444 Produce Degradation: Reaction Pathways and their Prevention
treatment of the lettuce. Takeuchi et al. (2000) studied the extent of attachment of
E. coli O157:H7, Listeria monocytogenes, Salmonella Typhimurium, and Pseudomo-
nas fluorescens on iceberg lettuce using confocal scanning laser microscopy
(CSLM). These researchers observed that a significantly larger amount E. coli
O157:H7 and L. monocytogenes cells adhered to cut edges, whereas Pseudomonas
fluorescens adhered preferentially to intact surfaces. Salmonella Typhimurium
adhered equally to both intact and cut surfaces. On the intact leaf surface the extent
of adhesion of each organism in descending order was Pseudomonas fluorescens
followed by E. coli O157:H7, L. monocytogenes, and S. Typhimurium. Some cells
of each organism adhered to and subsequently penetrated subsurface tissues through
the cut edges of the lettuce leaves.
Because leaf surfaces are generally hydrophobic due to the presence of a waxy
cuticle (Romberger et al., 1993), it can be assumed that the hydrophobicity of
bacterial cells helps them to adhere to leaves or other hydrophobic surfaces.
Pseudomonas fluorescens readily adhered to the intact surface of lettuce leaves
compared to the edges of cut surfaces (Seo and Frank, 1999). Spores of Pasteuria
species have been reported to adhere rapidly to the waxy cuticle of plants and
nematodes (Mendoza de Gives et al., 1999). Both hydrophobic and electrostatic
interactions seemed to be involved in the mechanism of rapid attachment of Pasteuria
penetrans spores (Afolabi et al., 1995; Davies et al., 1996). The hydrophobicity of
plant surfaces including the surfaces of leaves can be evaluated by measuring the
“contact angle” of water droplets on the leaf surface. The contact angle is the angle
formed between the advancing edge of a water droplet and a solid surface (Rent-
schler, 1971). There is an inverse relationship between the contact angle and the
extent of attachment between a liquid and a solid surface; therefore, the lower the
extent of attachment between liquid droplet and the surface (larger contact angle)
the greater the hydrophobicity. Generally, the leaves of many plants are hydrophobic
and have contact angles larger than 80 or 100° (Neinhuis and Barthlott, 1997). In
this regard, Beattie (2002) suggested that even the most hydrophobic of bacterial
cell surfaces are unlikely to be sufficiently hydrophobic to promote rapid adhesion
to leaf surfaces.
14.3.2 INVOLVEMENT OF BACTERIAL APPENDAGES
Although bacterial cell surface charge and hydrophobicity may contribute to bacterial
adhesion to plant surfaces, bacterial appendages such as pili or other protein struc-
tures help to facilitate this process. Generally, the surfaces of both plants and bacterial
cells are negatively charged. Therefore, electrostatic repulsion will occur once these
surfaces come in close proximity to each other. The rapid attachment of bacterial
cells on leaves may be mediated by pili or via carbohydrates or outer membrane
proteins on the bacterial cell surface. These cell surface adhesions bridge the gap
created by electrostatic repulsion (Romantschuk et al., 1996). The involvement of
pili in relatively rapid attachment of Pseudomonas syringae and Pseudomonas
phaseolicola to leaf surfaces has been demonstrated (Romantschuk et al., 1993;
Suoniemi et al., 1995).