Figure 1.
Effects of dietary restriction (DR) on axenic medium (AXM) plates, and rescue by addition of Escherichia coli. A, Fecundity of wild-type Caenorhabditis elegans at 20°C on AXM plates with addition of different numbers of radiation-arrested E. coli K12 (10 kGy). Data shown are the average of results from eight partially overlapping replicate trials. There was a small replicate effect on fecundity (variance between replicates = 0.028 ± 0.02). For each concentration, 50–90 adults were scored. Decreasing the concentration of bacteria on the plates below 2 × 108 to 2 × 109 cells caused a substantial decline in progeny production (p < .0001 for 2 × 107 vs 2 × 108 cells, and for 2 × 106 vs 2 × 108 cells). A higher concentration of E. coli similarly caused a reduction of fecundity (p < .0001 for 2 × 1010 vs 2 × 109 cells). B, Survival of N2 at 20°C on AXM plates with different concentrations of radiation-arrested E. coli K12 (10 kGy). There is no difference in life span between worms grown on 2 × 1010 and 2 × 109 cells (mean adult life span on 2 × 1010 cells = 23.4 ± 0.6 days, N = 77 (dead)/22 (censored); mean adult life span on 2 × 109 cells = 24.3 ± 0.8 days, N = 48/44; p = .39). When the E. coli concentration on the plates was diluted to 2 × 108 or 2 × 107 cells, life span was extended substantially, but to the same extent (mean adult life span on 2 × 108 cells = 31.7 ± 0.8 days, N = 81/19; mean adult life span on 2 × 107 cells = 32.9 ± 1.2 days, N = 70/30; p < .0001 for both comparisons). Three other replicate experiments showed largely similar results (data not shown). Censored worms were mainly a consequence of crawling off the plates in search of food on low concentrations of E. coli, and of crawling in the agar at higher E. coli concentrations. Note that worms, at a density of 6–8 per plate, were maintained on the same plates throughout life; thus at low bacterial cell densities, it is possible that E. coli density declined over time. C, Maximal worm volume at 20°C on AXM plates with different concentrations of radiation-arrested E. coli K12 (10 kGy). Average of four partially overlapping replicate trials. There was no effect of replicate on body size (variance between replicates = 0). Body size is maximal on the highest food concentration and declines substantially with decreasing E. coli concentration on the plates (p < .0001). *Indicates significant differences as compared to 2 × 1010 cells (p < .0001 for comparison with 2 × 106 and 2 × 107 cells/plate and p = .0004 for comparison with 2 × 108 cells/plate). Note that the volumes on the two lowest bacterial concentrations are probably underestimated due to the combination of substantial asynchrony on plates with such low food concentration on the one hand, and the bulk measurement of the length and width of the nematodes on the other hand. Length and width were affected in a similar fashion (data not shown). D, Survival of N2 at 22.5°C on nematode growth medium (NGM) and AXM plates with or without a rescuing concentration of carbenicillin-arrested E. coli OP50. Worms maintained on AXM with carbenicillin but without E. coli (mean adult life span = 31.7 ± 0.4 days, N = 68/82) were longer lived than animals grown on NGM or AXM with E. coli (mean adult life span on NGM with OP50 = 20.3 ± 0.3 days, N = 133/25; mean adult life span on AXM with OP50 = 19.5 ± 0.3 days, N = 121/29; p < .0001 for comparison between AXM with and without E. coli). The axenic condition without bacteria and without carbenicillin was included as a test for possible toxic effects of the antibiotic. The results show that carbenicillin does not affect the long life span of axenic cultured worms (mean adult life span on AXM = 31.7 ± 0.3 days, N = 75/68; p = .93). Similar results were obtained in a second trial (data not shown).