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Pathogen Biology

The causal agent of blackleg is Erwinia carotovora subsp. atroseptica, a Gram-negative, rod-shaped bacterium closely related to enteric bacteria of importance as human and animal pathogens. Bacteria in the genus Erwinia, however, are not known to be harmful to humans or animals. When grown on a medium containing sodium polypectate, the blackleg bacterium develops pits or craters in the medium due to the excretion of pectolytic enzymes that liquefy the pectate (Figure 8). The pectolytic enzymes are, in fact, an important component of the pathogenicity factors of this and related bacteria. In recognition of the unique pectolytic activity of these bacteria it has been proposed to place them in a separate genus, Pectobacterium. Furthermore, it has recently been suggested that on the basis of its genetic composition, the blackleg bacterium be considered a unique species, Pectobacterium atrosepticum.

Figure 8

Several other subspecies of E. carotovora that cause disease in other crops have been described. These subspecies can cause decay of potato tuber slices but do not cause the blackleg disease. A new strain of E. carotovora has been described recently which causes a blackleg-like disease of potato in Brazil. Preliminary results suggest that the atroseptica subspecies does not occur on potato where the newly-described subspecies, tentatively named brasiliensis, occurs.

The most distinguishing feature of the blackleg bacterium is its pectolytic enzyme activity but in contrast to many of the other pectolytic bacteria it does not grow above 36°C/97°F. It is facultatively anaerobic meaning that it can grow both with and without the presence of oxygen. It is motile with numerous peritrichous flagella. Most strains of E. carotovora subsp. atroseptica belong to serogroup I, although several other serogroups occur. Because of the relatively uniform serological type, serological methods such as enzyme linked immunoassay (ELISA) and immunofluorescence can be used for its detection. Molecular methods including the polymerase chain reaction (PCR) are also available for detection and identification of this bacterium.

The blackleg bacterium can be isolated on a selective medium such as the crystal violet pectate medium (CVP, Figure 9) most efficiently at about 22°C/72°F. Colonies of pectolytic erwinia can be readily identified on this medium with a dissecting microscope using oblique illumination, that is, by shining a light through the bottom of the Petri plate from an angle. Colonies transferred from CVP grow on general bacteriological media such as nutrient agar. Pathogenicity of isolates can be easily determined on young (10-15 cm/4-6 in. high) potato plants by stabbing into the stem a toothpick smeared with bacterial cells. Symptoms of blackleg develop within two weeks (Figure 9).

Figure 9