The effect of transport temperature and time on the recovery of antimicrobial-resistant Enterobacterales in stool

Surveillance for antibiotic-resistant (AR) bacteria is challenging. We evaluated AR Enterobacterales survival in stool over various transport conditions. Stool in Cary-Blair medium was spiked with AR Enterobacterales, held at 3 °C, 20 °C, or 37 °C, and cultured on days 3, 8, and 15. Stool from US international travelers sent through the US mail was also screened. We compared recovery rates using Fisher's exact tests and linear regression models. AR Enterobacterales recovery reduced with time (86% versus 75% versus 61% at days 3, 8, and 15; Beta for linear trend=-0.02, r²=0.99, P=0.02) and colder temperatures [56% (3 °C) versus 89% (20 °C) versus 86% (37 °C); P=0.003]. Traveler sample recovery also reduced with transport time (Beta for linear trend=-0.03, r²=0.70, P=0.01) but not with season [20% (cold) versus 22% (warm), P=0.7]. AR Enterobacterales are found over variable transport conditions, providing rationale for expanding surveillance sample processing timelines.

Latent class comparison of test accuracy when evaluating antimicrobial susceptibility using disk diffusion and broth microdilution to testEscherichia coliandMannheimia haemolyticaisolates recovered from beef feedlot cattle

The study objective was to use Bayesian latent class analysis to evaluate the accuracy of susceptibility test results obtained from disk diffusion and broth microdilution using bacteria recovered from beef feedlot cattle. Isolates ofEscherichia coliandMannheimia haemolyticawere tested for susceptibility to ampicillin, ceftiofur, streptomycin, sulfisoxazole, tetracycline, and trimethoprim-sulfamethoxazole. Results showed that neither testing method was always or even generally superior to the other. Specificity (ability to correctly classify non-resistant isolates) was extremely high for both testing methods, but sensitivity (ability to correctly classify resistant isolates) was lower, variable in the drugs evaluated, and variable between the two bacterial species. Predictive values estimated using Bayesian Markov chain Monte Carlo models showed that the ability to predict true susceptibility status was equivalent for test results obtained with the two testing methods for some drugs, but for others there were marked differences between results obtained from disk diffusion and broth microdilution tests.

Methodological comparisons for antimicrobial resistance surveillance in feedlot cattle

BACKGROUND

The purpose of this study was to objectively compare methodological approaches that might be utilized in designing an antimicrobial resistance (AMR) surveillance program in beef feedlot cattle. Specifically, four separate comparisons were made to investigate their potential impact on estimates for prevalence of AMR. These included investigating potential differences between 2 different susceptibility testing methods (broth microdilution and disc diffusion), between 2 different target bacteria (non-type-specific E. coli [NTSEC] and Mannheimia haemolytica), between 2 strategies for sampling feces (individual samples collected per rectum and pooled samples collected from the pen floor), and between 2 strategies for determining which cattle to sample (cattle that were culture-positive for Mannheimia haemolytica and those that were culture-negative).

RESULTS

Comparing two susceptibility testing methods demonstrated differences in the likelihood of detecting resistance between automated disk diffusion (BioMIC®) and broth microdilution (Sensititre®) for both E. coli and M. haemolytica. Differences were also detected when comparing resistance between two bacterial organisms within the same cattle; there was a higher likelihood of detecting resistance in E. coli than in M. haemolytica. Differences in resistance prevalence were not detected when using individual animal or composite pen sampling strategies. No differences in resistance prevalences were detected in E. coli recovered from cattle that were culture-positive for M. haemolytica compared to those that were culture-negative, suggesting that sampling strategies which targeted recovery of E. coli from M. haemolytica-positive cattle would not provide biased results.

CONCLUSIONS

We found that for general purposes, the susceptibility test selected for AMR surveillance must be carefully chosen considering the purpose of the surveillance since the ability to detect resistance appears to vary between these tests depending upon the population where they are applied. Continued surveillance of AMR in M. haemolytica recovered by nasopharyngeal swab is recommended if monitoring an animal health pathogen is an objective of the surveillance program as results of surveillance using fecal E. coli cannot be extrapolated to this important respiratory pathogen. If surveillance of E. coli was pursued in the same population, study populations could target animals that were culture-positive for M. haemolytica without biasing estimates for AMR in E. coli. Composite pen-floor sampling or sampling of individuals per-rectum could possibly be used interchangeably for monitoring resistance in E. coli.