Behavioural and genetic evidence for C. elegans' ability to detect volatile chemicals associated with explosives

Abstract

Background: Automated standoff detection and classification of explosives based on their characteristic vapours would be highly desirable. Biologically derived odorant receptors have potential as the explosive recognition element in novel biosensors. Caenorhabditis elegans' genome contains over 1,000 uncharacterised candidate chemosensory receptors. It was not known whether any of these respond to volatile chemicals derived from or associated with explosives.

Methodology/principal findings: We assayed C. elegans for chemotactic responses to chemical vapours of explosives and compounds associated with explosives. C. elegans failed to respond to many of the explosive materials themselves but showed strong chemotaxis with a number of compounds associated with commercial or homemade explosives. Genetic mutant strains were used to identify the likely neuronal location of a putative receptor responding to cyclohexanone, which is a contaminant of some compounded explosives, and to identify the specific transduction pathway involved. Upper limits on the sensitivity of the nematode were calculated. A sensory adaptation protocol was used to estimate the receptive range of the receptor.

Conclusions/significance: The results suggest that C. elegans may be a convenient source of highly sensitive, narrowly tuned receptors to detect a range of explosive-associated volatiles.

Figure 1. C. elegans chemotaxis indicates the presence of a high affinity receptor for cyclohexanone.

Log concentration-response curves for the chemotaxis of wild-type N2 nematodes to cyclohexanone (blue) compared with the responses to the AWA-directed odorant diacetyl (pink) and AWC-directed odorant benzaldehyde (green). Each bar represents the mean ± sem of at least four independent assays involving two plates each.

Figure 2. The C. elegans chemotactic response to cyclohexanone is primarily mediated via the AWC neurones.

(A) Chemotactic responses of wild-type N2 and cell specific mutants odr-7(ky4), odr-1(n1936), odr-1(n1936) odr-7(ky4) and ceh-36(ky646) to 1∶100 cyclohexanone. (B) Log-concentration chemotactic responses of wild-type and cell specific mutants to cyclohexanone. N2 (blue), odr-1(n1936) (pink), odr-7(ky4) (green), odr-1(n1936) odr-7(ky4) (pink line), and ceh-36(ky646) (purple line). Each bar represents the mean ± sem of at least six (A) or four (B) independent assays. Statistics: * p<0.05 and ** p<0.01 in a one way ANOVA and Dunnett's post test comparing all means to the wild-type (N2) mean.

Figure 3. Genetic analysis of cyclohexanone receptor transduction pathway.

(A) Chemotactic responses of wild-type N2 and mutants defective in one or more G_α subunits: odr-3(n2150), gpa-3(pk35), and gpa-3(pk35) odr-3(n1605) to 1∶100 cyclohexanone. (B) Log cyclohexanone concentration-response data for wild type N2 (blue), mutant odr-3(n2150) (pink) and gpa-3(pk35) odr-3(n1605) (green). (C) Chemotactic responses of mutant lines defective in various guanylate cyclase or ion-channel subunits: odr-1(n1936), daf-11(m47), tax-2(ks10), tax-4(ks28), and osm-9(ok1677) to 1∶100 cyclohexanone. Each bar represents the mean ± sem of at least six (A, C) or four (B) independent assays. Statistics: * p<0.05 and ** p<0.01 in a one way ANOVA and Dunnett's post test comparing all means to the wild-type (N2) mean.

Figure 4. Cyclohexanone receptor selectivity probed by odour adaptation.

Wild-type N2 worms were adapted to 1 µl of 1/10 (A) or undiluted (B) cyclohexanone for 60 minutes prior to chemotaxis to the specified odorants being measured. Control worms were treated identically except that odorant was not present during the adaptation period. Dilutions of test odorants were: cyclohexanone 1∶100, benzaldehyde 1∶100, isoamyl alcohol 1∶100, cyclohexanol 10 mg/ml, butanone 1∶1000, diacetyl 1∶1000. Bars represent the mean ± sem of at least six independent assays. Statistics: * p<0.05 and ** p<0.01 comparing the mean of adapted to the mean of unadapted nematodes. Numbers (A) indicate the adapted response as a percentage of the control, unadapted response to the test odorant.