Introduction and aim: Multi-drug resistant Gram-negative bacteria represent a big issue in the management of diseases. Aim of our study was to evaluate the in-vitro activity of N-acetil-cysteine (NAC) alone or in combination with a single antibiotic or more antibiotics.
Material and methods: Fifteen Acinetobacter baumannii (Ab) and 10 Klebsiella pneumoniae (Kp) both resistant to carbapenems isolated from different types of infections, were included. The antibacterial activity of NAC and of the single antibiotics was tested by MIC and MBC measurements. Synergy testing of NAC in association with antibiotics was evaluated by qualitative methods (chekerboard FIC index) and quantitative tests (Time Killing Curves TKC, bactericidal activity). Morphological changes at scanning electron microscopy (SEM) were also examined.
Results: MICs 50/90 (μg/ml) for Ab and Kp were the following: MEM (meropenem) 128/512 and 256/1024, COL (colistin) 0.25/4 and 256/1024, RIF (rifampicin) 2/4 and 32/256 respectively, TIG (tigecycline) 0.5/1 for both, A/S (ampicillin/sulbactam) only for Ab 64/256 and ERT (ertapenem) only for Kp 1024/2048, NAC 2.5/5 and 5/5 mg/ml for Ab and Kp. As for the strains of Ab, NAC alone was bactericidal at 2.5 mg/ml (1/2 MIC) after 2 hrs up to the end of the experiment (24 hrs). The association of NAC resulted as being 100% synergic with MEM and A/S, 93% with RIF whereas was indifferent with COL and TIG. Following the TKC, NAC was able to completely restore the activity of MEM and A/S at concentrations easily achievable in the serum (8-16 μg/ml) which, alone, don’t result effective. As for the strains of Kp, a marked synergism between MEM at 8 μg/ml and NAC at 0.5 MIC was detected. The association COL (2 μg/ml)+0.5 MIC MER (512 μg/ml) was inactive whereas in the presence of NAC at sub-MIC and MEM at 8 μg/ml resulted as fully synergistic. TKC showed that the addition of NAC to MEM at concentrations without bactericidal effect alone, induced a fast and lasting bactericidal activity that persisted overtime. Morphological alterations at SEM (bacterium lengthening and formation of outer membrane vescicles) were strongly evident in the cultures with antibiotics + NAC or with NAC alone compared with the controls.
Conclusions: In summary, NAC not only seems to show a considerable activity against difficult-to-treat bacteria but results as being able to restore the efficacy of the antibiotics associated, at concentrations achievable in the serum.
Maria Teresa Mascellino
