MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: Antibiotic resistance patterns in E. coli and Salmonella isolates recovered from commercially vailable compost
Submitted to: International Association for Food Protection Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 9, 2010
Publication Date: August 1, 2010
Citation: Ferguson, S.E., Mudd, C.L., Ingram, D.T., Millner, P.D., Sharma, M. 2010. Antibiotic resistance patterns in E. coli and Salmonella isolates recovered from commercially vailable compost. [abstract]. International Association for Food Protection Proceedings. p.91.
Introduction: Compost is used by both conventional and organic farming practices as an eco-friendly means to enhance soil properties and to reduce fertilizer inputs. Inadequate composting may lead to residual human pathogens (e.g. Escherichia coli O157:H7 and Salmonella spp.) in the final compost product. Since compost is often prepared from manures and/or biosolids from therapeutically treated animals and humans, it is possible that residual human pathogens may become resistant to these antibiotics.
Purpose: We determined the extent to which E. coli and Salmonella isolates from commercially available biosolids-based and yardwaste-based compost samples were resistant to selected antibiotics.
Methods: Individual E. coli (n=184) and Salmonella (n=43) isolates, previously quantified and confirmed using standard MPN and biochemical techniques, were selected from composts obtained from eight commercial operations (four yardwaste and four biosolids-based) across the U.S. For each isolate, the Kirby Bauer method was used to determine antibiograms to the following antibiotics (n=14): amoxicillin/clavulinic acid, ampicillin, chloramphenicol, ciprofloxacin, doxycycline, erythromycin, kanamycin, nalidixic acid, novobiocin, oxytetracycline, rifampin, streptomycin, sulfamethoxazole/trimethoprim, and tetracycline. All assays were performed in duplicate.
Results: Overall, antibiotic resistance to one or more antibiotics was observed in 31.5% (58 of 184) of the E. coli isolates. Resistance was seen in 7.6%, 2.2%, 1.6% and 1.1% of the E. coli isolates to two, three, four and five antibiotics, respectively. For the Salmonella isolates, 60.5% (26 of 43) were resistant to one or more antibiotics with 4.7% of the isolates being resistant to four antibiotics. E. coli and Salmonella isolates were most frequently resistant to tetracycline, followed by ampicillin and doxycycline. Yard-waste and biosolids-based compost appear equally susceptible for the contamination of antibiotic resistant E. coli.
Significance: We present the first study to report the existence of antibiotic-resistant E. coli and Salmonella in commercially available compost. This research suggests the possibility that compost may be a vector for the introduction of antibiotic-resistant E. coli and Salmonella into the pre-harvest environment. Use of compost that contains antibiotic-resistant E. coli and Salmonella has the potential to contaminate food crops in which it is used, and to exacerbate foodborne infections in which antibiotic treatment may be required.