Table 3 Examples of study types evaluating the emergence of antimicrobial resistance in natural bacterial background populations after food producing animals were exposed to antimicrobial drugs
Reference | Study | Key findings | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Typea | species | bacterial target | antimicrobial drug | administration route | study outcomes related to antimicrobial resistance | study design | susceptibility testing | controls | ||
Grade level: Controlled trial | ||||||||||
[65] | RCT | cattle | E. coli | chlortetracycline (with or without sulfamethazine), monensin, tylosin, virginiamycin; all as growth promoters | feed | emergence of antimicrobial resistance | 6 treatments, 5 pens per treatment, 10 steer per pen | Phenotypic agar dilution | negative control | exposure to chlortetracycline and sulfamethazine increased the prevalence of resistance; diet had an important impact on shedding of resistant strains |
[66] | RCT | cattle | E. coli | florfenicol | s.c. injection | emergence of antimicrobial resistance | 42 pens (8–10 cows per pen), 2 cows per pen enrolled in study | Phenotypic disk diffusion | negative control | after antimicrobial treatment, increased antimicrobial resistance; prior treatment predictive of resistance; animal source and previous management impact results |
[67] | RCT | Cattle | fecal bacteria | ceftiofur, tetracycline | s.c. injection | antimicrobial resistance | 176 steers; 2 replicates of 88 steers each, 8 pens of 11 steers per replicate; total of 4 treatment groups | Genotypic PCR based | positive and negative controls | Initial and subsequent antimicrobial treatments led to increased frequency of resistance genes |
[68] | RCT | pigs | Enterococci Staphylococcus hyicus | tylosin (as growth promoter) | feed | erythromycin resistance levels | 2 trials a 10 pigs, n = 5 per group | Phenotypic | negative control | Prevalence of resistance in enterococci immediately increased 2.4× in response to tylosin exposure; resistance in Staphylococcus hyicus occurred more gradually, at a rate of 8% per day and 5× over 20 days |
[130] | RCT | pigs | aerobic gram negative bacteria; Salmonella | chlortetracycline | feed | antimicrobial resistance | 3 farms (convenience sample); 12, 2 and 8 barns per farm | Phenotypic broth dilution | negative control | antimicrobial exposure was associated with increased prevalence of resistance in aerobic gram-negative bacteria |
GRADE level: observational study | ||||||||||
[69] | cohort | pigs | coliform bacteria | olaquindox | feed | emergence of resistance | 12 farms | Phenotypic | neighboring farms that do not use olaquindox | prevalence and level of resistance increased on farms that used it, and to lesser extent on adjacent farms |
[70] | CS | pigs | E. coli | various | feed | emergence of antimicrobial resistance | 34 farms | Phenotypic | negative control | antimicrobial use increases the emergence of resistance |
[71] | CS | pigs | E. coli | various | various | antimicrobial resistance | 47 farms | Phenotypic | negative control | antimicrobial use (in weaner rather than finisher pigs) was associated with resistance; some evidence for cross- resistance and co-selection |
GRADE level: other | ||||||||||
[72] | correlation study | pigs, cattle, poultry | E. coli | various | various | correlation between antimicrobial use and resistance | 7 countries | phenotypic | comparison across countries | use of antimicrobial drugs is strongly correlated with resistance in E. coli |
[73] | correlation study | Poultry | Enterococci, Vancomycin-resistant | avoparcin | various | Comparison of resistance prevalence before and after avoparcin ban | Italy | phenotypic | comparison over time | prevalence of vancomycin-resistant Enterococci decreased in poultry meat samples after the avoparcin ban |