conventionally produced fruits and vegetables. Alfalfa sprouts were selected because there have been several reports warning about food-borne illnesses that are linked to consuming fresh sprouts (Breuer et al., 2001; Taormina et al., 1999). Preparation Standard microbiology protocols were used for the preparation of growth media. The details for preparing materials and identifying bacteria were based on protocols provided by the Bacteriological Analytical Manual Online (www. cfsan.fda.gov/~ebam/bam-toc.html) and the supplier of the dehydrated culture media (Carolina Biological Supply Co.). Two growth media were used: nutrient broth-yeast extract agar (NBY) and potato dextrose agar (PDA) supplemented with two antibiotics, ampicillin and streptomycin (PDA-AS). NBY was used for bacteria because it allows for good growth of most bacteria with characteristic colony morphology. PDA is a common growth media for isolating fungi. The addition of the antibiotics in PDA-AS is to inhibit bacterial growth, allowing for better development of the fungi. The fungi and bacteria were cultured at 28[degrees] C in a standard laboratory incubator. Alternatively, they can be kept at room temperature, although it will require a longer incubation period. Bacterial growth was apparent after 24 hours. Most fungi were visible on PDA-AS plates within 72 hours. For this laboratory session, two to four students were assigned to a group working with fruits or vegetables. For example, both organic and conventionally farmed produce were tested by Group 1 (apples and peaches), Group 2 (carrots), and Group 3 (tomatoes). …