Abstract
As a neutrophilic bacterium, Helicobacter pylori is growth deficient under extreme acidic conditions. The gastric pathogen is equipped with an acid survival kit, regulating urease activity by a pH-gated urea channel, opening below pH 6.5. After overcoming acid stress, the bacterium’s multiplication site is situated at the gastric mucosa with near neutral pH. The pathogen exhibits exceptional genetic variability, mainly due to its capability of natural transformation, termed competence. Using single cell analysis, we show here that competence is highly regulated in H. pylori. DNA uptake complex activity was reversibly shut down below pH 6.5. pH values above 6.5 opened a competence window, in which competence development was triggered by the combination of pH increase and oxidative stress. In contrast, addition of sublethal concentrations of the DNA-damaging agents ciprofloxacin or mitomycin C did not trigger competence development under our conditions. An oxygen-sensitive mutant lacking superoxide dismutase (sodB) displayed a higher competent fraction of cells than the wild type under comparable conditions. In addition, the sodB mutant was dependent on adenine for growth in broth and turned into non-cultivable coccoid forms in its absence, indicating that adenine had radical quenching capacity. Quantification of periplasmically located DNA in competent wild type cells revealed outstanding median imported DNA amounts of around 350 kb per cell within 10 min of import, with maximally a chromosomal equivalent (1.6 Mb) in individual cells, far exceeding previous amounts detected in other Gram-negative bacteria. We conclude that the pathogen’s high genetic diversity is a consequence of its enormous DNA uptake capacity, triggered by intrinsic and extrinsic oxidative stress once a neutral pH at the site of chronic host colonization allows competence development.
Highlights
The human stomach is a hostile niche
The microaerobic pathogen is equipped with a set of oxidative stress enzymes, including catalase (KatA), alkyl hydroperoxide reductase (AhpC) and superoxide dismutase (SodB), which are important for host colonization [8,9,10]
In contrast to all other known bacteria, DNA uptake in H. pylori during natural transformation is established by a type IV secretion system that is encoded by two separate operons, comB2-B4 and comB6-B10 [12, 17]
Summary
The human stomach is a hostile niche. After overcoming the acid barrier, bacterial colonization at the gastric mucosa represents a constant battle with the host immune system. Helicobacter pylori are genetically highly variable, microaerobic Gram-negative bacteria that successfully colonize the human gastric mucosa of half the world’s population, with variable prevalence in different human populations [4]. H. pylori is a neutrophilic organism, but highly adapted to acid survival For this purpose, the pathogen has a potent urease enzyme, which is tightly regulated via substrate accessibility by a pH-regulated channel [6, 7]. Direct exchange of DNA between H. pylori cells by conjugation-like mechanisms were shown to occur [13,14,15] and were discussed to play a yet underestimated role in genetic diversity of the gastric pathogen [16]. Monitoring fluorescent DNA during natural transformation was subsequently applied to other Gram-negative bacteria, like Vibrio and Neisseria [21, 22]. A maximum of 40 kb of imported DNA was detected in the bacterial periplasmic space of Neisseria, limited by the binding capacity of the periplasmic DNA-binding protein ComE (ComEA homologue) [21], which is absent in H. pylori [23]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
