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An experimental study of postmortem decomposition of methomyl in blood
Yuka Kawakami1, Chiaki Fuke1, Maki Fukasawa1
1Department of Legal Medicine, Graduate School of Medicine, University of the Ryukyus, 207 Uehara, Nishihara, Okinawa Prefecture 903-0215, Japan.
Abstract:
Methomyl (S-methyl-1-N-[(methylcarbamoyl)oxy]thioacetimidate) is a carbamate pesticide. It has been noted that in some cases of methomyl poisoning, methomyl is either not detected or detected only in low concentrations in the blood of the victims. However, in such cases, methomyl is detected at higher concentrations in the vitreous humor than in the blood. This indicates that methomyl in the blood is possibly decomposed after death. However, the reasons for this phenomenon have been unclear. We have previously reported that methomyl is decomposed to dimethyl disulfide (DMDS) in the livers and kidneys of pigs but not in their blood. In addition, in the field of forensic toxicology, it is known that some compounds are decomposed or produced by internal bacteria in biological samples after death. This indicates that there is a possibility that methomyl in blood may be decomposed by bacteria after death. The aim of this study was therefore to investigate whether methomyl in blood is decomposed by bacteria isolated from human stool. Our findings demonstrated that methomyl was decomposed in human stool homogenates, resulting in the generation of DMDS. In addition, it was observed that three bacterial species isolated from the stool homogenates, Bacillus cereus, Pseudomonas aeruginosa, and Bacillus sp., showed methomyl-decomposing activity. The results therefore indicated that one reason for the difficulty in detecting methomyl in postmortem blood from methomyl-poisoning victims is the decomposition of methomyl by internal bacteria such as B. cereus, P. aeruginosa, and Bacillus sp.
Insights
Methomyl pesticide can decompose in blood after death, making it hard to detect. This study found that gut bacteria, including Bacillus cereus and Pseudomonas aeruginosa, break down methomyl into dimethyl disulfide (DMDS).
Area of Science:
- Forensic Toxicology
- Microbiology
- Environmental Science
Background:
- Methomyl, a carbamate pesticide, is often undetectable or present in low concentrations in postmortem blood samples.
- Higher methomyl concentrations in vitreous humor compared to blood suggest postmortem decomposition.
- Previous research indicated methomyl decomposition in pig organs, but not blood, leaving the cause unclear.
Purpose of the Study:
- To investigate the bacterial decomposition of methomyl in human blood.
- To identify specific bacteria responsible for methomyl degradation.
- To explain the low detection rates of methomyl in postmortem blood.
Main Methods:
- Incubation of methomyl with human stool homogenates.
- Isolation and identification of bacterial species from stool.
- Assaying bacterial isolates for methomyl-decomposing activity.
- Detection of decomposition products, such as dimethyl disulfide (DMDS).
Main Results:
- Methomyl decomposition was confirmed in human stool homogenates, producing DMDS.
- Three bacterial species—Bacillus cereus, Pseudomonas aeruginosa, and Bacillus sp.—exhibited methomyl-decomposing activity.
- These bacteria are implicated in the degradation of methomyl in biological samples.
Conclusions:
- Internal bacteria, particularly Bacillus cereus, Pseudomonas aeruginosa, and Bacillus sp., are responsible for methomyl decomposition in postmortem blood.
- Bacterial degradation of methomyl contributes to the challenges in its detection in forensic toxicology cases.
- This finding provides a crucial explanation for methomyl underdetection in victims of poisoning.
