Mycoplasma mycoides subsp. mycoides small colony (SC) type

Other Scientific Names

• Mycoplasma mycoides mycoides SC
• Mycoplasma mycoides subsp. mycoides SC
• Mycoplasma mycoides subspecies mycoides SC

English acronym

• MmmSC

• Domain: Bacteria
•     Phylum: Firmicutes
•         Class: Mollicutes
•             Order: Mycoplasmatales
•                 Family: Mycoplasmataceae
•                     Genus: Mycoplasma
•                         Species: Mycoplasma mycoides subsp. mycoides small colony (SC) type

Like all members of the Class Mollicutes, M. m. mycoides SC is small, simple and self-replicating, and distinguishable from walled bacteria because of a number of unique properties. The mollicutes lack a cell wall and the genetic machinery to synthesise one; they have low guanine/cytosine contents (less than 30%); they have small genomes and consequently small numbers of synthesised proteins. Attempts to measure the genome of M. m. mycoides SC strains have given disparate results depending on the technique used. Early work, using DNA thermal renaturation kinetics, estimated the size of the PG1 prototype strain as 760 kbp (Askaa et al., 1973) and 810 or 923 kbp (Razin et al., 1983). However, Pyle et al. (1990) gave a genome size value of 1280 kbp for this strain using the more reproducible pulse field gel electrophoresis (PFGE) technique.

Attempts to clarify the taxonomic position of the subspecies of M. mycoides have proved difficult. Results from DNA hybridisation studies, using single type strains, revealed a relatedness value (based on percentage homology) of 0.82 between M. m. mycoides SC and LC and 0.7 between M. m. mycoides SC and M. m. capri (Askaa et al., 1978). However, large differences in percentage hybridizations were seen in experiments between M. m. mycoides SC and M. m. capri depending which genomic DNA was tritium labelled. Protein studies gave a slightly different picture. Costas et al. (1987) examined cellular proteins of 26 isolates belonging to the cluster by SDS-PAGE and showed protein patterns to be generally similar with many shared bands. However, numerical analysis of the patterns and using a correlation coefficient revealed four distinct groups or phenons at a similarity level of 70%:

M. m. mycoides SC strains

M. m. mycoides LC and M. m. capri

M. capricolum and F38

BG7

A very close relationship between M. m. mycoides LC and M. m. capri was revealed as there were difficulties in distinguishing isolates from each subspecies even at the 75% level. More extensive studies confirmed the inseparability of M. m. mycoides LC and M. m. capri using protein analysis but showed that in most cases serological tests, in particular immunofluorescence, could distinguish the subspecies designations (Leach et al., 1989). Interestingly, several strains were serologically intermediate between the two subspecies being cross reactive with both.

Early attempts to classify the mollicutes using immunological and molecular biological approaches provided useful taxonomic data but provided no evolutionary information. More recent attempts at establishing phylogenetic relationships have concentrated on the highly conserved mycoplasmal ribosomal (r) RNA which, because of very tight structural constraints, has changed much less than the bulk of the genome. Complete sequences are known for some 450 bacteria. Weisburg et al. (1989) determined the small subunit (16S) rRNA sequences of over 40 species of mollicutes and specific walled bacteria. These workers confirmed the view that the initial event in mollicute phylogeny was the formation of the Acholeplasma branch from clostridial ancestors and later to ancestors of spiroplasmas probably without reductions in genome size. By repeated independent genomic reductions, the Spiroplasma branch led to Mycoplasma and Ureaplasma species. Systematic sequence analysis revealed that M. m. mycoides belonged to the spiroplasma group, one of the five groups of mollicutes. M. capricolum subspecies capricolum was also a member of this group which largely consisted of Spiroplasma spp. No other members of the M. mycoides group were analysed. The authors concluded that while phenotypically very different from bacteria, the mollicutes appear normal at the molecular level.

Over one hundred years since the cause of CBPP was discovered it is still not possible to say with certainty how M. m. mycoides SC causes disease. The elucidation of factors playing key roles in the pathogenicity of M. m. mycoides SC is hindered by the high cost of experimental infection studies using cattle and the absence of any small animal model in which disease is reproduced. Nevertheless, experimental infections have shown virulence differences amongst African strains (Provost et al., 1987) and morbidity and mortality are lower in European than in African disease outbreaks (Nicholas et al., 1996). The production of prolonged mycoplasmaemia after inoculation in mice has been used as an indication of differing virulence of some strains of M. m. mycoides SC (March and Brodie, 2000). The difficulty of reproducing disease symptoms in experimental infections using European M. m. mycoides SC strains (Abdo et al., 1998) may also indicate a reduced virulence compared to African strains. However, the pathological lesions caused during natural outbreaks in both Africa and Europe are identical and follow a similar pattern: during new outbreaks, many cattle show acute and sub-acute lesions while chronic lesions, sequestra predominate in later outbreaks. The striking difference is the almost complete lack of mortality and obvious morbidity seen in European outbreaks which must be due in part to improved husbandry and better health (Nicholas and Palmer, 1994).

The possible roles of the carbohydrate cell capsule and hydrogen peroxide production in M. m. mycoides SC infection were reviewed by Egwu et al. (1996). Capsules are generally considered to contribute to pathogenicity by promoting binding to host tissue surfaces and enhancing resistance to phagocytosis. In addition, there is some evidence that the capsule of M. m. mycoides SC might have a direct toxic effect on host cells and its structural similarity to bovine pneumogalactan further suggests that it might induce auto-immune reactions. Analysis of concentrated and partially purified capsular material from European M. m. mycoides SC strains are consistent with earlier work for the Australian strains V5 and Gladysdale (Plackett et al., 1963) which indicated that the major carbohydrate (>90%) is galactose. There was no evidence to indicate that the capsule was comprised of several sugars (N-acetylglucosamine, fructose, fucose, glucosamine, glucose and mannose) in approximately equal proportions, as recently suggested by March et al. (1999a; 1999b).

The production of hydrogen peroxide and other active oxygen species appears to be an important factor in mycoplasma pathogenicity (Tryon and Baseman, 1992). In mycoplasmas, hydrogen peroxide production may accompany the metabolism of sugars or certain organic acids to acetate plus carbon dioxide. However, in M. mycoides, only traces of hydrogen peroxide are formed (Miles et al., 1991).

Disease(s) associated with this pathogen is/are on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's Handistatus database on disease occurrence. Please see the AHPC library for further information from OIE, including the International Animal Health Code and the Manual of Standards for Diagnostic Tests and Vaccines. Also see the website: www.oie.int.