Survival of Salmonella spp. and fecal indicator bacteria in Vietnamese biogas digesters receiving pig slurry

Abstract

Small-scale biogas digesters are widely promoted worldwide as a sustainable technology to manage livestock manure. In Vietnam, pig slurry is commonly applied to biogas digesters for production of gas for electricity and cooking with the effluent being used to fertilize field crops, vegetables and fish ponds. Slurry may contain a variety of zoonotic pathogens, e.g. Salmonella spp., which are able to cause disease in humans either through direct contact with slurry or by fecal contamination of water and foods. The objective of this study was to evaluate the survival of Salmonella spp. and the fecal indicator bacteria, enterococci, E. coli, and spores of Clostridium perfringens in biogas digesters operated by small-scale Vietnamese pig farmers. The serovar and antimicrobial susceptibility of the Salmonella spp. isolated were also established. The study was conducted in 12 farms (6 farms with and 6 farms without toilet connected) located in Hanam province, Vietnam. Sampling of pig slurry and biogas effluent was done during two seasons. Results showed that the concentration of enterococci, E. coli, and Clostridium perfringens spores was overall reduced by only 1-2 log10-units in the biogas digesters when comparing raw slurry and biogas effluent. Salmonella spp. was found in both raw slurry and biogas effluent. A total of 19 Salmonella serovars were identified, with the main serovars being Salmonella Typhimurium (55/138), Salmonella enterica serovar 4,[5],12:i:- (19/138), Salmonella Weltevreden (9/138) and Salmonella Rissen (9/138). The Salmonella serovars showed similar antimicrobial resistance patterns to those previously reported from Vietnam. When promoting biogas, farmers should be made aware that effluent should only be used as fertilizer for crops not consumed raw and that indiscriminate discharge of effluent are likely to contaminate water recipients, e.g. drinking water sources, with pathogens. Relevant authorities should promote safe animal manure management practices to farmers and regulations be updated to ensure food safety and public health.

Keywords: Biogas; Effluent; Fecal indicators; Food safety; Salmonella; Vietnam.

Fig. 1

Schematic illustration of the biogas digester system operated by Vietnamese small-scale pig farmers (1: inlet tank; 2: inlet pipe; 3: digester tank; 4: gas collection pipe; 5: outlet pipe; 6: compensation tank; 7: overflow pipe; 8: effluent tank).

Fig. 2

Box plots showing concentration of fecal indicator bacteria (a–c) and prevalence of Salmonella spp. (d) in slurry and biogas effluent illustrated by season, commune and toilet connected. The concentration of bacteria or spores detected in slurry and biogas effluent is presented as cfu/mL or pfu/mL. The horizontal line within the box shows the median, the box the interquartile range and the whiskers show the adjacent values (i.e. up to 1.5 times the interquartile range below the 25th percentile or above the 75th percentile). Values outside this range are shown by markers.

Fig. 3

Distribution of antimicrobial-resistant Salmonella spp. by source (slurry, biogas effluent) and season (summer, winter), n = number of Salmonella spp. isolates that are resistant to a specific antimicrobial.