ABSTRACT
Internal yellowing (IY) caused by Enterobacter cloacae and characterized by yellow discolored tissue surrounding the papaya (Carica papaya L.) seed cavity, diffuse margins, and the presence of a distinctly rotten odor was first reported in 1987 (3). Here we report the formation of atypical internal yellowing (AIY) in ripe papaya caused by the bacterium Enterobacter sakazakii. In surveys conducted from 2006 to 2007, 'Kapoho Solo' papayas grown in the Puna District of Hawaii Island were obtained from various packinghouses. After incubation at 27°C, the papayas were bisected and examined for symptoms of IY. Among papayas that were asymptomatic for IY, a dull, greenish yellow discoloration of the flesh with a distinct margin extending from the seed cavity into the pericarp was noted, along with a pungent odor. These symptoms occurred in 5 of the 500 fruit surveyed and bacterial populations were 102 to 103 CFU/g. Discolored tissue was aseptically excised, weighed, macerated, serially diluted in sterile distilled water (SDW), and plated onto modified peptone yeast extract medium (PT-M4) (4). The plates were incubated at 30°C for 24 to 48 h until single colonies were evident. After 48 h, colonies on PT-M4 were orange-red, convex and circular, and surrounded by a somewhat opaque 1-mm margin. After single colony purification, five strains were obtained. The strains, inoculated into oxidation/fermentation-glucose tubes and API 20E strips (bioMerieux, Inc., Durham, NC) incubated at 30°C, were shown to be facultative anaerobes and identified as E. sakazakii with a 98.4% certainty. Colonies plated onto tryptic soy agar (TSA) and incubated for 72 h at 25°C produced yellow pigmentation, indicative of E. sakazakii. Amplification by PCR with E. sakazakii-specific primers (2) yielded a 929-bp fragment, which was absent with E. cloacae and Pseudomonas aeruginosa template DNA. To confirm pathogenicity, cell suspensions at 109 CFU/ml of putative E. sakazakii strains RK07-05, RK07-06, and RK07-07 and E. cloacae (3) were inoculated by injection (0.5 ml per site) into one-third-ripe 'Kapoho Solo' papayas (six fruit per strain, inoculated at duplicate sites) and incubated at 27°C for 4 days. Control sites were injected with 0.5 ml of SDW. Fruit inoculation experiments were repeated. E. cloacae-inoculated sites produced typical IY as previously described (3), while the sites inoculated with the three E. sakazakii strains produced greenish yellow tissue (26% mean incidence), symptomatic of AIY. Control sites did not produce IY or AIY. Koch's postulates were fulfilled, and the identification of reisolated bacterial strains was confirmed with API 20E, PCR, and pigment production on TSA. Although less prevalent (1% incidence) than the typical IY produced by E. cloacae (3), E. sakazakii has the potential to affect quality and food safety of fresh and processed papaya products. E. sakazakii has been implicated in a severe form of neonatal meningitis, sepsis, and necrotizing enterocolitis (1). Research into the transmission and infection of papaya of this cross-domain pathogen merits further study. References: (1) D. H. Adamson. Clin. Microbiol. Newsl. 3:19, 1981. (2) A. Lehner et al. BMC Microbiol. 4:43, 2004. (3) K. A. Nishijima et al. Plant Dis. 71:1029, 1987. (4) K. A. Nishijima et al. Plant Dis. 88:1318, 2004.
