HDM extracts are made from animal material; house dust mites. All living creatures have other creatures with them, in symbiosis or parasites etc. When making an extract of living material we have to be aware of impurities of these kind. Most well know impurities are bacteria fungi and yeast among others. An often forgotten group which is present in animal material is mycoplasma. Sterilisation of HDM by heating is not possible because the proteins are denaturized. The other solution for diminishing of impurities is then by filtration e.g. with a 0.22 µM filter. But unfortunately this filter size is not sufficient for mycoplasma.
A vague memory of a lecture a long time ago for some, a true nightmare for others, mycoplasma contamination has been responsible for many a headache in labs for decades. Mycoplasmas are incredibly small and are invisible to even optical microscopy. In earlier times the knowledge of mycoplasma has been limited due to their difficult nature of growing but with PCR technologies the knowledge is extending.
Contamination with mycoplasma
The tiny bacteria, mycoplasma, lack a cell wall making them resistant to a lot of common antibiotics and many are parasitic in nature, attaching to- and eventually invading other cells. This means a contamination of a model with mycoplasma is difficult to detect and to treat which can result in them skewing results without the researcher knowing.
Besides the contamination within animals the mycoplasma can also contaminate people and it is not easy to recognise and detect this contamination. This is known as Mycoplasma pneumoniae which is a very small bacterium in the class Mollicutes. It is a human pathogen that causes the disease mycoplasma pneumonia, a form of atypical bacterial pneumonia related to cold agglutinin disease.
Mycoplasma in mouse models
Multiple research groups from universities all over the world are using different murine models for asthma research. Studies have shown that 5 different mycoplasma species are identified in laboratory mice. These 5 species include M. arthritidis, M. collis, M. muris, M. neurolyticum and M. pulmonis with mice and rats as primary hosts. (Masoumalinejad et al., 2018). Infection with these Mycoplasmas has effect on immune responses and may predispose to other infections and can also alter the physiological response of mice in experiments. Animals with this infection may be clinically ill, which makes them unfit for use. Transmission of these strains occurs through direct contact and aerosol (Homberger & Thomann, 1994, p. 118).
Masoumalinejad, Z., Zinatizadeh, M. R., & Tahmasebiabdar, N. (2018). A Review of Mycoplasma in Laboratory Mice. Modern Medical Laboratory Journal, 02(01), 15–19. https://doi.org/10.30699/mmlj18.104.22.168
Homberger, F. R., & Thomann, P. E. (1994). Transmission of murine viruses and mycoplasma in laboratory mouse colonies with respect to housing conditions. Laboratory Animals, 28(2), 113–120. https://doi.org/10.1258/002367794780745263
HDM extracts tested for mycoplasma
Most of our new house dust mite extract are analysed on the presence of mycoplasma, we are using a PCR method for the detection of mollicute species. Our extracts are tested for the following species: M. arginine, M. orale, M. hyorhinis, M. fermentans, M. genitalium, A laidlawii, M. hominis, M. pirum, M. pneumoniae, M. salivarium, U. urealyticum, M. synoviae and M. bovis. The results of this test are also mentioned on our certificates of analysis:
Mycoplasmas are prevalent on mucosal surfaces of the urogenital and respiratory tracts of humans and animals. Despite being host-dependent for nutrients, at least 4 Mycoplasma species (see Table 1: (Razin, 2002)) are considered human Mycoplasmas and 20 have been identified as cell culture contaminants (Nikfarjam & Farzaneh, 2012; WHO, 2019). The 4 pathogenic species are described in in the following paragraph:
- M. pneumoniae is a well-recognized Mycoplasma specie that often gives rise to pneumonia in children and young adults (Sutherland, Brandorff, & Martin, 2004), which can lead to a mild respiratory tract infection or a severe pneumonia. They can only survive by adhesion to the epithelium of the respiratory tract by adhesion proteins (Gillespie et al., 2015). The expectation is that this M. pneumoniae is present in the HDMs, because they also play a role in respiratory infections.
- M. hominis is related to genital infections in men, gynaecological infections and pregnancy related disorders (Blazek, Schmitt, Krafft, & Hadding, 1990; Uphoff & Drexler, 2002)
- M. genitalium is a sexually transmitted Mycoplasma specie of which the clinical relevance has only recently been recognized. It can lead to urethritis and infertility (Herrmann, 2002).
- M. fermentas can be found in the genital area and is linked to rheumatoid arthritis. Some strains can lead a to respiratory infection that harms other organs (Yáñez et al., 2013).
Note. Reprinted from “Molecular Biology and Pathogenicity of Mycoplasmas”, by Razin, S., 2002, p. 47, New York, NY: Free Press. Copyright 2002 by Kluwer Academic Publishers.