Effect of heptachlor and related compounds on growth of Staphylococcus aureus.

Abstract

Previous work in this laboratory has shc~a~ that the grouch of Staphylococcus aureus in broth media was affected by as little as 3 ug per milliliter of heptachlor (2,4). In addition, pesticides containing 72% heptachlor were found to be more inhibitory than those containing 99.8% heptachlor (4). This study was made to determine the cause of the greater inhibition of 72% heptachlor and to obtain additional information on factors affecting the inhibition of S. aureus by heptachlor and related compounds. Methods and ~terials Pesticides. The following pesticides were used in this study: purified chlordane, 99.8% ganlma chlordane, 99.5% nonachlor and 72, 73, 74, 99 and 99.8% heptachlor. Stock solutions of each pesticide were made to contain 3 n~ per milliliter of absolute alcohol. The stock solutions were diluted with alcohol so that addition of 2 ml or less to 50 to 300 ml of sterile media would result in concentrations of 5 to 50 ug per ml of media. The pesticides were added to the sterile media prior to inoculation with S. aureus. Organism and cultural procedure. A 15 to 18 hour trypticase soy broth (TSB) culture of S. aureus (Departr~nt of Animal Sciences, Food Science Sect[on,~rsity of Kentucky) was used to inoculate flasks containing the desired pesticide and either TSB or skinmtilk. Control flasks contained media with and without alcohol and were treated the same as flasks ccntaining pesticides. All flasks were incubated at 37 C either in a water bath or incubator. In one part of the study, the flasks were shaken at 140 oscillations per minute during incubation in a water bath. Determination of growth. Determinations of numbers of bacteria were made every 2 hours during the initial 12 to 24 hours, and then daily for the duration of the incubation period. Numbers of S. aureus were determined by making standard plate counts on trypticase soy agar and incubating the plates at 37 C for 48 hours (i). In several trials, growth also was determined by following change in turbidity at 600 mu with a Klett-S~mnerson