Correlation between antibiotic resistance, phage-like particle presence, and virulence in Rhodococcus equi human isolates

Abstract

Rhodococcus equi is a gram-positive coccobacillus that appears to be emerging as a pulmonary pathogen in AIDS patients. In four human clinical isolates, two antibiotic resistance phenotypes were found to coexist: one β- lactam resistant and the other β-lactam susceptible. In vitro, β-lactam- resistant mutants were obtained at a frequency of 1 x 10-5 to 5 x 10-5 from β-lactam-susceptible strains on cephalothin-containing plates. Neither β-lactamase nor plasmid DNA was detected in β-lactam-resistant or - susceptible strains. The penicillin-binding protein patterns for the two types of strains were identical. Electron microscopy revealed that the β- lactam-resistant strains possessed cell-surface-associated appendages and produced phage-like particles. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of total cell protein showed at least three additional bands of 42, 39, and 30 kDa found only in the β-lactam-resistant strains. Testing for virulence in Swiss mice revealed that (i) phage-like-particle-producing strains had lower 50% lethal doses when injected intravenously in euthymic and nude mice than the non-phage-like-particle-producing strains did and (ii) intravenous inoculation of a sublethal dose (5 x 106 CFU) in nude mice led to chronic infection by the phage-like-particle-producing bacteria only. Finally, in vitro growth curves indicated that the phage-like-particle- producing strains possessed an ecological selection advantage. These results suggest that, among R. equi human isolates, the antibiotic resistance phenotype is associated with virulence and may be phage mediated.