Chemical, microbiological and antimicrobial resistance changes in vermicomposts of pig manure by the effect of the initial carbon:nitrogen ratio

Abstract

Vermicomposting is a helpful alternative to recover nutrients and reduce pathogenic bacteria from animal manure; however, the optimal initial carbon: nitrogen ratio (CNR) to maximize the nutritional value and minimize the microbial load and antimicrobial resistance factors (AMR) of the final product is unknown. The objective of the study was to evaluate the effect of the initial CNR (15, 31, 46, and 70) of the pre-composting of vermicompost prepared with swine manure on the changes over time in nutrient content, total aerobic bacteria (TAB), coliforms, fungi and yeasts and AMR factors in E. coli. Samples were taken for chemical (days 1, 28, and 140) and microbiological analysis (days 1, 7, 28, 56, 84, and 140), and the presence of AMR (days 1 and 140). Data were analyzed using a mixed model with repeated observations over time and by linear regression analysis. Organic matter, organic carbon, N-NH⁴⁺, and CNR decreased and total nitrogen increased with time (P < 0.01). TAB, coliform, fungi, and yeast counts showed decreasing cubic responses with time and were comparable at 140 days among vermicompost with different initial CNR (P < 0.01). E. coli was isolated from 10 initial manure samples; the relative frequency and percentage of E. coli isolates with AMR were highest for ampicillin and carbenicillin (7/10, 70%) and intermediate for cephalothin and chloramphenicol (4/10, 40%) and sulfamethoxazole/trimethoprim (3/10, 30%). Five E. coli isolates were shown to be multiresistant to antibiotics (MMR). Although no E. coli was isolated from the final vermicompost, the bacteria Acinetobacter anitratus was found in three (3/12) samples and was also found to be MMR. In conclusion, regardless of the initial CNR, vermicomposts with high nutritional value were obtained, with a lower load of potentially pathogenic bacteria and free of E. coli carrying AMR. However, prior to its use as organic fertilizer, the presence of AMR factors in other bacteria should be evaluated to reduce the risks of their spread through soil and water, and of human infections.