Antibiotic Resistance of Fecal Coliforms from Swine Fed Subtherapeutic and Therapeutic Levels of Chlortetracycline2

Abstract

The effect of feeding subtherapeutic (27.5 micrograms/g of diet for 85 d) and therapeutic (220 micrograms/g of diet for 14 d, followed by an antibiotic-free diet for 71 d) levels of chlortetracycline (CTC) on the antibiotic resistance of fecal coliforms of pigs from two herds (36 pigs/herd) with different histories of antibiotic exposure when housed in a newly constructed confinement facility was determined. The CTC-resistant coliforms were higher (65 vs 51%) for antibiotic (AB) pigs than for nonantibiotic (NAB) pigs after they had been fed an antibiotic-free diet for 21 d. Percentages of isolates resistant to ampicillin, kanamycin, neomycin and tetracycline and multiple antibiotic resistance were greater (P less than .05) in AB pigs after 21 d. Feeding subtherapeutic CTC resulted in a linear increase in CTC-resistant coliforms with time on experiment (P less than .03, NAB; P less than .06, AB). The CTC-resistant coliforms increased during the 14 d that therapeutic CTC was fed, then they decreased during the 71 d that the antibiotic-free diet was fed, resulting in a quadratic response with time (P less than .03, AB). Feeding subtherapeutic CTC resulted in a greater increase in CTC-resistant coliforms in AB (47%) than in NAB (23%) pigs. The CTC-resistant coliforms decreased after the therapeutic group had been returned to the antibiotic-free diet (P less than .05, NAB). Feeding CTC caused greater changes in the precentages of isolates from NAB pigs that were resistant to selected antibiotics and in multiple antibiotic resistance than in isolates from AB pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Antimicrobial Drug Resistance Genes Do Not Convey a Secondary Fitness Advantage to Calf-Adapted Escherichia coli

“…Food animal production consumes large amounts of antimicrobial drugs and thus can serve as an important arena for emergence and maintenance of AR (1,3,46). Nevertheless, it is not clear that eliminating all nontherapeutic antimicrobial use in animal production will result in a significant reduction in the prevalence of AR bacteria (7,10,24,25,37,41). This is because many factors unrelated to antimicrobial use are also implicated in the increase in prevalence of AR, including animal stress (30,31), close linkage to other selectively advantageous genes (2,9,16,21), plasmid addiction (14,17), and secondary advantage conveyed by the AR genes (15,27,36).…”

“…One strategy to decrease the prevalence of AR bacteria is to discontinue using antimicrobial drugs for growth promotion and prophylaxis in food animals (4,11,22,43,44). Unfortunately, this strategy has produced mixed results that are dependent on the duration and amount of antimicrobial drug use (10,16,24,25,37,38,41). While pathogenic bacteria are the primary concern, commensal bacteria are an important reservoir for AR genes.…”

Changes in Tetracycline Susceptibility of Enteric Bacteria following Switching to Nonmedicated Milk Replacer for Dairy Calves

“…Our findings concerning antimicrobial susceptibility and the intervention agree with earlier studies where removal of antimicrobial agents from cattle management resulted in increased susceptibility in enteric bacteria (1) and in another study where increasing calf exposure to penicillin in milk was associated with decreasing susceptibility (24). In an early study using therapeutic and subtherapeutic levels of chlortetracycline in swine feed, investigators found that feeding subtherapeutic levels of chlortetracycline (27.5 g/g feed) for longer periods (84 days) decreased Tet susceptibility in fecal coliforms in comparison to using therapeutic levels (220 g/g feed) for shorter periods (14 days) and that susceptibility increased after therapeutic levels were discontinued (25). The finding that oxytetracycline use in adult cattle decreased the risk of susceptibility in the herd level multivariable analysis also supports the theory that drug resistance will be sustained in the presence of selective pressure (26).…”

Tetracyclines in veterinary medicine and bacterial resistance to them

“…In 1969, the Swann report published in Great Britain, recommended the exclusion of the antimicrobial agents from animal feed that were used in human and/or animal therapy (Swann, 1969). Numerous studies described the effect of long-term usage of subtherapeutic doses of tetracyclines resulting in the increased level of resistant gut bacteria or pathogens (Smith and Tucker, 1975;Hooper and Hirsh, 1977;Langlois et al, 1984;Hinton et al, 1985). In contrast to the United States, the application of tetracyclines as growth promoters is not allowed neither in the European Union nor in the Czech Republic (Council Directive 70/524 EEC, 1970;Prescott et al, 2000), and since 1975, no tetracycline has been used for the growth promotion in Europe (Schwarz and Chaslus-Dancla, 2001).…”