Abstract
BackgroundContagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. mycoides small-colony type (SC) is among the most serious threats for livestock producers in Africa. Glycerol metabolism-associated H2O2 production seems to play a crucial role in virulence of this mycoplasma. A wide number of attenuated strains of M. mycoides subsp. mycoides SC are currently used in Africa as live vaccines. Glycerol metabolism is not affected in these vaccine strains and therefore it does not seem to be the determinant of their attenuation. A non-synonymous single nucleotide polymorphism (SNP) in the bgl gene coding for the 6-phospho-β-glucosidase (Bgl) has been described recently. The SNP differentiates virulent African strains isolated from outbreaks with severe CBPP, which express the Bgl isoform Val204, from strains to be considered less virulent isolated from CBPP outbreaks with low mortality and vaccine strains, which express the Bgl isoform Ala204.ResultsStrains of M. mycoides subsp. mycoides SC considered virulent and possessing the Bgl isoform Val204, but not strains with the Bgl isoform Ala204, do trigger elevated levels of damage to embryonic bovine lung (EBL) cells upon incubation with the disaccharides (i.e., β-D-glucosides) sucrose and lactose. However, strains expressing the Bgl isoform Val204 show a lower hydrolysing activity on the chromogenic substrate p-nitrophenyl-β-D-glucopyranoside (pNPbG) when compared to strains that possess the Bgl isoform Ala204. Defective activity of Bgl in M. mycoides subsp. mycoides SC does not lead to H2O2 production. Rather, the viability during addition of β-D-glucosides in medium-free buffers is higher for strains harbouring the Bgl isoform Val204 than for those with the isoform Ala204.ConclusionOur results indicate that the studied SNP in the bgl gene is one possible cause of the difference in bacterial virulence among strains of M. mycoides subsp. mycoides SC. Bgl does not act as a direct virulence factor, but strains possessing the Bgl isoform Val204 with low hydrolysing activity are more prone to survive in environments that contain high levels of β-D-glucosides, thus contributing in some extent to mycoplasmaemia.
Highlights
Contagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. mycoides small-colony type (SC) is among the most serious threats for livestock producers in Africa
We have recently identified by polymerase chain reaction (PCR) amplification and restriction enzyme analysis (PCR-REA) a genetic difference in a structural gene which divides M. mycoides subsp. mycoides SC strains in two groups, one including virulent African field strains and the other comprising the T1derived vaccine strains T1/44 and T1/Sr50, as well as European and Australian strains and the type strain PG1 [35]
Genetic variability of the bgl gene in M. mycoides subsp. mycoides SC The 9.8 kb genomic region containing the genes involved in disaccharide uptake and metabolism in M. mycoides subsp . mycoides SC type strain PG1, whose genome sequence was published by Westberg et al [10], was considered in this study
Summary
Contagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. mycoides small-colony type (SC) is among the most serious threats for livestock producers in Africa. Contagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. Mycoides small-colony type (SC) is among the most serious threats for livestock producers in Africa. Mycoides small-colony type (SC), a Mycoplasma species causing one of most severe infectious animal diseases as defined by the World Organization for Animal Health and included in the list of diseases notifiable to the Office International des Epizooties (OIE), does not show classical virulence genes such as toxin, cytolysin and invasin genes [10]. Mycoides SC causes contagious bovine pleuropneumonia (CBPP), the most serious cattle disease in Africa since the successful control of rinderpest [11]. Mycoides SC or by a substance released by these mycoplasmas [27]; and vi) toxic metabolic side products such as H2O2 [28,29,30,31], which is translocated efficiently to the host cells where it can cause cell death [32] Virulence of M. mycoides subsp. mycoides SC seems to be determined by intrinsic functions such as: i) capsular polysaccharide (CPS) that seems to be involved in serumresistance [13,14,15,16,17]; ii) lipoproteins that are expected to play a role as triggers in mechanisms of pathogenicity and in the release of pro-inflammatory cytokines [18,19,20,21,22]; iii) yet unknown but necessary adhesion factors that may play a central role in the intimate interactions of pathogenic mycoplasmas with mammalian cells for long periods triggering a cascade of signals which are transduced to the host cell and induce inflammation [23]; iv) repeating elements in variable membrane proteins of mycoplasmas that are suggested to increase the pathogen's ability to adhere to host cells and to evade the host immune response [24,25,26]; v) immunomodulating factors that can cause apoptosis of the mononuclear cells triggered by live M. mycoides subsp. mycoides SC or by a substance released by these mycoplasmas [27]; and vi) toxic metabolic side products such as H2O2 [28,29,30,31], which is translocated efficiently to the host cells where it can cause cell death [32]
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