Abstract

By investigating the resistance characteristics of bacteria from organic and conventional keeping systems of laying hens, it was to be determined to what extent different rearing systems influence bacterial resistance patterns. For this purpose, samples from 10 organic and 10 conventional flocks were investigated 4 times between January 2004 and April 2005. In total, 799 cloacal swabs and 800 egg samples were taken and examined. The isolation and identification of Salmonella spp., Listeria spp., E. coli/Coliforms, Campylobacter spp., and Enterococcus spp. was performed with standardized, cultural methods. Selected isolates of the genera Salmonella (n = 44), Listeria (n = 13), Campylobacter (C. jejuni, n = 218; C. coli, n = 46), Escherichia (E. coli, n = 545; E. fergusonii n = 18; E. hermanii n = 1 ) Citrobacter (Citrobacter freundii n = 9), Enterobacter (Enterobacter cloacae n = 5; Enterobacter sakazakii n = 3; Enterobacter gergoviae n = 2; Enterobacter asburiae n = 1), Pantoea (Pantoea agglomerans n = 2) and Enterococcus (E. faecalis, n = 361; E. faecium, n = 57; other enterococci [E. nonfaecalis/nonfaecium], n = 569) were tested for their resistance behaviour with respect to 29 different antibiotics by means of microdilution. During the bacteriological investigation of the cloacal swabs, prevalences were found for all bacteria groups in the same order of magnitude in the different rearing systems (Salmonella spp. 3.5 % (organic [org.]) vs. 1.8 % (conventional [con.]); Listeria spp.: 1.3 % (org.) vs. 1.6 % (con.); Campylobacter spp : 34.8 % (org.) or 29.0 % (con.); E. coli: 66.4 % (org.) vs. 72.0 % (con.); Enterococcus spp.: 95.5 % (org.) vs. 97.5 % (con.). Eggs were generally infected with less bacteria, most of which were of the genera Enterococcus and Escherichia, whereas Listeria, Salmonella and Campylobacter were only rarely isolated from the samples. Salmonella of the serovar type S. Typhimurium were resistant to up to nine antibiotics; Salmonella of the serogroup B were resistant to up to 6 different antibiotics. All Salmonella isolates proved to be resistant towards spectinomycin. A high percentage of C. jejuni and C. coli isolates showed resistance to flourquinolones; a similar resistance was observed in the case of ampicillin and doxycycline. The Listeria isolates were mostly sensitive towards the tested antibiotics, and only a few strains showed resistance to clindamycin and imipenem. E. coli isolates showed a high resistance prevalence to ß-lactames, doxycycline, streptomycin, and cefaclor. High percentages (54.8 %) of E. faecalis were found to be resistant to doxycycline; macrolides were also only marginally effective. The investigated E. faecium isolates proved to have high resistance rates to clindamycin, fosfomycin and erythromycin, while a significant percentage (9.1 %) of E. faecium had already been classified as resistant to the reserve antibiotic synercid. Other enterococci showed higher resistance rates to doxycycline, erythromycin, fosfomycin, and rifampicine. There were no glykopeptide-resistant enterococci. The analysis of the prevalence rates of sensitive and resistant isolates depending on the keeping system showed a correlation between rearing system and resistance rates: In the case of E. faecalis, a significantly lower prevalence of resistance to tylosin, streptomycin and doxycycline was determined among isolates from organic farms, while significantly higher amounts of isolates were found to be sensitive to enrofloxacin and ciprofloxacin; however, when tested on rifampicin and imipenem, E. faecalis behaved contrary to this. E. coli isolates from organic layers showed significantly lower resistance rates or significantly higher amounts of sensitive isolates with regard to nine agents, while in the case of E. coli from conventional rearing systems, these results could only be observed for two antibiotics. In the case of C. jejuni, significantly better rates were observed for isolates from organic flocks with regard to imipenem and amoxicillin/clavulanic acid, whereas fosfomycin favoured isolates from conventional layer flocks. In the case of 8 antibiotics, the amounts of sensitive and resistant enterococci of the E. nonfaecalis/nonfaecium group originating from organic farms were found to be higher and lower respectively compared to the isolates from conventional farms. Overall, the comparison of the amounts of sensitive and resistant bacteria isolates and the mean MIC values showed that the isolates from organic layer rearing systems scored much better statistically than those from conventional systems. The results show that organic layer flocks reduce the amount of bacteria resistant to antibiotics, as both the resistance rates of the selected bacteria to certain antibiotics as well as the mean MIC values for certain antibiotic agents were lower in organic systems than in conventional ones. Thus, organic livestock farming contributes towards securing the continued effectiveness of anti-infectives.