Tricarboxylic Acid Cycle and Glyoxylate Bypass

l-Serine potentiates fluoroquinolone activity against Escherichia coli by enhancing endogenous reactive oxygen species production

BACKGROUND

The increase in multiple antimicrobial-resistant bacteria seriously threatens global public health. Novel effective strategies are urgently needed. l-Serine was reported as the most effective amino acid inhibitor against bacterial growth and can sensitize Escherichia coli cells to gentamicin. It is currently unknown whether l-serine affects other type of antibiotics such as β-lactams and fluoroquinolones.

METHODS

Using E. coli, we studied the combination of l-serine with diverse antibiotics against laboratory and clinical E. coli cultures and persisters. The intracellular NAD(+)/NADH level and ROS were determined using kits. Total cellular iron was determined by using a colorimetric ferrozine-based assay.

RESULTS

Exogenous l-serine sensitized E. coli ATCC 25922 and clinically isolated fluoroquinolone-resistant E. coli to fluoroquinolones. This potentiation is independent of growth phase. Addition of serine increases the production of NADH. The underlying mechanism of this strategy is that the combination of serine with ofloxacin or moxifloxacin increases the NAD(+)/NADH ratio, disrupts the Fe-S clusters and increases the production of endogenous reactive oxygen species. Furthermore, we used a serine and ofloxacin or moxifloxacin combination in vitro to combat bacterial persister cells, compared with antibiotic treatment alone; combinational treatments of persister cells with antibiotics and l-serine resulted in a significantly greater decrease in cell viability.

CONCLUSIONS

To our knowledge, this is the first report that l-serine can potentiate the action of ofloxacin or moxifloxacin against Gram-negative bacteria and could constitute a new strategy for the eradication of bacterial infections.

Characterization of acetic acid-detoxifying Escherichia coli evolved under phosphate starvation conditions

BACKGROUND

During prolonged incubation of Escherichia coli K-12 in batch culture under aerobic, phosphate (Pi) starvation conditions, excess glucose is converted into acetic acid, which may trigger cell death. Following serial cultures, we isolated five evolved strains in two populations that survived prolonged incubation.

METHODS

We sequenced the genomes of the ancestral and evolved strains, and determined the effects of the genetic changes, tested alone and in combination, on characteristic phenotypes in pure and in mixed cultures.

RESULTS

Evolved strains used two main strategies: (1) the constitutive expression of the Trk- and Kdp-dependent K(+) transport systems, and (2) the inactivation of the ArcA global regulator. Both processes helped to maintain a residual activity of the tricarboxylic acid cycle, which decreased the production of acetic acid and eventually allowed its re-consumption. Evolved strains acquired a few additional genetic changes besides the trkH, kdpD and arcA mutations, which might increase the scavenging of organophosphates (phnE (+), lapB, and rseP) and the resistance to oxidative (rsxC) and acetic acid stresses (e14(-)/icd (+)).

CONCLUSIONS

Evolved strains rapidly acquired mutations (phnE (+) lapB rpoS trkH and phnE (+) rseP kdpD) that were globally beneficial to growth on glucose and organophosphates, but detrimental to long-term viability. The spread of these mutant strains might give the ancestral strain time to accumulate up to five genetic changes (phnE (+) arcA rsxC crfC e14(-)/icd (+)), which allowed growth on glucose and organophosphates, and provided a long-term survival. The latter strain, which expressed several mechanisms of protection against endogenous and exogenous stresses, might provide a platform for producing toxic recombinant proteins and chemicals during prolonged incubation under aerobic, Pi starvation conditions.

Genes responsible for anaerobic fumarate and arginine metabolism are involved in growth suppression in Salmonella enterica serovar Typhimurium in vitro, without influencing colonisation inhibition in the chicken in vivo

From a collection of over 2800 Salmonella enterica subspecies Enterica serotype Typhimurium F98 Tn5-TC1 insertion mutants 14 were identified as expressing growth-non-suppressive phenotype under strict anaerobic conditions. Sequence analysis of regions flanking the Tn insertions revealed that most of the selected mutants were defective in genes contributing to the anaerobic fumarate uptake and generation (insertions in dcuA, dcuB and aspA), or to the anaerobic L-arginine utilisation pathway (insertions in STM4467 encoding a putative arginine deiminase, and in between speF encoding ornithine decarboxylase and kdpE coding a response regulator protein). Mutants defective in flagellum synthesis (flhA) were also identified. In contrast to the in vitro results, all the mutants colonised 1-day-old chicks efficiently and suppressed the super-infection of chicks by the parent strain. This clearly indicates that neither of the metabolic pathways mentioned above nor motility play essential roles in lower intestinal tract colonisation.