# Review on Silver Nanoparticles as a Novel Class of Antibacterial Solutions
## 4 Silver Nanoparticles and Their Antibacterial Role
### Laboratory Methods for Quantification of Antibacterial Activity  
Antibacterial activity is observed using: zone of inhibition plates, minimal inhibitory concentration test, minimal bactericidal concentration test.  
Zone of inhibition (ZOI) plates is a test used for assessing antibiotic sensitivity of bacteria. Antibiotics are placed in agar plates and then left to incubate. After the incubation  
Figure 3: Silver nanoparticles’ toxic action.  
period, the growth inhibition or bacterial destruction is measured by examination of the empty zone surrounding the antibiotic (inhibition zone) [9].  
Several advantages make the zone inhibition test one of the most used laboratory methods, such as being a low-price and fast method, suitable for water soluble antimicrobial efficiency, usable for testing liquids, coated surfaces as well as antimicrobial impregnated solids. However, a number of disadvantages of zone of inhibition plates test have also been identified. The method does not directly assess bacterial death, but only indirectly by measuring the growth inhibition zone. Moreover, it cannot be used for viruses, it is not a real quantitative method although it does allow measurement of the diameter of the zone.  
Minimal inhibitory concentration (MIC) is the lowest concentration of a chemical which prevents the growth of bacteria. In order to determine MIC, solutions with different concentrations are incubated with bacteria and the results measured using agar dilution or broth micro dilution [53]. Relatively easy preparation and execution, small scale experiment adequacy, requirement of little preparation, as well as possible low test turnaround times are some of the advantages. Minor variations (incubation period, concentration) lead to large variation of MIC, bacteria were prevented from growing (but not killed), meaning the initial quantity of bacteria can still be present--are just a few disadvantages of MIC.  
Minimal bactericidal concentration (MBC) is the lowest concentration of a chemical that results in bacteria death. A similar approach to MIC is used to determine MBC, resulting in a protocol that presents several advantages. It allows the determination of minimum concentration of agent that achieves bacteria death, can be used to rank antimicrobial agents by potency. The test parameters are easy to control and the assay can test multiple antimicrobial agents under the same conditions and observe their effect.  
However, some disadvantages of MBC include the impossibility of individual bacteria assay. Also, tests will not determine the needed concentration to counterattack bacteria in short time and it has been demonstrated that different growth media will impact MBC values.  
Due to a more facile protocol, MIC is easier to be determined as compared to MBC, and it is therefore used clinically. In normal conditions, the antimicrobial effect of drugs is similar for comparable concentration thanks to the host immune system fighting against bacteria (in non-multiplication conditions). When MBC is much higher than MIC, the toxicity increases and may be harmful for the human cell [54].