**4. Conclusions**

These results indicate that infections due to multidrug resistant microorganisms could be treated by the use of a synergistic combination of antimicrobial drugs and AgNPs. Further studies are necessary to evaluate the specific mechanisms of action, which could help predict undesirable off-target interactions, sugges<sup>t</sup> ways of regulating a drug's activity, and identify novel therapeutics to this health problem.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6382/7/2/50/s1, Table S1: Resistance profile of the set of clinical strains, Table S2: Standardization of the method MIC.

**Author Contributions:** F.M.-G. is the intellectual author of the project who designed the microbiological experiments. G.A.M.-C. designed the experiments related to the synthesis and characterization of silver nanoparticles and contributed to the analysis and interpretation of the data. A.F. performed the isolation of the pathogens and the sensitivity profile. M.L.-C. performed the microbiological experiments and the statistical analysis of the data. C.C. designed the experiments related to the study of the cytotoxic activity of silver nanoparticles and K.I.V. performed these experiments. M.M. oversaw the follow-up of the patients. F.M.-G. and M.L.-C. wrote this paper.

**Acknowledgments:** This work was supported by the Fondo de Apoyo a la Investigación (FAI) UASLP, C16-FAI-09-23.23 and Doctorado en Ciencias Odontológicas, Facultad de Estomatología, UASLP.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix FIC Calculation**

One strategy used to overcome the resistance mechanism of the microorganisms is the use of the combination of drugs. The way to measure their effectiveness is through the calculation of the Fractional Inhibitory Concentration (FIC) index.

TheFICindexwas calculatedasfollows:

FIC of AgNPs = MICAgNPs in combination/MICAgNPs alone FIC of Antibiotic = MICAntibiotic in combination/MICAntibiotic alone FIC index = FIC of AgNPs + FIC of Antibiotic

The FIC index was interpreted as follows: FIC < 0.5, Synergy (S); 0.5 ≤ FIC <1, Partial Synergy (PS); FIC = 1, Additive (AD); 2 ≤ FIC < 4, Indifferent (I); FIC > 4, Antagonism (AN).
