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Mycoplasma hyopneumoniae is a member of a class of bacteria called the Mollicutes. These organisms are characterized by the absence of a cell wall, small genome size and structural simplicity. They possess limited metabolic activities and are dependent on nutrients from their environment [1].
Mycoplasma hyopneumoniae is the primary causative agent of porcine enzootic pneumonia (EP). This highly contagious chronic respiratory disease is of global significance and is highly prevalent within pig herds; it is estimated that the disease is present in more than 80% of UK pig population [2]. EP is characterized by high morbidity, but low mortality, and causes retarded growth, poor food conversion and increased susceptibility to other respiratory diseases within the infected pig [3]. This disease is of particular interest as it results in considerable economic losses, and research is ongoing into ways for limiting its impact.
Isolation of M. hyopneumoniae by culture of diseased lung tissue is an important part of this research. However, this can be very challenging and time consuming as M. hyopneumoniae is highly fastidious and other mycoplasmas, especially the fast-growing M. hyorhinis, easily overgrow and contaminate the primary culture [3].
Within the Don Whitley Scientific contract research laboratory, we have developed extensive expertise over many years working with this organism. The liquid growth medium we use for isolation and culture was first developed by Niels Friis (1975) and contains all the complex nutrients this bacterium requires for growth. A solid Friis agar medium can also be used; however, colony growth is not visible with the naked eye and can only be visualised under a microscope. Typical colony morphology is described as “fried egg” in appearance as the centre of the colony grows down into the agar.
Once a Mycoplasma has been isolated, we make a definitive identification using a duplex PCR to determine whether it is M. hyopneumoniae, M. hyorhinis or a mixed culture of both [4]. Any organisms isolated in pure culture are maintained in frozen storage, so we have a large collection of Mycoplasma cultures available for further research.
One way of treating and controlling the disease is via the use of antibiotics. We have collaborated with several agencies to monitor the antibiotic susceptibility of M. hyopneumoniae strains within different European geographical regions. The minimum inhibitory concentration (MIC) of an antimicrobial agent is determined for each antibiotic using broth microdilution methodology [5]. It should be noted that a standardised methodology specifically for mycoplasmas has yet to be described and the methodology developed at DWS is based on CLSI guidelines. We use Friis medium for our M. hyopneumoniae susceptibility testing and have invested considerable time and effort in optimizing the various components of this medium to achieve optimal growth and reproducible MIC results.
All the laboratory staff at DWS are proud to be involved with this important research and hope to continue to enhance our understanding of this organism.
Written by DWS Microbiologist, Alison Cheetham.
References:
- Methods in Molecular Biology Volume 104. Mycoplasma Protocols. Edited by R. Miles and R. Nicholas (1998).
- The National Animal Disease Information Service.
- https://www.nadis.org.uk/disease-a-z/pigs/enzootic-pneumonia/
- Diagnosis and Differentiation of Mycoplasma hyopneumoniae and Mycoplasma hyorhinis infections in Pigs by PCR amplification of the p36 and p46 genes. Caron, J., Ouardani, M., and Dea, S. Journal of Clinical Microbiology, Apr 2000, p 1390-1396.
- An improved multiplex PCR for diagnosis and differentiation of Mycoplasma hyopneumoniae and Mycoplasma hyorhinis. Barate, A.K et al Korean J Vet Res (2012) 52(1):39-43.
- Antimicrobial susceptibility monitoring of Mycoplasma hyopneumoniae isolated from seven European countries during 2015-2016. Anno de Jong et al Vet Microbiol. 2021 Feb.
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