Abstract
Human diarrheal isolates of Aeromonas hydrophila were found to be invasive in HEp-2 cell monolayers. Microscopic examination of two isolates from patients with symptoms of dysentery revealed that infected HEp-2 cells contained up to 50 bacteria localized within areas of the cytoplasm. A quantitative invasion assay was developed, using viable counts of total intracellular bacteria. Five fecal isolates of A. hydrophila were classified as invasive, with an average of 0.1 up to 2 bacteria per HEp-2 cell, as compared to 7 for an invasive Escherichia coli strain. 3 other fecal isolates, 1 reference strain, and 10 water isolates of A. hydrophila were similar to a noninvasive E. coli strain, with less than 0.008 bacteria per HEp-2 cell. All isolates were screened for plasmid DNA; no common plasmid was found in the invasive strains, and the loss of a 6.0-megadalton plasmid from one of these strains had no observable effect on invasiveness. Thus some A. hydrophila isolates are capable of cellular penetration and replication, and this may be an important chromosomally determined virulence property of the organism.