Survival and replication of Rhodococcus equi in macrophages

• Published in: Infection and Immunity 1994-10, Vol.62 (10), p.4167-4175
• Main Authors: Hondalus, M.K. (Temple University School of Medicine, Philadelphia, PA.), Mosser, D.M
• Format: Article
• Language: English
• Subjects: Animals; Bacteriology; Biological and medical sciences; CABALLOS; Cell Line; CHEVAL; CORYNEBACTERIUM EQUI; DIFERENCIAS BIOLOGICAS; DIFFERENCE BIOLOGIQUE; EXPERIMENTACION IN VITRO; EXPERIMENTATION IN VITRO; Female; Fluorescence; Fundamental and applied biological sciences. Psychology; Horses; MACROFAGOS; MACROPHAGE; Macrophages - microbiology; Macrophages, Alveolar - microbiology; Mice; Mice, Inbred BALB C; Microbiology; Pathogenicity, virulence, toxins, bacteriocins, pyrogens, host-bacteria relations, miscellaneous strains; PLASMIDE; PLASMIDIOS; PODER PATOGENO; POUVOIR PATHOGENE; Rabbits; RATON; Rhodococcus equi; Rhodococcus equi - growth & development; SOURIS; SUPERVIVENCIA; SURVIE
• ISSN: 0019-9567; 1098-5522
• DOI: 10.1128/iai.62.10.4167-4175.1994

Rhodococcus equi is a facultative intracellular bacterium of macrophages that can cause serious pneumonia in both young horses and immunocompromised people. Essential to understanding rhodococcus pathogenesis is a quantitative documentation of the intracellular events that follow macrophage phagocytosis of the organism. By using a bacterial immunofluorescence staining assay, we verified the intracellular survival and replicative potential of R. equi in both murine peritoneal macrophages and equine alveolar macrophages in vitro. Following an initial lag period of 6 to 12 h, the intracellular numbers of R. equi begin to rise, often reaching macrophage-compromising levels by 48 h. A quantitative determination of bacterial growth by a novel image analysis cytometry technique confirmed our fluorescence microscopic results. By 48 h postinfection, bacterial numbers had increased by more than fivefold, and the majority of infected macrophages in the monolayer contained 10 or more bacteria per cell. The intracellular organisms were viable, as evidenced by the ability to incorporate radiolabeled uracil. The use of these techniques has identified differences in the in vitro replicative capacities of a virulent strain and an avirulent strain of R. equi. A clinical isolate of R. equi expressing a 17-kDa virulence-associated plasmid-encoded antigen was able to survive and replicate within macrophages, whereas an avirulent, non-plasmid-containing strain replicated poorly. These results suggest that plasmid-encoded bacterial virulence factors may contribute to the ability of R. equi to replicate within its host cell, the macrophage