Mycoplasma hominis: a difficult pathogen? [letter to the editor]

We read with interest the article by Hooper and colleagues in the January 2013 issue of The Biomedical Scientist. We believe it sets out very well the issues concerning the detection of Mycoplasma hominis, a mycoplasma that is not the most difficult to isolate and identify when forethought is given to its possible existence in certain clinical situations. However, we wish to highlight a couple of points with regard to best laboratory practice. First, although mycoplasmas evolved from Gram-positive bacteria, during this process they lost the ability to form a bacterial cell wall and, therefore, do not take up the Gram stain. This feature is widely known and the use of the Gram stain serves no purpose. Thus, it is inappropriate to suggest that it can be used as an alternative to Giemsa stain. If the latter is not available, Dienes stain (mainly Methylene Blue) is an excellent substitute for defining colonial morphology. Second, the incorporation of thallium acetate in mycoplasma medium to suppress bacterial growth was largely abandoned years ago following the poisoning of several laboratory staff who suffered mental disturbance and hair loss. A broad-spectrum penicillin is commonly used as mycoplasmas are unaffected due to the aforementioned absence of a bacterial cell wall.