(p)ppGpp and moonlighting RNases influence the first step of lipopolysaccharide biosynthesis in Escherichia coli

The outer membrane (OM) protects Gram-negative bacteria from harsh environmental conditions and provides intrinsic resistance to many antimicrobial compounds. The asymmetric OM is characterized by phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet. Previous reports suggested an involvement of the signaling nucleotide ppGpp in cell envelope homeostasis in Escherichia coli. Here, we investigated the effect of ppGpp on OM biosynthesis. We found that ppGpp inhibits the activity of LpxA, the first enzyme of LPS biosynthesis, in a fluorometric in vitro assay. Moreover, overproduction of LpxA resulted in elongated cells and shedding of outer membrane vesicles (OMVs) with altered LPS content. These effects were markedly stronger in a ppGpp-deficient background. We further show that RnhB, an RNase H isoenzyme, binds ppGpp, interacts with LpxA, and modulates its activity. Overall, our study uncovered new regulatory players in the early steps of LPS biosynthesis, an essential process with many implications in the physiology and susceptibility to antibiotics of Gram-negative commensals and pathogens.

Sonography of acute appendicitis in children: 7 years experience

OBJECTIVE

The objective of this prospective study was to determine the accuracy in diagnosing appendicitis in children by using high-resolution graded compression sonography.

MATERIALS AND METHODS

During a 7-year period, 3859 children (mean age 9.9 years) with clinically suspected appendicitis were evaluated by sonography with the graded compression technique. The ultrasound results were correlated with the intraoperative and histopathological findings or clinical outcome.

RESULTS

High-resolution sonography was performed in 3859 children. Of the 610 patients who underwent a laparotomy, 494 had histologically proven acute or perforated appendicitis (prevalence 13%). In these children, sensitivity, specificity and overall accuracy of sonography were 90%, 97% and 96%, respectively. The reasons for false-negative and false-positive results are discussed.

CONCLUSION

Although the use of ultrasound to diagnose acute appendicitis in children has excellent results, the decision for surgery remains a clinical one because of the continuing false-negative and false-positive results from sonography.