First morphological characterization of 'Candidatus Mycoplasma turicensis' using electron microscopy

Abstract

At least three haemotropic mycoplasmas have been recognized in cats: Mycoplasma haemofelis (Mhf), 'Candidatus Mycoplasma haemominutum' (CMhm) and 'Candidatus M. turicensis' (CMt). The latter was originally identified in a Swiss pet cat with haemolytic anaemia and shown to be prevalent in domestic cats and wild felids worldwide using molecular methods. So far, there has been no confirmatory morphological evidence of the existence of CMt presumably due to low blood loads during infection while CMhm has only been characterized by light microscopy with discrepant results. This study aimed to provide for the first time electron microscopic characteristics of CMt and CMhm and to compare them to Mhf. Blood samples from cats experimentally infected with CMt, CMhm and Mhf were used to determine copy numbers in blood by real-time PCR and for transmission and scanning electron microscopy. High resolution scanning electron microscopy revealed CMt and CMhm to be discoid-shaped organisms of 0.3 μm in diameter attached to red blood cells (RBCs). In transmission electron microscopy of CMt, an oval organism of about 0.25 μm with several intracellular electron dense structures was identified close to the surface of a RBC. CMhm and CMt exhibited similar morphology to Mhf but had a smaller diameter. This is the first study to provide morphological evidence of CMt thereby confirming its status as a distinct haemoplasma species, and to present electron microscopic features of CMhm.

Fig. 1

(a–d) SEM images of RBCs from blood anticoagulated with Alsever's solution collected from Cat 1 on day 23 after experimental infection with CMt. Discoid-shaped organisms, about 0.3 μm in diameter, are attached to the surface of RBCs (indicated by an arrow in a). Bars represent 1 μm.

Fig. 2

SEM image of RBCs from lithium-heparin-anticoagulated blood collected from Cat Z 19 days after experimental infection with CMt. Circular contrast-free regions of about 0.3 μm on two RBCs are visible. Bar represents 1 μm.

Fig. 3

(a–d) SEM images of RBCs from blood anticoagulated with Alsever's solution collected from Cat Q1 on day 56 after experimental infection with CMhm. Discoid-shaped organisms, about 0.3 μm in diameter, are attached to the surface of RBCs. A circular contrast-free region of about 0.3 μm in diameter is seen on the surface of a RBC in Fig. 4 d. Bars represent 1 μm.

Fig. 4

(a, b) SEM images of RBCs from blood anticoagulated with Alsever's solution collected from Cat QLA5 10 days after experimental infection with Mhf. Several discoid-shaped organisms of about 0.5 μm in diameter are attached to RBCs. An organism in binary fission is indicated by an arrow. Bars represent 1 μm.

Fig. 5

TEM images. (a) Lithium-heparin-anticoagulated blood collected from Cat Z on day 19 after experimental infection with CMt. An oval structure of about 0.25 μm in length with intracellular round electron dense particles is located close to the surface of a RBC. Bar represents 0.2 μm. (b, c) Blood sample anticoagulated with Alsever's solution and collected from Cat QLA5 10 days after experimental infection with Mhf. Structures of about 0.5–0.6 μm in diameter are tightly attached to the surface of RBCs. Bars represent 0.5 μm.