The effects of Vermicomposting on Population and Community Structure of Enterococci in a Residential Community

The effects that vermiculture (VC) treatment had on population and community structure of enterococci in domestic biosolids in a residential resort in southeast Queensland were investigated by enumerating and typing enterococci with a biochemical fingerprinting method. Vermiculture beds with a substrate consisting of non-amended biosolids from an activated sludge sewage treatment plant were established. Various treatments (earthworms added and earthworms excluded) beds were sampled on 3 occasions. Enterococci numbers were significantly lower in the earthworm treatments than those without. Of the 300 isolates biochemically fingerprinted, 106 different biochemical phenotypes (BPTs) were identified. The overall mean diversity of enterococci in all beds was high (0.87 ± 0.13; maximum is 1), indicating a diverse enterococci population. The overall mean population similarity (Sp) value amongst beds was quite low (0.16 ± 0.10) also indicating a diverse population. Antibiotic resistance patterns (ARPs) of the 106 BPTs revealed that 83.6% were resistant to rifampicin, 59.6% were resistant to erythromycin, 21.9% were resistant to tetracycline, 7.7% were resistant to ampicillin and 2.9% were resistant to gentamicin and vancomycin. The results indicate that the current VC methods used in the residential resort to create worm castings for potting mixture may not satisfactorily pasteurize the biosolids and significant number of pathogenic microorganisms could be present. There remains a potential health risk to workers and guests and routine monitoring of pathogens is recommended for improved management of stockpiled biosolids.