The intracellular bacteria from naturally occurring porcine proliferative enteropathy were cultured in an in vitro
cell culture systea, the norphology of the bacteria, the pathogenicity of the organism in vivo, in hamsters, and the
morphological events of infection in both cultured cells and hamsters were investigated.
Bacteria purified from intestinal lesions of the disease were cultured in a rat enterocyte, IEC-18 cell line. Infection
was enhanced by centrifugation of the bacteria onto the cell monolayer incubated at reduced oxygen tension in a
aicroaerobic atmosphere. However, centrifugation was not necessary for infection to occur. Bacterial infection can be
passaged and maintained several times.
Morphological observations of the bacteria grown in cell culture pelleted by centrifugation revealed that the bacterium
measure from 0.1 to 0.3pm in width and 0.7 to 2.0pm in length. The bacteria were pleomorphic with a wavy trilaminar outer
membrane and an often indistinct cytoplasmic membrane generally clearly separated by a periplasmic space. The pleomorphic
bacteria differed in the internal structure and in the electron density of their cytoplasm; some were electron-dense and
some were electron-lucent. The internal structure of the former was amorphous and consisted of numerous granules,
presumably bacterial ribosomes whereas the latter demonstrated a central reticulate appearance with less dense peripheral
granules. Bacterial divisions were consistently seen in the electron-lucent form and were by transverse septation.
IEC-18 cells were used as an in vitro model to study the cellular events of intestinal cell infection. Cells were
artifically infected either by centrifugation or spontaneously. A method of bacterial attachment and entry into the host
cell was observed in both methods of infection. In ccntrifuged infection, bacteria were seen attached by an electron-dense
cap projection of the cell membrane. In contrast, bacteria were seen closely apposed to the cell membrane in noncentrifuged infection. However, in both cases, attachment of the bacteria was followed by entry into and escape from
endocytic vacuoles free into the cytoplasm. Bacteria then multiplied to large numbers to fill the cytoplasm and were
eventually released by extrusion from the host cell cytoplasm.
The aetiology and pathogenesis of the disease in hamsters infected with the pig-derived bacteria was revealed.
Hamsters developed marked hyperplasia of the crypt epithelial cells associated with numerous bacteria in the apical
cytoplasm when dosed with the bacteria cultivated in cell culture. Cellular events of infection, morphologically similar
to that observed in the in vitro model were evident.
This study revealed that the intracellular bacteria from porcine proliferative enteritis can be cultured and passaged
in rat enterocytes in vitro and suggest that the in vitro model is a relevant model of in vivo infection, in hamsters as
both models showed similar stages in the pathogenesis of infection and proved that the porcine-derived intracellular
bacteria grown in IEC-18 cells are pathogenic in hamsters.