**2.2 Classification of MDR**

Many different definitions for multidrug-resistant (MDR), extensively drugresistant (XDR) and pandrug-resistant (PDR) bacteria are being used to characterize the different patterns of resistance. Was defined as acquired non-susceptibility to at least one agent in three or more antimicrobial categories, XDR was defined

*Antimicrobial Resistance in* Staphylococcus aureus *DOI: http://dx.doi.org/10.5772/intechopen.96888*

as non-susceptibility to at least one agent in all but two or fewer antimicrobial categories (i.e. bacterial isolates remain susceptible to only one or two categories) and PDR was defined as non-susceptibility to all agents in all antimicrobial categories. MDR is a frequently encountered phenomenon in *S. aureus* which can be broadly classified as primary MDR, secondary MDR and clinical MDR (**Figure 1**) [1, 4–6]. Survival of the microorganisms has gradually developed extensive resistance against varied antimicrobial drugs. Also, there is a failure of many clinical trials which are not always due to the occurrence of resistance but all due to poor bioavailability of drugs, very poor immune system, excessive-high metabolism of drugs.

## **2.3 Mechanism of multi-drug resistance (MDR)**

Before studying the resistance of *S. aureus*, it is very important to take a look upon all the possible biochemical mechanisms of resistance that the microbes show. Microorganisms have the ability to employ several ways to develop multi drug resistance [2]. The resistance of all these antimicrobial agents against the pathogenic microorganisms are taking a rise in some patients due to prolong use of the antimicrobial agents by these patients. Below, is the schematic diagram of all methods of resistance mechanism (**Figure 2**). Microorganisms for the need of the survival undergoes mutational changes either in their chromosomal DNA/RNA which confers the resistance. One of the famous examples is the resistance against methicillin in *Staphylococcus aureus*. The cell wall of the microbes plays a vital role as a barrier and helps in their survival but due to alteration in the chromosomal DNA or genetic mutations the compositions of the cell wall or the plasma membrane changes and this in turn encourage the resistance phenomenon.

Drug Efflux Pumps are one of the major ways for the MDR mechanism. ABC transporters (ATP Binding Cassette) are membrane proteins which are commonly defined as drug efflux pumps that specifically helps in the transport of the drugs in the cell. The P-glycoprotein or multi-resistant protein (MRP) damages

**Figure 1.** *Classification of MDR.*

*Insights Into Drug Resistance in* Staphylococcus aureus

**Figure 2.** *Multi drug resistance mechanism.*

the permeability and influences the ATP-dependent efflux of the drugs which is responsible for decreasing the intracellular concentrations [7–9].
