Institute for Medical Microbiology and Hygiene, University of Cologne, Goldenfelsstrasse 19- 21, 50935 Cologne, Germany
Tel: +49-221-478-3060; Fax: +49-221-478-3067.
Sir,
We read with interest the recent correspondence in the journal from Gilbert & Brown, 1 in which they comment on the activity of antibiotics in biofilms.
In general, the dominant mechanisms of biofilm resistance may be related to the suppression of bacterial growth within the biofilm, to physicochemical interaction of the glycocalyx with certain antibiotics (via dipole- dipole-, H-, and ionic-bonds, and complexes) and to changes in the cell envelope following adhesion to hard and soft tissues. Suppressed growth rate and cell wall alterations subsequent to cell density transcriptional activation may be ultimately responsible for phenotypic resistance of adherent bacteria to certain antibiotics. 2 In addition to the cited Sorbarod technique, other models and methods for understanding specific aspects of antimicrobial resistance of sessile bacteria are useful to elucidate specific resistance mechanisms. Thus, for testing the effects of antibiotics on slow-growing bacteria we use phosphate-buffered saline (PBS) as a non-proliferating medium at 37°C.
We agree with Gilbert & Brown, 1 that antibiotics other than ß-lactams would have markedly different results when the Sorbarod technique is employed. Relatively thick, hydrated, polyanionic-gelled polysaccharides and glycoproteins may act like ion exchange resins adsorbing cationic aminoglycosides until all binding sites are saturated. 3 Uptake of more hydrophilic antibiotics into bacteria varies with the drug's charge, size, and hydrophilicity and with the number of porin proteins, which changes with varying metabolic activity. 4 Ciprofloxacin, though highly effective against growing and non-growing Gram-negative bacilli, 5 had no detectable efficacy in an animal model with infected foreign bodies. 6
Extracts of exopolysaccharide from slime-positive strains of Staphylococcus epidermidis 7 antagonized the antimicrobial efficacy of vancomycin but not that of rifampicin in a dose-dependent fashion. This could explain the increased level of resistance to vancomycin of organisms embedded in a biofilm. Obst et al. 8 also reported incomplete sterilization of S. epidermidis biofilms when vancomycin was suspended in peptone water or buffered peritoneal dialysis fluid, demonstrating the influence of the microenvironment. When rifampicin was tested in combination with nafcillin, vancomycin, clindamycin, pefloxacin, ciprofloxacin, trimethoprim, teicoplanin or erythromycin in vitro, the bactericidal activity of nafcillin, vancomycin and teicoplanin was significantly reduced and rifampicin-resistant strains emerged in combination with trimethoprim. 9
It is difficult to deduce a general structure- activity relationship but some pharmacodynamical features may be important for the choice of biofilm-active drugs: intracellular accumulation, direct membrane damage, penetration of infected tissues, leukocytes, biofilms and lipophilicity may be advantageous for killing adherent, metabolically- inactive bacteria.
Lipophilic rifampicin with a high antistaphylococcal activity may be a drug that shows most of these properties. 10 Recently, some unpublished investigations on lipophilic drug combinations indicated that the addition of rifampicin to bacteriostatic agents such as erythromycin and fusidic acid resulted in antimicrobial activities more effective than the individual agents towards stationary growth phase bacteria in PBS (Figure). Addition of the bactericidal antibiotic mupirocin to bacteria in the stationary state was also effective. The molecular level of additive and synergic antimicrobial effects of rifampicin with erythromycin, mupirocin and fusidic acid was assumed to be a sequential activity of these substances on the RNA- polymerase dependent protein-synthesis level. Due to the highly lipophilic nature of the drugs, intracellular accumulation may enhance the post-antibiotic effect and therefore the killing efficacy. Lipophilic drug combinations containing rifampicin tested in this study may be beneficial for the outcome of implant infections by microorganisms in a stationary growth phase.
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References
1
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Gilbert, P. & Brown, M. R. W. (1998). Biofilms and ß-lactam- activity. Journal of Antimicrobial Chemotherapy 41, 5712.
2 . Copper, M., Batchelor, S. M. & Prosser, J. I. (1995). Is cell density signalling applicable to biofilms? In The Life and Death of Biofilm (Wimpenny, J., Handley, P., Gilbert, P. & Lappin-Scott, H., Eds), pp. 937. Bioline Press, Cardiff.
3 . Wagman, G. H., Bailey, J. V. & Weinstein, M. J. (1975). Binding of aminoglycoside antibiotics to filtration materials. Journal of Antimicrobial Agents and Chemotherapy 7, 31619.
4 . Livermore, D. M. (1991). Antibiotic uptake and transport by bacteria. Scandinavian Journal of Infectious Diseases, Suppl. 74, 1422.
5 . Zeiler, H. J. & Grohe, K. (1984). The in vitro and in vivo activity of ciprofloxacin.European Journal of Microbiology and Infectious Diseases 3, 33943.
6 . Widmer, A. F., Frei, R., Rajacic, Z. & Zimmerli, W. (1990). Correlation between in vivo and in vitro efficacy of antimicrobial agents against foreign body infections. Journal of Infectious Diseases 162, 96102.[ISI][Medline]
7 . Farber, B. F., Kaplan, M. H. & Clogston, A. G. (1988). Staphylococcus epidermidis extracted slime inhibits the antimicrobial action of glycopeptide antibiotics. Journal of Infectious Diseases 161, 3740.
8 . Obst, G., Gagnon, R. F., Harris, A., Prentis, J. & Richards, G. K. (1989). The activity of rifampin and analogs against Staphylococcus epidermidis biofilms in a CAPD environment model. American Journal of Nephrology 9, 41420.[ISI][Medline]
9 . Hackbarth, C. J., Chambers, H. F. & Sande, M. A. (1986). Serum bactericidal activity of rifampin in combination with other antimicrobial agents against Staphylococcus aureus. Journal of Antimicrobial Agents and Chemotherapy 29, 61113.
10 . Butts, J. D. (1994). Intracellular concentrations of antibacterial agents and related clinical implications. Clinical Pharmacokinetics 27, 6380.[ISI][Medline]