Bacteriology : Helicobacter pylori

BACTERIOLOGY : Helicobacter   pylori

H. pylori has morphologic and growthsimilarities to thecampylobacters,with whichtheywere originally classified. The cells are slender, curved rods with polar flagella. The cell wall structure is typical of other Gram-negative bacteria, although Helicobacter LPS may be less toxic than its enteric counterparts. Growth requires a microaerophilic atmosphere and is slow (3 to 5 days).

A number of unique bacteriologic features have been found in H. pylori. The most distinctive is a urease whose action allows the organism to persist in low pH environ-ments by the generation of ammonia. The urease is produced in amounts so great (6% of bacterial protein) that its action can be demonstrated within minutes of placing H. pylori in the presence of urea. Another secreted protein called the vacuolating cytotoxin (VacA) causes apoptosis in eukaryotic cells it enters generating multiple large cytoplas-mic vacuoles. The vacuoles are felt to be generated by the toxin’s formation of channels in lysosomal and endosomal membranes.

Most H. pylori strains also contain a 30 gene PAI, so called because the guanine cytosine content of the PAI differs from the rest of the genome. This suggests the PAI is a genetic cassette acquired from some unknown organism in the distant past. Most of the PAI genes code for elements of a contactsecretionsystem, which in other bacteria trans-fers DNA or proteins across the outer membrane to the extracellular space or into other cells. The cells receiving the products of these secretion systems include bacterial, plant, and epithelial cells. In H. pylori, the secretion system injects VacA and a protein Cag, also coded in the PAI, into epithelial cells. Once in the cell, Cag induces changes in mul-tiple cellular proteins and has a strong association with virulence (see Pathogenesis).