Carbon dioxide occupies a central position in the physiology of
Helicobacter pylori owing to its capnophilic nature, the large amounts of
carbon dioxide produced by urease-mediated
urea hydrolysis, and the constant bicarbonate supply in the
stomach.
Carbonic anhydrases (CA)
catalyze the interconversion of
carbon dioxide and bicarbonate and are involved in functions such as CO(2) transport or trapping and pH homeostasis.
H. pylori encodes a periplasmic alpha-CA (alpha-CA-HP) and a
cytoplasmic beta-CA (beta-CA-HP). Single CA inactivation and double CA inactivation were obtained for five
genetic backgrounds, indicating that
H. pylori CA are not essential for growth in vitro. Bicarbonate-carbon dioxide exchange rates were measured by
nuclear magnetic resonance spectroscopy using
lysates of parental strains and CA
mutants. Only the
mutants defective in the alpha-CA-HP
enzyme showed strongly reduced exchange rates. In
H. pylori,
urease activity is essential for acid resistance in the
gastric environment.
Urease activity measured using crude cell extracts was not modified by the absence of CA. With intact CA
mutant cells incubated in acidic conditions (pH 2.2) in the presence of
urea there was a delay in the increase in the pH of the incubation medium, a
phenotype most pronounced in the absence of
H. pylori alpha-CA. This
correlated with a delay in acid
activation of the
urease as measured by slower
ammonia production in whole cells. The role of CA in vivo was examined using
the mouse model of
infection with two mouse-adapted
H. pylori strains, SS1 and X47-2AL. Compared to colonization by the
wild-type strain, colonization by X47-2AL single and double CA
mutants was strongly reduced. Colonization by SS1 CA
mutants was not significantly different from colonization by
wild-type strain SS1. However, when
mice were infected by SS1 Delta(beta-CA-HP) or by a SS1 double CA
mutant, the
inflammation scores of
the mouse gastric mucosa were strongly reduced. In conclusion, CA contribute to the urease-dependent response to acidity of
H. pylori and are required for high-grade
inflammation and efficient colonization by some strains.
