2.2.4. Enterobacter cloacae

Another bacterium that has also shown cross-kingdom pathogenesis is the Gram-negative bacterium Enterobacter cloacae. It is an important nosocomial pathogen responsible for bacteremia, lower respiratory tract infections, skin, and soft-tissue infections, as well as urinary tract infections. E. cloacae synthesizes a Shiga-like toxin II-related cytotoxin, which has been implicated in an infant case of haemolytic-uremic syndrome. Although there is evidence that E. cloacae has evolved to colonize the human host, it has also been identified as the causal agent of grey kernel disease of macadamia (Macadamia integrifolia). The onset of grey kernel disease not only affects the quality of the kernels produced by the tree, but also results in grey discoloration and a foul odor. E. cloacae also causes bacterial soft rot disease in dragon fruit (Hylocereus spp.), bacterial leaf rot in Odontioda orchids, and is also responsible for internal yellowing disease in papaya and onions.

Enterobacter bulb decay develops after onions are harvested, cured, and stored. The decay usually occurs in a few scales of the onion bulb and the tissue develops a brown color giving the bulb a dirty ring appearance when cut in half. If storage lots of onions have a high enough incidence of Enterobacter bulb decay (>2–5%), the whole lot cannot be sold and results in a significant loss to the grower. The mechanism of how E. cloacae causes bulb decay, as well as the specificity of the virulence factors employed in each of these hosts are unknown and, as a result, the development of disease control methods for bulb decay are limited.