RESISTANCE OF SALMONELLAE ISOLATED IN 1962 TO TETRACYCLINE, CHLORAMPHENICOL, AND AMPICILLIN

Antibiotic Administration Routes and Oral Exposure to Antibiotic Resistant Bacteria as Key Drivers for Gut Microbiota Disruption and Resistome in Poultry

Previous studies have identified oral administration of antibiotics and gut-impacting drugs as critical drivers for fecal antibiotic resistance (AR) and microbiome disruption in lab mice, but the practical implications of these findings have yet to be validated in hosts nurtured in conventional environment. Using ampicillin (Amp) as a way to extrapolate the general effect of antibiotics, this project examined the impact of drug administration routes on fecal microbiota and resistome using poultry raised in a teaching farm. AR genes were found to be abundant in the feces of young Leghorn chicks without previous antibiotic treatment. In chickens seeded with bla _CMY-2 ⁺ Escherichia coli, 300 mg/kg body weight of Amp was orally administered for 5 days. This led to the fecal microbiota switching from Firmicutes occupied (95.60 ± 2.62%) and Lactobacillus rich, to being dominated by Proteobacteria (70.91 ± 28.93%), especially Escherichia/Shigella. However, when Amp was given via muscle injection, Firmicutes was mostly retained (i.e., from 83.6 ± 24.4% pre- to 90.4 ± 15.2% post-treatment). In control chickens without seeding with bla _CMY-2 ⁺ E. coli, oral Amp also led to the increase of Proteobacteria, dominated by Klebsiella and Escherichia/Shigella, and a reduction of Firmicutes. Specifically within Firmicutes, Enterococcus, Clostridium, etc. were enriched but Lactobacillus was diminished. The fecal resistome including Amp^r genes was more abundant in chickens receiving oral Amp than those treated with muscle injection, but the difference was primarily within 1 log. The data illustrated that both drug administration routes and pre-existing gut microbiota have profound impacts on gut microbiome disruption when antibiotic treatment is given. In hosts nurtured in a conventional environment, drug administration route has the most evident impact on gut microbiota rather than the size of the targeted bla _CMY-2 ⁺ gene pool, likely due to the pre-existing bacteria that are (i) less susceptible to Amp, and/or (ii) with Amp^r- or multidrug resistance-encoding genes other than bla _CMY-2 ⁺. These results demonstrated the critical interplay among drug administration routes, microbiota seeded through the gastrointestinal tract, AR, gut microbiota disruption, and the rise of common opportunistic pathogens in hosts. The potential implications in human and animal health are discussed.

Effects of different levels of chlortetracycline in the diet of turkey poults artifically-infected with Salmonella typhimurium

Two separate experiments were conducted to assess the shed rate and duration of shed of S . typhimurium organisms from turkey poults orally infected with chlortetracycline-sensitive S. typhimurium in relation to chloretetracycline (CTC) given in the feed at 0, growth promotant, subtherapeutic and therapeutic levels; the emergence of resistant S. typhimurium organisms in reference to the diet given; in vitro transfer of drug resistance from thses resistant S. typhimurium donor cultures to multiply-sensitive E. coli recipients; and phage type changes, is any, of these S. typhimurium isolates. The results showed that increasing CTC in the diet from 0 to the three levels of antibiotic supplementation, appeared to (a) reduce shed and duration of shed corresponding to each level used; (b) cause a minimal development of drug resistance and its transfer (usually at sub-therapeutic levels of CTC supplementation) for the duration of the experiment; and (c) induce phage type changes in some of the S. typhimurium isolates. These phage type changes question the validity of using phage typing as a tool in epidemiological investigations.