Culture's growing conditions impact bacterial adhesion.

The ability of pathogens to adhere to the surfaces of fruits and vegetables is a major concern in the industry today. Modeling this adhesion is crucial because models can provide data on the long-term survivability of pathogens on surfaces.

Adhesion may be enhanced or hindered by the nutritional factors under which a culture is grown, or by conditions present in the environment. The objective of scientists was to compare the adhesion and desiccation of liquid- and solid-media grown cultures of E. coli 0157:H7 932 on membrane filters.

Cultures were grown on tryptic soy broth (TSB)--agar or liquid broth--to the stationary phase. Then they were harvested and re-suspended in bovine serum albumin (BSA). Resuspended cultures (50 mL) were inoculated onto polycarbonate filters and held at room temperature for up to 24 hours. Individual filters were stomached in 100 mL of TSB for 2 minutes. The recoveries were determined by plating the materials on PCA.

Preliminary results for hydrophilic filters show that liquid- and solid-media-grown cultures of E. coli O157:H7 had similar recoveries after 1 hour, with log losses being minimal: less than 0.5 log. However, after 24 hours, liquid-media-grown cultures showed greater losses--1.5 logs to 2.5 logs--when compared to solid-media-grown cultures of near equal cell concentrations. Decreasing the inoculum concentration in liquid-media-grown cultures led to less recoverability in general, and more erratic results.

The increased recoverability of solid-media-grown culture shows that this approach is preferable to the liquid-culture modeling technique. The low recovery rate from liquid-media-grown cultures suggests there was greater adherence of the microbes to the filter, possibly due to an environmentally triggered mechanism, or cell death.

The conditions under which a culture is grown (solid or liquid media) may affect the hydrophobicity of the cells and their binding to and recoverability from filters or fruit surfaces.

Further information. Richard Swiech, National Center for Food Safety and Technology, Illinois Institute of Technology, MF 305, Moffett Campus, 6502 S. Archer Rd., Summit-Argo, IL 60501; phone: [phone]; fax: [phone]; email: [e-mail].