Detection of a second mechanism of resistance to gentamicin in animal strains of Escherichia coli

Genetic mechanisms of antimicrobial resistance identified in Salmonella enterica, Escherichia coli, and Enteroccocus spp. isolated from U.S. food animals

The prevalence of antimicrobial resistance (AR) in bacteria isolated from U.S. food animals has increased over the last several decades as have concerns of AR foodborne zoonotic human infections. Resistance mechanisms identified in U.S. animal isolates of Salmonella enterica included resistance to aminoglycosides (e.g., alleles of aacC, aadA, aadB, ant, aphA, and StrAB), β-lactams (e.g., bla_{CMY−2, TEM−1, PSE−1}), chloramphenicol (e.g., floR, cmlA, cat1, cat2), folate pathway inhibitors (e.g., alleles of sul and dfr), and tetracycline [e.g., alleles of tet(A), (B), (C), (D), (G), and tetR]. In the U.S., multi-drug resistance (MDR) mechanisms in Salmonella animal isolates were associated with integrons, or mobile genetic elements (MGEs) such as IncA/C plasmids which can be transferred among bacteria. It is thought that AR Salmonella originates in food animals and is transmitted through food to humans. However, some AR Salmonella isolated from humans in the U.S. have different AR elements than those isolated from food animals, suggesting a different etiology for some AR human infections. The AR mechanisms identified in isolates from outside the U.S. are also predominantly different. For example the extended spectrum β-lactamases (ESBLs) are found in human and animal isolates globally; however, in the U.S., ESBLs thus far have only been found in human and not food animal isolates. Commensal bacteria in animals including Escherichia coli and Enterococcus spp. may be reservoirs for AR mechanisms. Many of the AR genes and MGEs found in E. coli isolated from U.S. animals are similar to those found in Salmonella. Enterococcus spp. isolated from animals frequently carry MGEs with AR genes, including resistances to aminoglycosides (e.g., alleles of aac, ant, and aph), macrolides [e.g., erm(A), erm(B), and msrC], and tetracyclines [e.g., tet(K), (L), (M), (O), (S)]. Continuing investigations are required to help understand and mitigate the impact of AR bacteria on human and animal health.

Survey of antimicrobial resistance in bacterial isolates from diseased cattle in France

Since 1982, a national veterinary network has been involved in the monitoring of resistance to antimicrobial agents in the main pathogenic bacteria isolated from diseased cattle in France. It is based on 40 regional veterinary diagnostic laboratories and managed by a central reference laboratory (CNEVA Lyon). Highly standardized methods are used in the diagnostic laboratories. This network collects up-to-date information on antimicrobial resistance in veterinary isolates and gathers strains for specific studies on fastidious bacteria and for the analysis of mechanisms of resistance to antibiotics. Such a permanent survey is essential to establish a rational veterinary antibiotic policy. It could be connected to other compatible systems developed in other fields such as human medicine, food, and environment, to evaluate the importance of resistance and R-factors spread for public health. The limits and perspectives of this surveillance system are discussed.

Minimum inhibitory concentrations of intestinal Escherichia coli from broiler chickens after oral administration of apramycin

This study describes the influence of apramycin on minimum inhibitory concentrations (MIC) of intestinal Escherichia coli in young broiler chickens, after oral administration of the antibiotic at a dosage equivalent to a prophylactic course of treatment for 10 d. The bacteria were isolated from cloacal swabs and caecal contents. MICs were determined by agar dilution procedures. MIC of apramycin for the investigated strains ranged from 1 microgram ml-1 to 16 micrograms ml-1. Strains obtained from undosed birds mainly had MIC values of 2 micrograms ml-1. MIC values of 8 micrograms ml-1 or more were recorded only among isolates obtained from chickens which had received apramycin. Administration of apramycin resulted in a slight but statistically significant increase in the average MIC. Statistically higher average MICs were recorded among isolates from cloacal swabs 10 d after withdrawal until the end of the experiment. For strains from caecal contents, this was demonstrated only on one sampling occasion, 15 d after withdrawal.

Bacterial resistance monitoring in animals: the French national experiences of surveillance schemes

In France, bacterial resistance monitoring in animals is based on a national network of local veterinary laboratories organised by CNEVA following two methodologies. As part of the epidemiological surveillance of Salmonella, LCHA in Paris has been receiving since 1969 most of the strains isolated from animals (sick and healthy carriers) but also from food, feed and environment. For many years sensitivity tests have been carried out on strains collected: animal ones, mainly from cattle and poultry, show multiple antibiotic resistance more frequently than those of food or environment. Taking into account of the importance of antibiotic resistance in E. coli and Salmonella isolated from sick calves, LPB in Lyon has set up since 1982 a national system for collecting and analysing antimicrobial sensitivity patterns of main bovine pathogens determined routinely by local veterinary laboratories. In order to ensure comparability and coherency of data from different laboratories it was necessary to introduce standard techniques and reference reagents. The major bacterial species being tested concern enteric, respiratory and mammary flora. This network is useful to detect new resistances. The resistant strains collected are useful to carry out studies on the mechanism of resistance and the efficacy of new molecules. Results constitute the basis of a real predictive epidemiology necessary to lay down a policy for rational use of antimicrobials in animal breeding and veterinary medicine.

Use of plasmid profiles in epidemiologic surveillance of disease outbreaks and in tracing the transmission of antibiotic resistance

Plasmids are circular deoxyribonucleic acid molecules that exist in bacteria, usually independent of the chromosome. The study of plasmids is important to medical microbiology because plasmids can encode genes for antibiotic resistance or virulence factors. Plasmids can also serve as markers of various bacterial strains when a typing system referred to as plasmid profiling, or plasmid fingerprinting is used. In these methods partially purified plasma deoxyribonucleic acid species are separated according to molecular size by agarose gel electrophoresis. In a second procedure, plasmid deoxyribonucleic acid which has been cleaved by restriction endonucleases can be separated by agarose gel electrophoresis and the resulting pattern of fragments can be used to verify the identity of bacterial isolates. Because many species of bacteria contain plasmids, plasmid profile typing has been used to investigate outbreaks of many bacterial diseases and to trace inter- and intra-species spread of antibiotic resistance.