Evolution of Penicillin-Binding Protein 2 Concentration and Cell Shape during a Long-Term Experiment withEscherichia coli

Abstract

Peptidoglycan is the major component of the bacterial cell wall and is involved in osmotic protection and in determining cell shape. Cell shape potentially influences many processes, including nutrient uptake as well as cell survival and growth. Peptidoglycan is a dynamic structure that changes during the growth cycle. Penicillinbinding proteins (PBPs) catalyze the final stages of peptidoglycan synthesis. Although PBPs are biochemically and physiologically well characterized, their broader effects, especially their effects on organismal fitness, are not well understood. In a long-term experiment, 12 populations of Escherichia coli having a common ancestor were allowed to evolve for more than 40,000 generations in a defined environment. We previously identified mutations in the pbpA operon in one-half of these populations; this operon encodes PBP2 and RodA proteins that are involved in cell wall elongation. In this study, we characterized the effects of two of these mutations on competitive fitness and other phenotypes. By constructing and performing competition experiments with strains that are isogenic except for the pbpA alleles, we showed that both mutations that evolved were beneficial in the environment used for the long-term experiment and that these mutations caused parallel phenotypic changes. In particular, they reduced the cellular concentration of PBP2, thereby generating spherical cells with an increased volume. In contrast to their fitness-enhancing effect in the environment where they evolved, both mutations decreased cellular resistance to osmotic stress. Moreover, one mutation reduced fitness during prolonged stationary phase. Therefore, alteration of the PBP2 concentration contributed to physiological trade-offs and ecological specialization during experimental evolution.Bacteria in nature must survive multiple stresses and repeated shifts between feast and famine conditions. When Escherichia coli experiences nutritional deprivation, its growth rate decreases sharply and the cells become more spherical and resistant to various other environmental stresses (27). Many of the morphological and physiological changes that occur during entry into "stationary" phase are associated with the cell envelope, especially the cell wall.Peptidoglycan is the major component of the bacterial cell wall, and it affects osmotic stability and cell shape (78). Many of the final steps of peptidoglycan synthesis are catalyzed in the periplasm by the penicillin-binding proteins (PBPs) (40, 82). These enzymes are named for their binding to ␤-lactam antibiotics that inhibit cell wall biosynthesis. Twelve PBPs have been identified so far in E. coli, and they are classified as high-molecular-weight (HMW) and low-molecular-weight (LMW) PBPs. The HMW PBPs assemble both the glycan chain and the peptide cross-links of peptidoglycan and are further divided into class A PBPs (PBP1a, PBP1b, and PBP1c), which are bifunctional enzymes that perform both transglycosylation and transpeptidation functions, and class B PBPs (PBP2 and PBP...

Nanofiber scaffolds influence organelle structure and function in bone marrow stromal cells

“…49 The E. coli increased their volume and some strains lost their characteristic rod-like morphology and evolved a spherical shape, which reduced their surface area to volume ratio and improved their fitness for the culture microenvironment. 50 A sphere has the lowest surface area to volume ratio for a given volume and elongated, high aspect ratio cells will have greater surface area FIGURE 7. The inter-organelle distance data in Figure 6 was used to generate mean x-y-z coordinates for the organelles using multidimensional scaling.…”

Specificity of genome evolution in experimental populations of Escherichia coli evolved at different temperatures

“…Four of the six lines that evolved at 42°C acquired mutations in mrdA (pbpA), whereas one 37°C line also did so (adjusted P = 0.0258). The mrdA gene encodes penicillin-binding protein 2, a transpeptidase involved in peptidoglycan synthesis, and mutations in this gene have also been shown to affect cell size and shape (31).…”

High-throughput, Highly Sensitive Analyses of Bacterial Morphogenesis Using Ultra Performance Liquid Chromatography

“…Bacteria-By far, the majority of HPLC muropeptide analyses have focused on E. coli strains in different growth conditions (13,19,25,26,31,(51)(52)(53)(54). A variety of laboratory strains have been used, with key quantities such as cross-linking varying over a range from 25% (25,52) to 37% (31).…”

“…Muropeptide precursors are synthesized in the cytoplasm, flipped across the inner membrane, and incorporated into an existing glycan strand by penicillinbinding proteins (12). In the rod-shaped bacterium Escherichia coli, mutations or changes to the expression levels of essential penicillin-binding proteins can result in subtle changes to rod dimensions (13) or cell rounding (13,14). In rod-shaped bacteria, cell-shape determination and maintenance involve the spatiotemporal regulation of cell wall growth, which requires the actin homolog MreB (15).…”