Role of Curli and Cellulose Expression in Adherence of Escherichia coliO157:H7 to Spinach Leaves

AbstractShiga-toxigenic Escherichia coliO157:H7 outbreaks have been linked to consumption of fresh produce. It is generally recognized that bacterial attachment to vegetal matrices constitutes the first step in contamination of fresh produce. Cellular appendages, such as curli fibers, and cellulose, a constituent of extracellular matrix, have been suggested to be involved in E. coliattachment and persistence in fresh produce. A comparative evaluation was conducted on the ability of Shiga toxin-producing E. coliO157:H7 strains EDL933 and 86-24, linked to two independent foodborne disease outbreaks in humans, and their mutants deficient in curli and/or cellulose expression to colonize and to firmly attach to spinach leaf. Inoculated spinach leaves were incubated at 22°C, and at 0, 24, and 48 h after incubation loosely and strongly attached E. coliO157:H7 populations were determined. Curli-expressing E. coliO157:H7 strains developed stronger association with leaf surface, whereas curli-deficient mutants attached to spinach at significantly (p<0.01) lower numbers. Attachment of cellulose-impaired mutants to spinach leaves was not significantly different from that of curliated strains. The relative attachment strength of E. coliO157:H7 to spinach increased with incubation time for the curli-expressing strains. Laser scanning confocal microscopy (LSCM) analysis of inoculated leaves revealed that curli-expressing E. coliO157:H7 were surrounded by extracellular structures strongly immunostained with anti-curli antibodies.Production of cellulose was not required to develop strong attachment to spinach leaf. These results indicate that curli fibers are essential for strong attachment of E. coliO157:H7 to spinach whereas cellulose is dispensable.