are the sources from where pathogenic strains acquired the genetic codes onto their mobile genetic elements (Schwarz et al., 2004). A well‐known AME is the N‐acetyl transferase, which acetylates the aminoglycosides. Apart from AMEs, chloramphenicol acetyltransferase (CAT) and antibiotic hydrolyzing enzyme β‐lactamases belong to the same group of enzymes that acts on the antibiotics and modifies them (Martinez, 2018; Schwarz et al., 2004).
1.5.1.2 Antibiotic Efflux
The second most common mechanism of antibiotic resistance is antibiotic efflux and permeability barrier. As we discussed earlier, the permeability barrier mechanism is mostly availed by the greatest number of Gram‐negative bacteria. The presence of an extra outer membrane in Gram‐negative bacteria exhibits a barrier against hydrophilic antimicrobial agents and antibiotics such as vancomycin. However, a mutation in the genes related to outer membrane such as porin or even change in their expression level makes them vulnerable to hydrophilic antibiotics (Li et al., 2012).
The antibiotic efflux pumps in bacteria are categorized into five different families: ATP‐binding cassette (ABC), major facilitator superfamily (MFS), resistance–nodulation–division (RND), small multidrug resistance (SMR), and multidrug and toxin extrusion (MATE) (Sun, Deng, & Yan, 2014). Among these, only ABC family proteins use ATP as an energy source for efflux whereas the rest couple the export of their substrate with ion gradients. The acquired determinants of the efflux system are generally located on the plasmids in the pathogenic bacteria such as Tet genes. At least 22 genes have been identified in both Gram‐positive and Gram‐negative bacteria (Roberts, 2005). In pathogenic bacteria, the resistance–nodulation–division (RND) pump systems are operative synergistically with the Tet pump systems. The simple Tet protein effluxes the tetracycline into periplasm where RND captures and exports it outside. This is the plausible reason for increase in the minimum inhibitory concentration of tetracycline against pathogenic bacteria (Lee et al., 2000).
1.5.1.3 Target Modification or Bypass or Protection
The resistant mechanism involving target modification and protection has been also observed in various clinically resistant strains of bacteria. A typical example of target modification is found in methicillin‐resistant Staphylococcus aureus (MRSA) strains. In MRSA strains, the resistance mechanism to β‐lactams is conferred by the exogenous penicillin‐binding protein (PBP) called PBP2a. The acquired PBP2a is devoid of trans‐glycosylase activity; hence, it acts along with native PBP2 to confer the β‐lactams. PBP2a coded by mecA gene is located in large mobile genetic elements called staphylococcal chromosomal cassette (Fishovitz et al., 2014; Liu et al., 2016). Another example of target modification is vancomycin resistance in enterococci. The acquired vancomycin resistance genes called “van gene clusters” are located on the mobile genetic elements. Among the different types of van clusters, vanA and vanB are the most effective resistant systems as they are found in critical clinical strains (Miller, Munita, & Arias, 2014).
1.6 Microbial Resistance: Role of NMs
Every year millions of people suffer from severe illness due to antibacterial resistance and the mortality rate is expected to reach 10 million by 2050 (Gupta et al., 2019). For patients having such infections, antibiotic therapy is the most common treatment used with removal of infected tissue in few cases. There are many disadvantages to such treatments. Along with the high cost, these treatments make the condition worse by increasing antibiotic tolerance of the surviving bacteria. Therefore, it becomes important to find new ways to tackle such problem. Recent advancement in the field of nanotechnology provides new prospects to develop novel NMs with antimicrobial properties. Various NMs have been synthesized to defend against multidrug resistance (MDR) and microbial resistance. NMs have the ability to act not only against bacteria but also as carrier for synthetic and natural antimicrobial compounds. Different NMs have different mechanisms to combat bacteria.
1.6.1 Overcoming the Existing Antibiotic Resistance Mechanisms
Most kinds of NMs can defeat a minimum of one among the prevalent resistance mechanisms mentioned in Section 1.5.1. These impacts are the consequences of NMs' bactericidal mode, which is based on their physicochemical properties. Unlike traditional antibiotics, the characteristic sizes of NPs are 1–100 nm, which gives them novel properties like better interactions with cells (Huh & Kwon, 2011). How the NMs interact with the cell barriers and disrupt the bacterial cell membrane is discussed in this section. Since few mutations cannot change bacterial cell membrane, it further reduces the chance of drug resistance.
Not only bacterial membrane but the hindrance of biofilm formation is also an important mechanism as biofilms develop bacterial resistance by providing shelter to microorganisms, thus escaping most of the antibiotics (Peulen & Wilkinson, 2011); also, they are breeding grounds for frequent resistant mutations (Khameneh et al., 2016). NMs play a vital role in the prevention of this biofilm formation and the size of these NMs determines the level of their effectiveness in the destruction of these biofilms.
1.6.1.1 Combating Microbes Using Multiple Mechanisms Simultaneously
The simple mechanism of action of traditional antibiotics is the main reason why bacterial resistance occurred in the first place. On contrary, NMs have different action mechanisms and can be designed to have multiple mechanisms that act simultaneously against microbes. Hence, it becomes difficult for microbes to develop resistance against NMs as it is unlikely to have many mutated genes.
1.6.1.2 Acting as Good Carriers of Antibiotics
As mentioned above, NMs can also act as a carrier for various antibiotics. The delivery mechanism of NMs is different from that of other available drug delivery systems. Various NMs that have attracted attention and are currently in common use for drug delivery are polymeric NMs, metallic NMs, ceramic‐based NMs, micelles, liposomal NMs (Daeihamed et al., 2017), and carbon‐based NMs (Ranghar et al., 2014). For being an efficient antibiotics delivery system NMs possess following advantages:
1 Size: The tunable ultrasmall size of NMs makes them a suitable delivery system as they can act against intracellular bacteria. Antibiotics have poor membrane transport, which makes it difficult for them to kill intracellular and drug‐resistant microbes. On the other hand, drug‐loaded NMs can easily pass cell membranes and act. Further, NMs can also enter host cells by phagocytosis and get released inside by endocytosis (Andrade et al., 2013).
2 Protection: When a direct drug goes inside the body, the chemical present inside could deteriorate drug molecules or microbes could develop resistance against them. Also, uptake of antibiotics in bacterial cells is very slow and less. NMs‐based carriers maintain the drug potency and protect it from resistance by microbes. The uptake rate of NMs can be manipulated as needed. For example, in gastrointestinal (GI) tract, dendrimers inhibit glycoprotein‐mediated efflux of drug (Liu, Tee, & Chiu, 2015).
3 Precision and security: Side effects of antibiotics and their targeted delivery is an important concern, which is difficult to achieve with conventional drug delivery. The targeted delivery of antibiotics to the infection site minimizes systemic side effects. NM‐based drug delivery helps in delivering the drug to targeted site and reduces the risk of side effects. With this, higher dose can be applied to the site of infection directly. This targeted drug delivery can be either active or passive. In active targeting, the NM's surface is modified to selectively recognize the signals on the target infected site, whereas, in passive targeting, permeation, and retention of drug‐loaded NMs is increased at the infection site. Drugs like vancomycin that have kidney toxicity but are good for Gram‐positive
