**3. Etiology of VAP**

VAP is usually caused by bacteria, whereas fungi and viruses are rarely involved [3, 6]. Generally, early-onset VAP is caused by pathogens more susceptible to antibiotics, including *Streptococcus pneumoniae*, *Haemophilus influenzae*, and methicillin-susceptible *Staphylococcus*  *aureus*. On the other hand, late-onset VAP is usually caused by antibiotic-resistant bacteria, such as *Pseudomonas aeruginosa*, *Acinetobacter* spp., methicillin-resistant *S. aureus* (MRSA), and extended-spectrum β-lactamase producing Enterobacteriaceae, such as *Klebsiella pneumoniae*, *Escherichia coli*, *Enterobacter* spp., among others [5–7]. However, some studies have reported that both susceptible and antibiotic-resistant microorganisms can have similar frequencies in early and late-onset VAP [22, 23].

magnesium, silver, titanium, or zinc) have been applied to medical devices such as endotracheal tubes [16, 17]. Other approach consists in the use of polymers that exhibit antimicrobial activity by themselves, with positively charged active groups (biguanide, cyclic *N*-halamine, quaternary ammonium, pyridinium or phosphonium salts, and polyionenes) or other polymers, such as synthetic poly(phenylene ethynylenes), polynorbornenes, and polymethacrylates that display similar antimicrobial activities of human peptides [17]. Both types of devices display advantages and disadvantages, but in the near future one expects to have nontoxic and biocompatible products available, which display broad-spectrum antibiofilm activities

The aspiration technique of endotracheal secretions also plays an important role as a risk factor for the establishment of VAP. The open method where a sterile aspiration probe is introduced has disadvantages such as loss of oxygenation, since the patient is temporarily disconnected from the ventilator and the system is opened with exposure of the patient, and

Tracheostomy is an indicated procedure after 2 weeks of translaryngeal intubation of critically ill patients. Apparently, early tracheostomy may be associated with a lower incidence of pneumonia when compared to the late procedure or nonprocedure [19]. Frequent reintubations are also associated with VAP because of the risk of aspiration of gastric contents through the use of the nasogastric tube, subglottic dysfunction, and lowering of the level of consciousness [5].

The VAP prevention guidelines recommend the placement of the patient in the bed between 30 and 45° semi-reclined [20]. The supine position to which the patient is subjected may lead to lesions such as atelectasis in the dorsal lung region, barotrauma in the ventral lung region [5]. Experiments performed on rats proved the advantage of lateral decubitus in improving gas exchange, reducing gastroesophageal reflux, and avoiding pulmonary infection by gastric aspiration due to gravity [21]. Recently, the semi-decubitus position (30–60°) was shown to reduce the risk of VAP compared to supine positioning (0–10°) [20].

Nasal feeding by nasogastric tube increases gastric secretions and pH, leading to colonization by Gram-negative bacilli. Aspiration of this gastric content increases the risk of VAP. The use of sedative medications used in therapeutic procedures can cause prolonged relaxation of the

In addition, in the neonatal intensive care unit (NICU), some risk factors are associated with characteristics peculiar to this age group, including: length of stay in the NICU, enteral and parenteral feeding, blood transfusion, low birth weight, prematurity, and bronchopulmonary

VAP is usually caused by bacteria, whereas fungi and viruses are rarely involved [3, 6]. Generally, early-onset VAP is caused by pathogens more susceptible to antibiotics, including *Streptococcus pneumoniae*, *Haemophilus influenzae*, and methicillin-susceptible *Staphylococcus* 

for the prevention of biofilm formation on endotracheal tubes [18].

the maximum duration of use of each circuit is not known [5].

muscles, increasing the risk of aspiration [5].

dysplasia [10, 13].

**3. Etiology of VAP**

108 Contemporary Topics of Pneumonia

In many cases, VAP can be caused by more than one pathogen (polymicrobial infection). This fact can be ignored sometimes when isolates are reported only as a percentage of the total number of isolates. In a recent study, performed in medical and surgical ICUs of a hospital in Spain, of 147 VAP patients, 32 (21%) had more than one pathogen associated. Interestingly, the clinical outcomes were not influenced by the polymicrobial etiology, when appropriate antibiotic therapy was administered [24].

The etiology of VAP varies in different countries and even between ICUs of the same city, distinct patients groups (like the ARDS patients, immunocompromised, and so on), or settings of the same hospital [25]. However, among Gram-negative bacteria, a high frequency is generally reported for *P. aeruginosa*, *Acinetobacter* spp., and Enterobacteriaceae members. Among Gram-positive isolates, *S. aureus* and *Streptococcus* spp. are considered as important pathogens [3, 5, 14, 25–27]. **Table 1** shows a list of the most frequently and also some uncommon microorganisms detected in VAP patients.



in the ICU increase the risk of *A. baumannii* bacteremia: mechanical ventilation, central venous

Microbiology of Ventilator-Associated Pneumonia http://dx.doi.org/10.5772/intechopen.69430 111

The virulence of *A. baumannii* can be attributed to several factors: capacity to form biofilms; its ability to adhere, to colonize, and invade human epithelial cells; its antibiotic resistance mechanisms; and its ability to acquire foreign genetic material to promote its own survival under antibiotic and host selection pressures. Approximately 30% of *Acinetobacter* strains also produce an exopolysaccharide, which is a major virulence factor protecting bacteria from host

*K. pneumoniae* is generally considered an opportunistic pathogen that affects mainly immunocompromised individuals. It can be found normally in the intestine, oral cavity, and skin, as well as in hospital settings and medical devices [33]. *K. pneumoniae* is able to form biofilms in catheters and endotracheal tubes, which represent major sources of infection in patients with

Infections by *K. pneumoniae* that involves biofilm formation tend to be persistent or chronic, since the biofilm protects the pathogen of the host immune response and also of the antibiotic action [35]. An additional risk factor for chronic infections caused by nosocomial strains includes resistance to multiple antibiotics, making difficult for the choice of suitable antibiot-

*K. pneumoniae* has about 78 capsular serotypes (or K antigens) [37]. Some of them present an increase in the production of the capsule and present very viscous colonies, which are called hypermucoviscous. Such isolates have also been considered to be hypervirulent because cap-

A practical way to check if an isolate is hypermucoviscous is by using the string test. If there is the formation of a viscous chain greater than 5 mm after touching bacterial growth on agar and try to stretch it with a platinum loop. The degree of mucoviscosity correlates with the establishment of invasive infections. Hypervirulent *K. pneumoniae* is highly invasive and can affect previously healthy persons, causing fatal infections, including severe pneumonia among them [38]. The bacterium with the phenotype of hypermucoviscosity is capable of

The capsule consists of polysaccharides and is generally constituted by repeating units of three to six sugars [38]. The main functions assigned to it include: (1) protection of *K. pneumoniae* against opsonization and phagocytosis [40]; (2) interference with dendritic cells (DCs) maturation and, consequently, in the production of pro-Th1 cytokines mediated by DCs [41]; (3) anti-inflammatory effect by the inhibition of IL-8 expression [42–44]; and (4) reduction of the amount of antimicrobial peptides reaching the bacterial surface, thus, promoting resis-

In addition to capsule, iron acquisition is a virulence property that also contributes to the persistence of the microorganism in the patient body and, consequently, to VAP, since iron is essential for bacterial growth. Pathogenic members of the Enterobacteriaceae family usually display

sule is the most important virulence factor of *K. pneumoniae* [38].

spreading from one organ to other organs (metastatic spread) [39].

or urinary catheterization, and nasogastric tube use [31, 32].

defenses [28, 31].

**4.2.** *Klebsiella pneumoniae*

invasive devices [34].

ics for the treatment [36].

tance to them [45].

**Table 1.** Frequency of etiologic agents of VAP.
