Antimicrobial Resistance Patterns in Organic and Conventional Dairy Herds in Sweden

Abstract

Monitoring antimicrobial resistance (AMR) and use (AMU) is important for control. We used Escherichia coli from healthy young calves as an indicator to evaluate whether AMR patterns differ between Swedish organic and conventional dairy herds and whether the patterns could be related to AMU data. Samples were taken twice, in 30 organic and 30 conventional dairy herds. Selective culturing for Escherichia coli, without antibiotics and with nalidixic acid or tetracycline, was used to estimate the proportions of resistant isolates. Microdilution was used to determine the minimum inhibitory concentrations (MICs) for thirteen antimicrobial substances. AMU data were based on collection of empty drug packages. Less than 8% of the bacterial growth on non-selective plates was also found on selective plates with tetracycline, and 1% on plates with nalidixic acid. Despite some MIC variations, resistance patterns were largely similar in both periods, and between organic and conventional herds. For most substances, only a few isolates were classified as resistant. The most common resistances were against ampicillin, streptomycin, sulfamethoxazole, and tetracycline. No clear association with AMU could be found. The lack of difference between organic and conventional herds is likely due to a generally good animal health status and consequent low AMU in both categories.

Keywords: AMR; MDR; antibiotic; antibiotic resistance; antibiotic use; environment; livestock.

Figure 1

Minimum inhibitory concentration (MIC) distribution of Escherichia coli isolates from calves, sampled in period 1, in 30 (148 isolates) organic (Org) and 30 (145 isolates) conventional (Conv) Swedish dairy herds, in a panel of 13 antimicrobial agents. Blue color indicates the range of concentrations tested. Vertical black lines are the epidemiological cut-off points, according to EUCAST. p values test the difference between organic and conventional herds with Fisher’s exact test. Yellow color denotes isolates from conventional herds and pink color shows MIC values.

Figure 2

Minimum inhibitory concentration (MIC) distributions of Escherichia coli isolates from calves, sampled in period 2, in 27 (127 isolates) organic (Org) and 27 (131 isolates) conventional (Conv) Swedish dairy herds, in a panel of 13 antimicrobial agents. Blue color indicates the range of concentrations tested. Vertical black lines are the epidemiologic cut-off points according to EUCAST. p values test the difference between organic and conventional herds with Fisher’s exact test. Yellow color denotes isolates from conventional herds and pink color shows MIC values.

Figure 3

Minimum inhibitory concentration (MIC) distributions of Escherichia coli isolates from environmental samples of farm manure (drainage and manure pit) at the beginning of period 2, in 27 (50 isolates) organic (Org) and 27 (53 isolates) conventional (Conv) Swedish dairy herds, in a panel of 13 antimicrobial agents. Blue color indicates the range of concentrations tested. Vertical thicker black lines are the epidemiologic cut-off points according to EUCAST. p values test the difference between organic and conventional herds with Fisher’s exact test. Yellow color denotes isolates from conventional herds and pink color shows MIC values.

Figure 4

Number of sampled herds with zero (0), low (1–25%), medium (26–60%) and high (>60%) proportions of Escherichia coli with antimicrobial resistance to any antimicrobial class (AMR) or to three or more antimicrobial classes (MDR) in Swedish organic and conventional dairy herds. Isolates from calf samples in period 1 (per 1) and period 2 (per 2) and environmental samples in period 2 (env).

Figure 5

Heatmaps showing the patterns of resistant and susceptible Escherichia coli isolates from Swedish organic and conventional dairy herds sampled in two different time periods. (A) conventional herds 1st sampling (B) conventional herds 2nd sampling (C) organic herds 1st sampling (D) organic herds 2nd sampling. Color scale illustrates average value for each herd and each antimicrobial substance tested, where 1 = resistant and 0 = susceptible. AMX = ampicillin, CAZ = ceftazidime, CHL = chloramphenicol, CIP = ciprofloxacin, CST = colistin, CTX = cefotaxime, FLOR = florfenicol, GEN = gentamycin, NAL = nalidixic acid, STR = streptomycin, SXT = sulfamethoxazole, TET = tetracycline, and TMP = trimethoprim.