23 Complins, Holybourne, Hampshire GU34 4EJ, UK
Sir,
In their recent article, Jonas et al.1 comment that, The use of cell lines has been questioned because of the marked effects seen with ß-lactamase inhibitors such as sulbactam, clavulanic acid and imipenem which have no effect in vivo. This statement could be regarded as a little misleading with regard to the agents mentioned. Imipenem is not used clinically as a ß-lactamase inhibitor and clavulanic acid has been shown to be effective in vivo in a model of Legionella pneumophila respiratory infection in rats.24 In the paper by Stokes et al.5 that was quoted by the authors, clavulanic acid demonstrated good activity against this organism both in vitro and in tissue culture (MRC-5 cells), whereas amoxycillin was ineffective in the tissue culture system. At the concentrations used by Jonas et al.,1 co-amoxiclav was ineffective against intracellular L. pneumophila. The use of different cell lines could explain this difference, as Jonas et al. used Mono-Mac 6 cells; however, they do not appear to have considered the stability of the antimicrobial agents in the culture medium. Clavulanic acid and amoxycillin were found by Stokes et al.5 to be unstable after 12 h of incubation in tissue culture medium; to allow for this inactivation, the medium was removed from the infected cell monolayers every 12 h up to 48 h and replaced with fresh tissue culture medium containing antibiotic. In addition, the concentrations used by Jonas et al. were rather low relative to observed concentrations in human serum after oral administration of standard doses of co-amoxiclav.
The in vivo activity of clavulanic acid in the presence or absence of amoxycillin was demonstrated in the studies of Smith et al. in a respiratory tract infection of L. pneumophila in weanling rats2 and in immunocompromised rats.3,4 Amoxycillin alone was ineffective. The activity of clavulanic acid was similar to that seen for erythromycin and was achieved at concentrations in serum that were similar to
those obtained in humans after typical therapeutic doses. These were interesting observations and, because the results of Smith and colleagues24 in vivo reflected the results of Stokes et al.5 for clavulanic acid, it was suggested that the activity of clavulanic acid against L. pneumophila might be of clinical significance when co-amoxiclav is used for empirical therapy of respiratory infections.
The activities of ciprofloxacin and the macrolide azithromycin against L. pneumophila as reported by Jonas et al.1 are not in question, particularly as the macrolide erythromycin has been the agent of choice for many years. The differences between the results of the two laboratories do, however, illustrate the difficulties in interpretation of experimental results of studies for which test conditions can significantly affect outcome.
Notes
J Antimicrob Chemother 2001;47:359-360
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References
1
.
Jonas, D., Engels, I., Daschner, F. D. & Frank, U. (2000). The effect of azithromycin on intracellular Legionella pneumophila in the mono-mac 6 cell line at serum concentrations attainable in vivo. Journal of Antimicrobial Chemotherapy 46, 38590.
2 . Smith, G. M., Abbott, K. H., Wilkinson, M. J., Beale, A. S. & Sutherland, R. (1991). Bactericidal effects of amoxycillin/clavulanic acid against a Legionella pneumophila pneumonia in the weanling rat. Journal of Antimicrobial Chemotherapy 27, 12736.[Abstract]
3 . Smith, G. M., Abbott, K. H. & Sutherland, R. (1992). Bactericidal effects of co-amoxiclav (amoxycillin/clavulanic acid) against a Legionella pneumophila pneumonia in the immunocompromised weanling rat. Journal of Antimicrobial Chemotherapy 30, 52534.[Abstract]
4 . Smith, G. M. & Sutherland, R. (1992). Activity of amoxycillin clavulanic acid against Legionella pneumophila in vitro and in an experimental respiratory infection model. Journal of Hospital Infection 22, Suppl. A, 617.[ISI][Medline]
5 . Stokes, D. H., Wilkinson, M. J., Tyler, J., Slocombe, B. & Sutherland, R. (1989). Bactericidal effects of amoxycillin/clavulanic acid against intracellular Legionella pneumophila in tissue culture studies. Journal of Antimicrobial Chemotherapy 23, 54756.[Abstract]