**3. Classification of bacterial pathogens**

The classification of infectious agents inregards to their infective lifestyles in the host and corresponding pathogenic indications must be precisely described [3]. In the life of a microbe, the intracellularity and extracellularity are unclear designations unless obviously related to the situation where it is living. For a microbial pathogen, what matters is whether intra-or extracellularity is in the basis of the *in vivo* life and in relationship with pathogenicity. Classically, infectious agents are indicated as extracellular and intracellular pathogens [4-6].

#### **3.1. Extracellular pathogens**

*Staphylococcus aureus*, *Streptococcus pyogenes*, *Pseudomonas aeruginosa*, *Escherichia coli* are typical examples of bacteria which have been considered extracellular pathogens, and lesion infec‐ tions, osteomyelitis, scarlet fever, specified forms of pneumonia, urinary tract infections are examples of infections caused by these pathogens [7]. To produce disease, extracellular pathogens utilize any portal of entry provided a satisfactory fluid medium be recognized at the site of lesion [4]. Extracellular pathogens utilize virulence mechanisms to avoid the antimicrobial capabilities of humoral immunity and phagocytosis thus advancing extracellular reproduction [8], in contrast with intracellular pathogens that promote the entry in to host cells containing macrophages and non-professional phagocytes such as epithelial cells [9].

#### **3.2. Intracellular pathogens**

and minimizes the undesirable side effects of drugs. The major goals in designing nanoparti‐ cles as delivery systems are to control particle size, surface properties and release of pharma‐ cologically active agents in order to achieve the site-specific action at the therapeutically optimal rate and dose regimen. This chapter focuses on nanoparticle-based drug delivery systems and clinical applications to treat a variety of bacterial infectious diseases and their

Infectious disease is a clinically obvious disorder resulting from the presence of a pathogenic agent which can either be a virus, bacterium, fungus or parasite. These diseases are also called communicable diseases due to their ability to get transferred from one person to another (malaria, tuberculosis) and also sometimes from one species to another (flu, influenza). Infectious diseases can be vastly classified as: 1) known diseases which are insistently there (e.g., dengue, malaria, tuberculosis); 2) new, previously unknown diseases (e.g., severe acute respiratory syndrome); and 3) diseases which threaten to enhance in the near future (e.g., avian influenza). These diseases own a great risk as more than half of the deaths happening world‐ wide can be attributed to these diseases, particularly in developing countries [1]. Parasitism is based on the benefits acquired by a pathogenic bacterium invading the host and causing an infection. A bacterial infection is the process occurring when the microbe manifests its pathogenicity, and thus its capacity of inducing disease, by invading and causing a damage (locally or systemically) of the host organism. Consequently, the infectious disease could result in an acute infection, with a short and severe course, or a chronic, low-grade and long lasting

The classification of infectious agents inregards to their infective lifestyles in the host and corresponding pathogenic indications must be precisely described [3]. In the life of a microbe, the intracellularity and extracellularity are unclear designations unless obviously related to the situation where it is living. For a microbial pathogen, what matters is whether intra-or extracellularity is in the basis of the *in vivo* life and in relationship with pathogenicity. Classically, infectious agents are indicated as extracellular and intracellular pathogens [4-6].

*Staphylococcus aureus*, *Streptococcus pyogenes*, *Pseudomonas aeruginosa*, *Escherichia coli* are typical examples of bacteria which have been considered extracellular pathogens, and lesion infec‐ tions, osteomyelitis, scarlet fever, specified forms of pneumonia, urinary tract infections are examples of infections caused by these pathogens [7]. To produce disease, extracellular

potential applications in the field of medicine and biology.

**3. Classification of bacterial pathogens**

**2. Types of infections**

156 Application of Nanotechnology in Drug Delivery

infection [2].

**3.1. Extracellular pathogens**

Classical examples of intracellular pathogens are *Brucella abortus*, *Listeria monocytogenes*, *Mycobacterium tuberculosis*, *Salmonella enterica*, and typical infectious diseases caused by them include brucellosis, listeriosis, tuberculosis, and salmonellosis [10]. Intracellular pathogenic bacteria have the ability to establish a relationship in the sensitive host which includes a stage of intracellular reproduction [11]. To establish an infection, these pathogens have to make contact with the appropriated type of host cell that provides suitable intracellular conditions for growth [4]. Bacteria such as *Mycobacterium, Legionella, Brucella* or *Listeria* have extended the ability to resist and replicate inside various mammalian cells including the aggressive phagocytic cells, which establish the first-line defense against invading pathogens [12].
