MOLLICUTES

Authors:
F T H THIAUCOURT - OIE Expert and Head of CIRAD CBPP Reference Laboratory, Veterinarian, PhD, HDR, TA A117 Campus de Baillarguet, Montpellier, Occitanie, 34398, France
L MANSO-SILVAN - Researcher at CIRAD ASTRE, DVM, PhD, TA A117 Campus de Baillarguet, Montpellier, Occitanie, 34398, France

The class mollicutes represents a unique category of bacteria, commonly known as “mycoplasmas”, characterized by their small cell size of 200 - 800 nm, reduced genome of 580 - 2200 kbp, and limited metabolic capabilities. As these bacteria do not possess a cell wall, they are bounded by a single plasma membrane and appear phenotypically Gram-negative. However, phylogenetic analysis based on 16S rDNA genes showed that they are related to Gram-positive bacteria such as Bacillus and Clostridium species.¹³

Mollicutes evolved from their common ancestor through a process of reductive evolution, undergoing successive genome deletions. The down-sizing of mycoplasma genomes must be seen in the context of a co-evolution of these bacteria with their hosts, which implied their adaptation to restricted ecological niches where they can scavenge metabolites they are no longer able to synthesize, and where they find environmental conditions that allow them to multiply and survive.¹¹ Mollicutes have also developed exquisite mechanisms to escape the adaptive response of their host. One of the first to be discovered was their ability to express highly variable surface lipoproteins, allowing them to adapt to inhibiting antibodies in vivo.³ Some mollicutes are able to bind and cleave host immunoglobulins as a possible strategy to evade phagocytosis.¹ Many of them produce capsular polysaccharides, which may play a role in the resistance to the innate response of the host and contribute to the formation of biofilms, enabling them to resist desiccation and protecting them from the effects of antibiotics.¹⁰ In spite of the numerous mycoplasma genomes sequenced, very few virulence mechanisms have been discovered in these bacteria.⁹ The production of hydrogen peroxide through the metabolism of glycerol seems to be quite conserved among pathogenic species,² whereas the release of mycoplasma lipopeptides and lipoglycans may also trigger a significant inflammatory response.⁶

Because of their apparent simplicity, mollicutes have attracted considerable attention for studying fundamental aspects of cellular life. A mycoplasma genome was amongst the first bacterial genomes to be fully sequenced in 1995⁴ and the first creation of a bacterial cell controlled by a chemically synthesized genome was that of a mycoplasma.⁵

Historically, the definition of mollicutes species was mainly based on a few biochemical tests, on immunological techniques such as the growth inhibition test and on genetic analyses such as DNA-DNA hybridization. This led to the description of numerous species whose genus names were adopted by resemblance to previously described mollicutes (e.g. Mycoplasma, Ureaplasma, Acholeplasma), while the species names often depended on the host they were isolated from. The advent of the polymerase chain reaction applied on 16S rDNA genes revolutionized bacterial phylogeny, as this technique could be applied on non-cultivable organisms. This allowed the grouping of species into four clades sharing common ancestors: the Spiroplasma, Hominis, Pneumoniae and AAP (Acholeplasma, Anaeroplasma Phyptoplasma) clades (https://services.cbib.u-bordeaux.fr/molligen/SPECIES/phylo.php). Based on 16S rDNA sequences, the former genera Eperythrozoon and Haemobartonella were rightly included in the class mollicutes and were assigned to the genus Mycoplasma,^{7, 8} which is not in agreement with the genetic distance existing with the closest Mycoplasma species.¹² This example illustrates that the present taxonomy of the mollicutes is not in agreement with its phylogeny and shows that species bearing the Mycoplasma genus name can be found in the various phylogenetic clades.

Mollicutes are found as saprophytes or pathogens in every possible host, from plants and arthropods to chordates, including reptiles, birds and mammals. Many of them can be found in livestock (Table 1).

Table 1 List of the main mollicute species found in livestock