Abstract
Contaminated poultry meat is considered to be the main source of human infection with Campylobacter spp., a pathogen that asymptomatically colonizes broiler chickens during fattening and contaminates carcasses during slaughter. To prevent or reduce the colonization of broiler flocks with Campylobacter spp., applying different organic acids, especially in combinations, via feed or drinking water seems to be a promising approach. However, only very few combinations of organic acids have been tested for their antibacterial efficacy against Campylobacter spp. Therefore, the in vitro susceptibility of 30 Campylobacter spp. isolates (20 C. jejuni and ten C. coli) to ten organic acids and ten combinations was determined. The testing of minimum inhibitory concentration (MIC) values was performed at pH 6.0 and 7.3 by using the broth microdilution method and included the following organic acids: Caprylic acid, sorbic acid, caproic acid, benzoic acid, ascorbic acid, propionic acid, acetic acid, formic acid, fumaric acid and tartaric acid and combinations thereof. The lowest MIC values were seen for caprylic acid (MIC range at pH 7.3: 0.5–2 mmol/L) and sorbic acid (MIC range at pH 7.3: 1–4 mmol/L). One to two dilution steps lower MIC values were determined at the lower pH value of 6.0. Furthermore, ten combinations consisting of three to five organic acids were developed. In addition to the tested antibacterial activity, other criteria were included such as approval as feed additives, reported synergistic effects and chemical properties. For nine of ten combinations, the MIC90 values of the organic acids decreased 1.25- to 241.5-fold compared to the MIC90 values for the individual substances. Furthermore, nine of ten combinations exhibited synergistic activities against two or more of the tested C. jejuni and C. coli isolates. A combination of caprylic acid, sorbic acid and caproic acid exhibited synergistic activities against the largest number of Campylobacter spp. isolates (six C. jejuni and four C. coli) with fractional inhibitory concentration (FIC) indices (∑FIC) ranging from 0.33 to 1.42. This study shows in vitro synergistic activities of different organic acids in combinations against the major Campylobacter species and could therefore be a promising basis for reducing Campylobacter spp. in vivo.
Highlights
Campylobacteriosis is one of the leading foodborne gastrointestinal diseases worldwide [1] and the most frequently reported zoonosis in the European Union with more than 246,000 reported cases in 2018 [2]
The antibacterial activities of organic acids both alone and in different combinations were determined by using the broth microdilution method
The minimum inhibitory concentration (MIC) values of several organic acids differed in part strongly to those determined for two C. jejuni strains in a previous study [11]
Summary
Campylobacteriosis is one of the leading foodborne gastrointestinal diseases worldwide [1] and the most frequently reported zoonosis in the European Union with more than 246,000 reported cases in 2018 [2]. Campylobacter (C.) jejuni and C. coli are the species that most frequently cause gastrointestinal diseases [3]. Comprehensive strategies for reducing microbial contamination of carcasses and controlling Campylobacter infections are urgently needed. Reducing Campylobacter spp. in primary production is considered to be most effective for minimizing human infections [4]. Different in vivo studies demonstrated the potential of organic acids to decrease the susceptibility for colonization or to reduce caecal concentrations of Campylobacter spp. when applicated via feed or water [6,7,8,9] as pointed out in the scientific opinion published by the European Food Safety Authority [4], results of previous in vivo experiments concerning the effectiveness of organic acids were inconsistent, indicating that further research using standardized methods is urgently needed
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