*3.5. Antimicrobial Activity of ACR and [Cu2(ACR)(NO3)2]*

Figure 4 plots the inhibition zones formed by the ACR and [Cu2(ACR)(NO3)2]) against three different model pathogens. Both the free ligand and [Cu2(ACR)(NO3)2] showed strong activity against the used Gram (+) *B. cereus* and the yeasts *Candida*; the complex exhibited about 30% higher activity than the ligand and standard drug. The low activity has been observed against Gram negative *P. aeruginosa*.

**Figure 4.** Zones of inhibition of the model strains by 0.06 g of the investigated ACR and [Cu2(ACR)(NO3)2]. G, Gentamicin; Ns, Nystatin.

The MICs of the compounds have been determined by serial dilution in MPB, and the results are shown in Table 2. Obviously, complexation with Cu(II) ions enhances the antimicrobial effect of the ligand to a different extent, depending on the tested strain.

**Table 2.** Minimum inhibitory concentration (MIC) values of the compounds ACR and [Cu2(ACR)(NO3)2] towards the tested strains.


MIC s of the [Cu2(ACR)(NO3)2] vary from 10 to 80 μg/mL, and from 40 to 120 μg/mL for the ligand. Both the ligand and its Cu(II) complex exhibit the highest effectiveness in inhibiting the growth of *B. cereus* (the lowest MICs 10 and 40 μg/mL, respectively) followed by *C. lipolytica* with MICs of 40 and 80 μg/mL, respectively. Gram (−) *P. aeruginosa* displays the highest resistance to the compounds with the highest MIC values. The observed difference in the susceptibility of both type of bacteria to the investigated substances is due to the different structure of the bacterial cell walls. [34].

Dendrimer interior is a binding site for Cu(II) ions, and after its complexation, the polarity of the Cu(II) is reduced which can be explained to the partial sharing of positive charges with the nitrogen groups that increase the lipophilicity of the [Cu2(ACR)(NO3)2] complex. The enhanced lipophilicity increases penetration of the metal complex through the lipid membranes and consequently blocking metal-binding sites in the targeted enzymes of the microorganisms [35].

According to Holetz theory MICs values can be used to classify the microbiological activity of the compounds against pathogenic microorganisms [36]. Compounds with MICs less than 100 μg/mL exhibit good antimicrobial activity, in the range 100 to 500 μg/mL have moderate activity, weak activity demonstrate compounds in the range from 500 to 1000 μg/mL, and compounds over 1000 μg/mL are considered inactive. Depending on those criteria, our results exhibit a good antimicrobial activity of the tested compounds against the used model strains.
