Infected tissue, which are observed only when fruit are cut open, occur in areas around and including portions of the seed cavity, usually near the calyx and middle sections of the fruit. Fruit ripeness is a factor in the incidence of internal yellowing with ripe and overripe fruit having the highest incidences of infection. Symptom expression also varies with fruit ripeness. Since then, recent surveys indicate that this level is often surpassed, depending on the time of year and the packinghouse. Production of lysine decarboxylase, hydrogen sulfide, urease, tryptophan deaminase and indole are all negative.
Acid is produced from many carbon sources.
In addition, E. cloacae is negative for phenylalanine deaminase and pectate degradation. Enterobacter cloacae grows well on standard bacteriological media on which yellow pigment and purple stain are not produced. It is suspected that after E. cloacae is transmitted to papaya flowers by fruit flies or other insects, the pathogen remains quiescent during fruit development until symptoms are expressed when the fruit are fully ripe. A report of E. cloacae isolated from homogenates of papaya seeds in 1972 suggests that this organism may have been present in a non-pathogenic form for many years. Monthly samplings from five papaya packinghouses, that process fruit from different areas on the island of Hawaii, indicate that the incidence of internal yellowing is sporadic and may be affected by environmental factors.