3.2 Effect of AgNPs on bacterial growth
Dose-dependent growth kinetics of bacterial strains can be used to assess the relative rate and extent of bactericidal activity of silver nanoparticles. Fig. 4(a–c) demonstrates the growth profiles of the three representative bacterial strains treated with various concentrations of 10 nm AgNPs. The results showed that for all the bacterial strains, introduction of silver nanoparticles affected the growth kinetics as compared to the negative control (culture grown in absence of AgNPs). Bacterial growth was reduced with an increase in AgNP concentration. At their respective MBC values, no visible bacterial growth was observed up to 24 hours, and thus it represents the bactericidal concentration for the specific bacterial strain. In the case of the E. coli MTCC 443 strain, introducing 10 and 20 mg ml1 Ag(10) caused -18% and -53% reduction in bacterial density as compared to the control sample. Further, increasing concentration of AgNP to 30 and 40 mgml1 caused absence of bacterial growth as these concentrations represent the MIC and MBC values respectively, for the E. coli MTCC 443 strain. Interestingly, for B. subtilis MTCC 441, an introduction of similar concentrations of Ag(10) i.e., 10 and 20 mg ml1 caused a reduction of -38% and -84% in bacterial density as compared to the control sample. Another gram positive bacteria S. aureus NCIM 5201 caused 14%, 67%, and 97% reduction in cell density for Ag(10) concentration of 20, 40 and 80 mg ml1, respectively. Notably, the relative order of bacterial sensitivity against 10 nm sized silver nanoparticles was found to be similar as reported earlier.