A proteomic analysis of erythromycin resistance in Streptococcus pneumoniae

Streptococcus pneumoniae is a significant human pathogen which is an important cause of pneumonia and bacteraemia. Over the past few years the incidence of antibiotic resistance among clinical isolates of S. pneumoniae has increased. Penicillin resistance is now widespread and the frequency of isolates that are resistant to erythromycin has risen. Erythromycin resistance in S. pneumoniae follows two basic patterns. The MLS erythromycin-resistant phenotype is due to the enzymatic methylation of ribosomal RNA that blocks erythromycin binding to the ribosome. Alternatively, in isolates of the M phenotype, a more recently documented mechanism, resistance is associated with an active efflux process that reduces intracellular levels of erythromycin. We used two-dimensional electrophoresis to examine the proteins synthesised by erythromycin-susceptible and -resistant S. pneumoniae. Erythromycin-resistant S. pneumoniae with the M phenotype showed a significantly increased synthesis of a 38,500 Dalton (pI 6.27) protein compared to susceptible isolates. Peptide mass mapping was used to identify the 38,500 Dalton protein as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It was demonstrated that S. pneumoniae synthesised at least three forms of GAPDH that differed in their isoelectric points. The form of GAPDH possessing the most basic pI showed the increased synthesis in the erythromycin-resistant S. pneumoniae isolates. Alterations in the synthesis of GAPDH were only found for those erythromycin-resistant isolates possessing the M phenotype. S. pneumoniae isolates with the MLS phenotype were indistinguishable from the susceptible strains using the analytical conditions employed for the current study. The possible role of GAPDH in erythromycin resistance of S. pneumoniae is considered.