[Importance of the antimicrobial mode of action of antibiotics for their combined effect with glycine in subinhibitory concentrations on microbial growth].

Following previous observations of an increase in microbial sensitivity to the bactericidal beta-lactams ampicillin and imipenem in the presence of glycine, the aim of the presented study was to examine if such an effect is due to the antimicrobial mode of action of an antibiotic and/or to its bactericidal or bacteriostatic capacity. Using growth curves as an experimental parameter the same synergistic glycine effect could be shown if tested concomitantly with a number of other antibiotics acting equally on bacterial cell wall formation as cefaclor, cefadroxil, or fosfomycin. This glycine effect is, therefore, associated with the antibiotic mode of action, but is independent of wether the antibiotics are beta-lactams or not (fosfomycin). In contrast, glycine had no particular effect in combination with antibiotics inhibiting protein synthesis (sisomicin, kanamycin, chloramphenicol, oxy-tetracycline) or nucleic acid polymerase activity (ciprofloxacin, cinoxacin; rifampicin being a certain exception) as well as with those acting on cytoplasmic and external membrane as polymyxin B. The synergistic effect of glycine and cell wall active antibiotics was interpreted predominantly by an action on carboxypeptidases.