Vermiculite is a compound formed by hydration of basaltic mineral; it has been used as a substrate for fecal culture, particularly in veterinary parasitology, as it has the advantages of being similar to soil and costing less3–7. It has also been used for growing insect8 or nematodes larvae in order to evaluate the larvicidal effects of fungi9–12.
Very few reports have compared the culturing of stools in vermiculite vs. that with addition of other media. Steffan et al. 13 compared the addition of vermiculite or polystyrene pellets to cultivate cattle feces and found that both significantly improved growth of larvae of Ostertagia ostertagi and Cooperia oncophora as compared to the cultivation of feces without additives.
Agyei14 compared the addition of vermiculite or sawdust to cultivate cattle feces to obtain L3 larvae ofHaemonchus, Trichostrongylus, Cooperia, and Oesophagostomum and concluded that the number of L3 larvae obtained from sawdust cultures was significantly higher than those obtained by other techniques; the former also did not require further elaborate apparatus.
As there have been no reports comparing the cultivation of feces in vermiculite and charcoal, we here compared vermiculite with charcoal as growth medium for larvae of Strongyloides venezuelensis. Feces of Wistar albino rats infected with S. venezuelensis (900 larvae, subcutaneously) were collected using an anticoprophagic cage. The eggs were quantified with a McMaster chamber. Two identical samples (in terms of weight and eggs per gram) of feces were homogenized with vermiculite (Vermiculite Insulation Thermo-Acoustic Ltd., Belo Horizonte, MG; granulation 1) or coal (Tobasa Bioindustrial, Tocantinópolis, TO; granulation 4) in disposable plastic cups (200ml) at a ratio of two parts of vermiculite or charcoal to one part stool (e.g., 5g of feces and 10g of vermiculite or charcoal). The cultures were incubated at 28°C for 48h, and were humidified daily with distilled water. The incubation time was based on the time used for cultivation of feces on coal for recovery of larvae of Strongyloides stercoralis2. Larvae were collected from the plastic cups using the Baermann-Moraes technique and counted in one aliquot of 50µL. The results of culturing in both media are summarized in Table 1. Of 17 cultures grown in vermiculite, 15 were positive, with recovery of filarial larvae and, exceptionally, free-living forms of the worm. In contrast, among the 17 cultures on coal, only four were positive. The two negative cultures in vermiculite were also negative in coal. When comparing the positive cultures, the mean number of larvae recovered was significantly higher in cultures containing vermiculite (Table 1).
TABLE 1- Comparison of feces culture in vermiculite or charcoal. Feces of rats infected with Strongyloides venezuelensiswere mixed with vermiculite or charcoal and incubated at 28°C for 48h.
| Culture medium | Positive/total cultures (17) | P* | Larvae (n)**mean ± SD | P*** | |
|---|---|---|---|---|---|
| n | % | ||||
| Vermiculite | 15 | 88.2 | 990.6 ± 307.5 | ||
| Charcoal | 4 | 23.5 | <0.001 | 215 ± 78.1 | 0.027 |
In conclusion, data presented here demonstrated that fecal cultures in the presence of vermiculite, assessed 48h after cultivation, yielded significantly better results than cultures grown in the presence of coal, allowing recovery of a larger number of larvae. In addition, the pellet obtained by the Baermann-Moraes method is cleaner, greatly facilitating the identification of larvae and their evolutionary forms, and improving their application for other purposes, such as obtaining antigens and inoculation into experimental animals.
