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The toxicity of Cr to soil bacterial isolates was studied by measuring turbidity of liquid cultures supplemented with Cr(VI) or Cr(III). Ten to 12 ppm Cr(VI) was found inhibitory to most isolates growing in either a soil‐extract medium and or in a semisynthetic medium. Gram negative bacteria were more affected than gram positive bacteria by Cr(VI) both at this level and at 1 ppm. Toxicity due to Cr(III) was not observed at similar levels.Nellis loam (Typic Eutrochrept) and Hinesburg fine sandy loam (Entic Haplorthod) samples were treated with 10 and 100 ppm Cr(VI) and 100 ppm Cr(III). All three treatments significantly decreased CO2 evolution from that of controls for the 3‐week duration of the experiment. Extractable Cr(VI) in these soils decreased rapidly during this time, indicating that reduction of the added Cr(VI) was occurring.The toxicity of low levels of Cr(VI) to gram negative bacteria in aqueous culture indicates that soil microbial transformations, such as nitrification, may be affected. The fact that both Cr(III) and Cr(VI) inhibited microbial activity in soil shows that more caution may be needed in handling Cr‐containing wastes, regardless of the form of Cr.

Abstract. A nonpigmenting strain of Serratia marcescens Bizio isolated from dead and apparently diseased wild apple maggot flies, Rhagoletis pomonella (Walsh), was shown to he pathogenic to healthy apple maggot flies upon ingestion. The microorganism was detected in live adult alimentary canal organs four days post ingestion hut produced death in some flies within 24 h when flies fed on a cell concentration of 4.7 x 104 cfu/ml and within 8 h when flies fed on filter-sterilized culture medium that previously contained a 21 h culture of S. marcescens. Increasing the cell concentration 10,000 fold did not lead to an increased rate of kill. Young flies (7-10 days old) were more susceptihle to infection leading to death than were older flies (21-28 days old). The potential use of S. marcescens cells as control agents against apple maggot flies is negated hy their pathogenicity to vertehrates; however, the potential use of toxic compounds produced hy this strain of S. marcescens is discussed.

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