In vitro activities of antimicrobial cationic peptides; melittin and nisin, alone or in combination with antibiotics against Gram-positive bacteria.

The In vitro activities of two antimicrobial cationic peptides, melittin and nisin alone and in combination with frequently used antibiotics (daptomycin, vancomycin, linezolid, ampicillin, and erythromycin), were assessed against clinical isolates of methicillin-susceptible Staphylococcus aureus, methicillin-resistant S. aureus and Enterococcus faecalis. Using the broth microdilution method, minimum inhibitory concentration (MIC) ranges of melittin and nisin against all strains were 2-8 µg/ml and 2-32 µg/ml respectively. In combination studies performed with the microdilution checkerboard method using a fractional inhibitory concentration index of ≤ 0.5 as borderline, synergistic interactions occurred more frequently with nisin-ampicillin combination against MSSA and nisin-daptomycin combination against E. faecalis strains. The results of the time-killing curve analysis demonstrated that the concentration dependent rapid bactericidal activity of nisin, and that synergism or early synergism was detected in most strains when nisin or melittin was used in combination with antibiotics even at concentrations of 0.5 × MIC.

In vitro activities of nisin alone or in combination with vancomycin and ciprofloxacin against methicillin-resistant and methicillin-susceptible Staphylococcus aureus strains.

BACKGROUND: We investigated the in vitro activities of nisin alone or in combination with vancomycin and ciprofloxacin against methicillin-resistant (MRSA) and -susceptible Staphylococcus aureus (MSSA) strains. METHODS: The minimum inhibitory concentrations were determined by microbroth dilution technique. Antibiotic combinations were assessed using the checkerboard technique. The time-kill curve method was used for determining the bactericidal activity of nisin alone and in combination. RESULTS: For both MSSA and MRSA strains, the minimum inhibitory concentrations of nisin ranged between 4 and 16 mg/l. With a fractional inhibitory concentration of ≥0.5 as borderline, synergistic interactions were seen in three of five isolates with nisin-ciprofloxacin compared to two of five isolates with nisin-vancomycin combinations against both MSSA and MRSA. No antagonism was observed. The results of time-kill curve analysis demonstrated concentration-dependent rapid bactericidal activity of nisin and synergism almost in all strains when nisin was used in combination with ciprofloxacin, and early synergistic interactions in some of the strains when it was used in combination with vancomycin. CONCLUSION: Nisin seems to be a good candidate for further investigations in the treatment of Gram-positive bacteria, alone or in combination with antibiotics.

In vitro activities of various antibiotics, alone and in combination with colistin methanesulfonate, against Pseudomonas aeruginosa strains isolated from cystic fibrosis patients.

BACKGROUND: The in vitro activities of various antibiotics, either alone or in combination with colistin methanesulfonate, were assessed using Pseudomonas aeruginosa strains isolated from cystic fibrosis patients. METHODS: Except for colistin methanesulfonate, minimum inhibitory concentrations were determined by microbroth dilution technique as described by the Clinical and Laboratory Standards Institute (CLSI); for colistin methanesulfonate, a modified method of the CLSI was used. Minimum bactericidal concentrations were determined as described by the CLSI. The in vitro activities of antibiotics in combination were determined by microbroth checkerboard technique, and results were interpreted by fractional inhibitory concentration index. RESULTS: According to MIC values, 100, 98, 96 and 84% of the isolates were found susceptible to amikacin, colistin methanesulfonate, meropenem and ceftazidime, respectively. The minimum bactericidal concentrations were generally equal to or twice as high as those of the minimum inhibitory concentrations. With a fractional inhibitory concentration index of < or =0.5 as borderline, synergistic interactions were more frequent with combinations where amikacin was involved than with those with colistin methanesulfonate. No antagonism was observed. CONCLUSION: The findings of this study may play a useful role in selecting the appropriate combinations when a single agent is inadequate to treat cystic fibrosis patients with P. aeruginosa infections.

CFTR is a pattern recognition molecule that extracts Pseudomonas aeruginosa LPS from the outer membrane into epithelial cells and activates NF-kappa B translocation.

Immune cells are activated during cellular responses to antigen by two described mechanisms: (i) direct uptake of antigen and (ii) extraction and internalization of membrane components from antigen-presenting cells. Although endocytosis of microbial antigens by pattern recognition molecules (PRM) also activates innate immunity, it is not known whether this involves extraction and internalization of microbial surface components. Epithelial cells on mucosal surfaces use a variety of receptors that are distinct from the classical endocytic PRM to bind and internalize intact microorganisms. Nonclassical receptor molecules theoretically could act as a type of endocytic PRM if these molecules could recognize, bind, extract, and internalize a pathogen-associated molecule and initiate cell signaling. We report here that the interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) and the outer core oligosaccharide of the lipopolysaccharide (LPS) in the outer membrane of Pseudomonas aeruginosa satisfies all of these conditions. P. aeruginosa LPS was specifically recognized and bound by CFTR, extracted from the organism's surface, and endocytosed by epithelial cells, leading to a rapid (5- to 15-min) and dynamic translocation of nuclear transcription factor NF-kappa B. Inhibition of epithelial cell internalization of P. aeruginosa LPS prevented NF-kappa B activation. Cellular activation depended on expression of wild-type CFTR, because both cultured Delta F508 CFTR human airway epithelial cells and lung epithelial cells of transgenic-CF mice failed to endocytose LPS and translocate NF-kappa B. CFTR serves as a critical endocytic PRM in the lung epithelium, coordinating the effective innate immune response to P. aeruginosa infection.