Submitted to: Journal of Applied Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/9/2006
Publication Date: 4/1/2007
Citation: Siragusa, G.R., Wise, M. 2007. Quantitative analysis of the gastrointestinal bacterial community of broiler chickens reared with and without antimicrobial growth promotants. Journal of Applied Microbiology. 102(4):1138-49. Interpretive Summary: Low levels of antibiotics, a.n.a. antibiotic growth promotants or AGP’s, are routinely administered to poultry and livestock reared on modern production farms. AGP’s increase growth rates and control animal diseases. AGP’s in animal agriculture has been associated with the spread of antibiotic resistant bacteria affecting humans and thus there is growing pressure from the public health authorities to reduce or eliminate their use in feeds. Unfortunately, the discontinuation of antibiotics in broiler chicken farming has been linked to diseases of the chicken gastrointestinal tract, specifically necrotic enteritis. Disturbances in the gut bacterial community are hypothesized to lead to an imbalance that may allow for overgrowth of pathogenic bacteria, like Clostridium perfringens, a causative agent of necrotic enteritis. The effects of antibiotics on the gut bacterial community were explored over a three week period using genetic tests that allowed for measurement of the population sizes of thirteen bacterial groups known to be important in this system. The bacteria associated with the middle portion of the gastrointestinal tract was dominated by lactobacilli and enteric bacteria, with the enteric group more numerous in birds raised without antibiotics. The progression of the major populations in the lower GI tract was similar in birds grown both with and without antibiotics. However, some numerically minor groups, were more numerous in the antibiotic-free birds.
Technical Abstract: Aims: To explore the effect of antimicrobial growth promotants (AGP) on the intestinal microflora of broiler chickens, the bacterial community of this environment was quantitatively profiled in birds reared with and without AGPs. Methods and Results: Quantitative, real-time PCR with group-specific 16S rDNA primer sets was used to enumerate the abundance of the following chicken gastrointestinal tract phylogenetic groups: the Clostridium leptum - Faecalibacterium prausnitzii subgroup (Clostridium genus cluster IV), the Clostridium coccoides - Eubacterium rectale subgroup (Clostridium cluster XIVa and XIVb), the Bacteroides group (including Prevotella and Porphyromonas), Bifidobacterium spp., the Enterobacteriaceae, the Lactobacillus group (including the genera Leuconostoc, Pediococcus, Aerococcus and Weissella), the Clostridium perfringens subgroup (Clostridium cluster I), Enterococcus spp., Veillonella spp., Atopobium spp., Campylobacter spp. and the domain Bacteria. A species-specific 5’ nuclease (TaqmanR) assay was also employed to specifically assess Clostridium perfringens abundance. Ten birds were sampled from each of two commercial chicken houses, one conventionally reared with AGPs and the other drug-free, at 7, 14 and 21 days of age. The ileal community was dominated by two large populations, the lactobacilli and the Enterobacteriaceae, with enteric signature sequences much more numerous in drug-free birds than those reared with AGPs at the 7 and 14 day time periods. Microflora successional patterns with both the conventional and drug-free ceca were similar to each other, with the Enterobacteriaceae sequences dominating at day 7, but being replaced by those from obligate anaerobes by day 14. Of note was the finding that all the day 14 and day 21 replicate cecal samples from the drug-free house were positive for Campylobacter spp. averaging >108 16S rDNA gene copies/ gm wet weight. Conclusions: Quantitative, real-time PCR indicates that the effects of AGPs on the chicken GI tract microbial community structure are most pronounced in the ileal region, but AGPs may be important in controlling Campylobacter colonization of the cecum. Significance and Impact of the Study: An understanding of the microbial ecology of the broiler chicken gut microflora is important in light of AGP withdrawal due to concerns regarding antibiotic resistance in human therapy.