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Intraphagocytic growth induces an antibiotic-resistant phenotype of Legionella pneumophila
J Barker1, H Scaife, M R Brown
1Department of Biomedical Sciences, Sheffield Hallam University, United Kingdom.
Abstract:
The antimicrobial susceptibilities of Legionella pneumophila isolates grown either in U937 human monocytic cells or in Acanthamoeba polyphaga were studied after release from the host cells without further subculture. Time-survival studies showed that exposure of L. pneumophila cells, gown exclusively in vitro, to 5 micrograms of rifampin per ml resulted in at least 99% killing after 6 h and no detectable survivors at 24 h. Similar rates of killing were observed for in vitro-grown cells tested by exposure to ciprofloxacin. Conversely, time-survival studies revealed that macrophage-grown and amoeba-grown cells were ca. 1,000-fold more resistant to the activities of both drugs. Macrophage-grown cells treated with 5 micrograms of rifampin per ml showed 70 and 62% survival after 6 and 24 h, respectively. Intracellularly grown legionellae were also highly resistant to erythromycin (8 microgram/ml). After 24 h of exposure to the drug, there was 70 and 60% survival for amoeba-grown and macrophage-grown legionellae, respectively, whereas in vitro-grown cells showed a 2-log10 reduction in viable count. when intracellularly grown L. pneumophila cells were subcultured in broth for 48 h, they reverted to the phenotype characteristic of in vitro growth. Morphologically, the cells were larger than their intracellularly grown counterparts and resistance characteristics were lost. The susceptibilities of the subcultured cells to all three drugs were similar to those of Legionella cells grown exclusively in vitro. In view of these findings, the successful treatment of Legionnaires disease may be related as much to the resistance phenotype induced by intramacrophage growth as to the ability of the antibiotic to enter phagocytic cells.
Insights
Legionella pneumophila grown inside host cells, like macrophages, become highly resistant to antibiotics. This resistance is lost upon subculturing, suggesting intracellular growth influences antibiotic susceptibility in Legionnaires disease treatment.
Area of Science:
- Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Legionella pneumophila is an intracellular bacterium responsible for Legionnaires disease.
- Understanding the factors influencing L. pneumophila's antibiotic susceptibility is crucial for effective treatment.
Purpose of the Study:
- To investigate the antimicrobial susceptibilities of L. pneumophila grown intracellularly versus in vitro.
- To determine if intracellular growth induces antibiotic resistance in L. pneumophila.
Main Methods:
- L. pneumophila isolates were grown in U937 human monocytic cells or Acanthamoeba polyphaga.
- Antimicrobial susceptibility testing was performed using rifampin, ciprofloxacin, and erythromycin.
- Time-survival studies assessed bacterial killing rates after drug exposure.
Main Results:
- In vitro-grown L. pneumophila were highly susceptible to rifampin, ciprofloxacin, and erythromycin.
- Macrophage- and amoeba-grown L. pneumophila exhibited significantly increased resistance (up to 1,000-fold) to these antibiotics.
- Subculturing intracellularly grown L. pneumophila in broth for 48 hours restored their in vitro susceptibility phenotype.
Conclusions:
- Intracellular growth of L. pneumophila induces a resistant phenotype.
- This induced resistance may significantly impact the efficacy of antibiotic treatment for Legionnaires disease.
- The ability of antibiotics to penetrate host cells may be less critical than the intracellularly induced resistance phenotype.