Occurrence of antibiotic-resistant enterobacteria in agricultural foodstuffs

Abstract

Antibiotic-resistant bacteria or their corresponding resistance determinants are known to spread from animals to humans via the food chain. We screened 20 vegetable foods for antibiotic-resistant coliform bacteria and enterococci. Isolates were directly selected on antibiotic-containing selective agar (color detection). Thirteen "common vegetables" (tomato, mushrooms, salad) possessed 10(4)-10(7) cfu/g vegetable of coliform bacteria including only few antibiotic-resistant variants (0-10(5) cfu/g). All seven sprout samples showed a some orders of magnitude higher contamination with coliform bacteria (10(7)-10(9) cfu/g) including a remarkable amount of resistant isolates (up to 10(7) cfu/g). Multiple resistances (up to 9) in single isolates were more common in sprout isolates. Resistant bacteria did not originate from sprout seeds. The most common genera among 92 isolates were: 25 Enterobacter spp. (19 E. cloacae), 22 Citrobacter spp. (8 C. freundii), and 21 Klebsiella spp. (9 K. pneumoniae). Most common resistance phenotypes were: tetracycline (43%), streptomycin (37%), kanamycin (26%), chloramphenicol (29%), co-trimoxazol (9%), and gentamicin (4%). The four gentamicin-resistant isolates were investigated in molecular details. Only three (chloramphenicol) resistant, typical plant-associated enterococci were isolated from overnight enrichment cultures. In conclusion, a contribution of sprouts contaminated with multiresistant, Gram-negative enterobacteria to a common gene pool among human commensal and pathogenic bacteria cannot be excluded.