Edwardsiella tarda osteomyelitis in a patient with SC hemoglobinopathy

Extraintestinal manifestations of Edwardsiella tarda infection

Summary Edwardsiella tarda, a member of the family Enterobacteriaceae, is a rare human pathogen. Gastroenteritis is the most frequently reported manifestation of E. tarda infection. In contrast, extraintestinal infection with E. tarda has rarely been reported. This study made a retrospective case and microbiological data review of patients with extraintestinal E. tarda infections to further understand this disease. This study retrospectively reviewed the charts of all isolates of E. tarda cultures from clinical specimens other than faeces at Chang Gung Memorial Hospital, Taoyuan, Taiwan from October 1998 through December 2001. Edwardsiella tarda was isolated from 22 clinical specimens from 22 hospitalised patients (13 females and nine males). The extraintestinal manifestations of E. tarda infection included biliary tract infection, bacteraemia, skin and soft tissue infection, liver abscess, peritonitis, intra-abdominal abscess, and tubo-ovarian abscess. The major underlying diseases predisposing to E. tarda extraintestinal infection were hepatobiliary diseases, malignancy and diabetes mellitus. The overall mortality rate of E. tarda extraintestinal infection in the present series was 22.7% (5/22), and four (40%) of 10 patients with bacteraemia expired. Although rare, human E. tarda extraintestinal infections can have diverse clinical manifestations and moreover may cause severe and life-threatening infections. Consequently, E. tarda should be considered a potentially important pathogen.

Pathogenic properties of Edwardsiella species

The pathogenic characteristics of 35 Edwardsiella strains from clinical and environmental sources were investigated. Overall, most Edwardsiella tarda strains were invasive in HEp-2 cell monolayers, produced a cell-associated hemolysin and siderophores, and bound Congo red; many strains also expressed mannose-resistant hemagglutination against guinea pig erythrocytes. Edwardsiella hoshinae strains bound Congo red and were variable in their invasive and hemolytic capabilities while Edwardsiella ictaluri strains did not produce either factor; neither E. hoshinae nor E. ictaluri expressed mannose-resistant hemagglutination nor elaborated siderophores under the tested conditions. Selected strains of each species tested for mouse lethality indicated strain variability in pathogenic potential, with E. tarda strains being the most virulent; 50% lethal doses in individual strains did not correlate with plasmid content, chemotactic motility, serum resistance, or expression of selected enzyme activities. The results suggest some potential important differences in pathogenic properties that may help explain their environmental distribution and ability to cause disease in humans.

Penetration and replication of Edwardsiella spp. in HEp-2 cells

The ability of 22 Edwardsiella strains to penetrate and replicate in cultured epithelial cells was initially evaluated by light microscopy methods and by the recovery of gentamicin-resistant (Gmr) bacteria from the Triton X-100 cell lysates of HEp-2-infected monolayers. Giemsa-stained HEp-2 cells revealed the presence of numerous internalized bacteria 3 h postinfection, often appearing as parallel rows of replicated bacteria within the cytosol and sometimes obliterating the cytoplasm because of the large numbers of bacilli present. Invasive bacteria were also sometimes found within cytoplasmic vacuoles in infected cells; thin-section electron micrographs of HEp-2-infected cells supported these conclusions. Results of light microscopy studies and cell lysate assays indicated that most Edwardsiella tarda (92%) and some Edwardsiella hoshinae strains were invasion positive on one or more occasions, while Edwardsiella ictaluri isolates were uniformly negative. HEp-2 invasion by E. tarda was a microfilament-dependent (cytochalasin B- and D-sensitive) process, with maximum numbers of Gmr CFU recorded between 3 and 6 h postinfection. The small percentage (0.01 to 1.0%) of the challenge inoculum recoverable as Gmr progeny 3 to 6 h postinfection was attributed to a strong cell-associated (not filterable) hemolysin that was produced by a majority (85%) of the E. tarda strains but not by E. ictaluri and only minimally by E. hoshinae. This cytolysin/hemolysin was responsible for the toxic effects observed in HEp-2 cells during the infection-replication process of edwardsiellae and appears to play a role in the release of internalized and replicated bacteria from infected cells. The results suggest an invasion strategy with some similarities to and differences from those of other recognized enteroinvasive pathogens.