Contemporary North American Mycoplasma hyopneumoniae MICs

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, the MycoLab at the University of Minnesota shares results from a project looking at Mycoplasma hyopneumoniae Minimum Inhibitory Concentrations (MICs) from various US isolates.

Key points

  • Antibiotic susceptibility profiles of eleven M. hyopneumoniae isolates circulating in the US within the most recent six years were obtained. 
  • Overall, a high in vitro efficacy of the tested antimicrobials against M. hyopneumoniae field isolates was observed.


Mycoplasma hyopneumoniae (M. hyopneumoniae) is the causative agent of enzootic pneumonia, which has a recognized negative effect on productive performance and economic revenue in finishing pigs (1). Control of M. hyopneumoniae infections is usually attempted in various ways, including optimization of management practices, use of vaccines, and treatment with antibacterial compounds. Moreover, protocols for eradication of M. hyopneumoniae incorporate the use of antimicrobials (2).

Information about antimicrobial susceptibility is largely limited, as M. hyopneumoniae is especially hard to culture and antibiograms are not routinely performed. Indeed, the most current publicly available data on antimicrobial susceptibility for M. hyopneumoniae isolates from the US is dated several decades ago (3). In this context, the aim of this study was to determine the in vitro susceptibility to different antibiotics of M. hyopneumoniae contemporary isolates originating from clinical cases in the US. 

Materials and Methods

Eleven M. hyopneumoniae field isolates were obtained from US swine clinical specimens dating within the last six years. Minimum inhibitory concentration (MIC) values of the examined antibiotics against the isolates were determined by microbroth dilution method (4). Briefly, 100 µL of the appropriate antimicrobial solution were distributed into the corresponding well of microtiter plates, with a final range of antimicrobials from 0.001 to 64 mg/L. The test was accomplished on 104 CCU/mL of each isolate. All isolates were tested in three independent replicates. For each isolate, a positive (growth) control was included by adding 100 µL of culture in a well with no antimicrobial. For negative (uninoculated) controls, two wells were filled with 200 µL of sterile medium. M. hyopneumoniae strain 11 (ATCC 25095) was used as reference for the MIC test. Plates were incubated at 37°C up to 14 days. 


The highest variance in the MIC values obtained was observed for tilmicosin and oxytetracycline, whereas the lowest variance was recorded for tylvalosin (Table 1). Compared with ATCC 25095, the M. hyopneumoniae US isolates MIC90 values were higher for all compounds but for tilmicosin, lincomycin, oxytetracycline and tiamulin. The highest MIC90 values for M. hyopneumoniae US isolates were achieved at ≤8 µg/mL for tilmicosin and oxytetracycline, whereas the lowest were recorded at ≤0.016 µg/mL for tylvalosin.

Table 1. MIC values for the reference strain (ATCC) and MIC range and MIC90 values (mg/L) for M. hyopneumoniae isolates in the US

Discussion and conclusions

This study presents the antibiotic susceptibility profiles of M. hyopneumoniae isolates circulating in the Midwest region of the US. Overall, a high in vitro efficacy of the tested agents against M. hyopneumoniae was observed. Results from this study represent a renewed step towards appropriate and accurate antibiotic treatment in the event of mycoplasma diseases in swine.


  1. Ferraz, M.E.S., et al. 2020. Prev Vet Med. 182:105091.
  2. Holst, S., et al. 2015. J Swine Health Prod. 23(6):321-330.
  3. Williams, P.P. 1978. Antimicrob Agents Chemother. 14:210-213.
  4. Klein, U.A., et al. 2017. Vet Microbiol. 204:188-193.