Abstract
The capsule (cps) locus of Streptococcus pneumoniae is flanked by the pbp2x and pbp1a genes, coding for penicillin-binding proteins, enzymes involved in cell wall synthesis that are targets for beta-lactams. This linkage suggested to us that selection for beta-lactam resistance might coselect for capsular transformants. The recombination event would then involve PBP genes, as well as the cps operon, and would change both the serotype and the resistance profile of the strain. We transformed beta-lactam-susceptible strain TIGR4 by using whole genomic DNA extracted from multidrug-resistant strain GA71, a serotype 19F variant of pneumococcal clone Spain(23F)-1, and selected beta-lactam-resistant transformants. Smooth colonies appearing on selective plates were subcultured, serotyped by the Quellung reaction, and genotyped to confirm the presence of the GA71 pbp2x-cps19-pbp1a locus in the TIGR4 genetic background by restriction fragment length polymorphism analysis of the whole locus and its flanking regions. The results showed that a new serotype, combined with resistance to beta-lactams, could emerge in a susceptible strain via a single transformation event. Quantitative analysis showed that transfer of the cps locus had occurred at an elevated rate in beta-lactam-selected transformants. This suggests that in natural settings selection by host immunity and selection by antibiotics may be interrelated because of "hitchhiking" effects due to linkage of resistance determinants and the capsule locus.
Highlights
Seventy-five years ago, in a milestone experiment, Griffith demonstrated the astonishing ability of Streptococcus pneumoniae to adapt under the pressure of the host immune system, by acquiring what we know to be DNA encoding biosynthetic enzymes for a polysaccharide capsule, which protects the organism against phagocytosis during infection [16]
The capsule locus of Streptococcus pneumoniae is flanked by the pbp2x and pbp1a genes, coding for penicillin-binding proteins, enzymes involved in cell wall synthesis that are targets for -lactams
We transformed the -lactam-susceptible, serotype 4 TIGR4 strain with whole genomic DNA extracted from GA71, a penicillin- and cefotaxime-resistant, serotype 19F variant of the pandemic Spain23F-1 clone
Summary
0.047 1.0 1.0 1.0 1.0 1.0 1.0 a The -lactam concentration used to select the transformants listed was 0.04 mg/liter for penicillin and 0.1 mg/liter for cefotaxime. b Recipient strain in transformation experiments. c Donor strain. 0.047 1.0 1.0 1.0 1.0 1.0 1.0 a The -lactam concentration used to select the transformants listed was 0.04 mg/liter for penicillin and 0.1 mg/liter for cefotaxime. Tams, requiring replacements in pbp2x or in both pbp1a and pbp2x, which flank the cps locus, suggested that it might be possible to select single-step capsular transformants in vitro under selective pressure of cefotaxime or penicillin. In case of selection for resistance to penicillin, a single event like this would require replacement of 42 kb or almost 2% of the TIGR4 strain genome, twice the size of any naturally transformed fragment of S. pneumoniae homologous DNA previously reported [4]. Some transposons of comparable size have been reported to integrate into the S. pneumoniae genome following transformation, their mechanism of integration into the host genome is different [36, 42].
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