**5. Antimicrobial susceptibility and management of patients**

Choosing an initial antibiotic for suspected VAP is a difficult task. A scheme of empiric antibiotic therapy must take into account that *S. aureus*, *P. aeruginosa*, *Acinetobacter* spp., and Enterobacteriaceae members together represent more than 80% of VAP cases worldwide and several strains are defined as MDR pathogens [26]. To provide suitable antibiotic exposure regarding the possibility of infection by MDR pathogens, the empiric therapy should contain multiple agents with broader spectrum of activity [25].

However, antibiotic choices should be based on local prevalence and the antimicrobial susceptibility profile of the usual pathogens, since data from guidelines or other hospitals can be ineffective [61]. For empiric MRSA coverage, vancomycin or linezolid are strongly recommended. On the other hand, if it is indicated as MSSA coverage, the following antibiotics should be used: piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem. Suspected etiology for MRSA or MSSA should be based on the presence of risk factors [61].

It has been reported that more than 50% of MRSA are also resistant to macrolides, lincosamides, fluoroquinolones, and aminoglycosides. This high level of resistance not only impedes successful therapy but also allows the microorganism to persist in the hospital, expanding its reservoir. So, vancomycin is the first-line treatment to VAP patients caused by MRSA. Nevertheless, some studies have described *S. aureus* strains with decreased susceptibility to vancomycin (vancomycin intermediate-resistant *S. aureus*, VISA). The acquired-resistance of MRSA to vancomycin is related to acquire mutations that appear in MRSA during vancomycin therapy [62, 63]. More recently, studies describing MRSA strains with high-level vancomycin resistance (vancomycin-resistant *S. aureus*, VRSA) were described. The mechanism of resistance is associated to the presence of transposon Tn1546, acquired from vancomycinresistant *Enterococcus faecalis*, which is known to alter cell wall structure and metabolism, but the resistance mechanisms in VISA and VRSA isolates are less well defined [62].

have been considered the agents of choice for infections caused by susceptible pathogens, but the rapid increase in carbapenem resistance rates has complicated this issue. Other mechanisms include: changes in OMPs, penicillin-binding proteins, and efflux pumps; resistance to aminoglycosides, mediated by aminoglycoside phosphotransferases, acetyltransferases, and adenyltransferases; resistance to quinolones, polymyxins, tetracyclines, among others [28].

Microbiology of Ventilator-Associated Pneumonia http://dx.doi.org/10.5772/intechopen.69430 115

A recent cohort study of bacteremia associated with pneumonia found that inappropriate initial antibiotic treatment seems to be the most important independent determinant of mortality and is the only identified mortality predictor amenable to intervention [67]. These Gramnegative bacteria are responsible for increasing numbers of infections encountered in hospitals, particularly among immunocompromised patients, and community-acquired infections are also increasing in prevalence. Furthermore, the impact of *P. aeruginosa* and *A. baumannii*

The diagnosis of VAP is usually based on clinical, radiographic, and microbiological criteria. Microbiological diagnosis is important in the management of VAP, since early diagnosis can influence clinical outcomes. The usual methods for microbiological diagnosis are based on quantitative or semiquantitative culture, but the results can take 48 h or more to be available. The Gram stain method has been used as screening of infection and to guide initial antibiotic therapy. However, utility of microscopy examination of respiratory secretions is still

Molecular methods can also be used to obtain results more quickly and initiate rational antibiotic therapy of patients with VAP. Many method formats are available for the detection of target genes for microbial identification and also for the detection of antimicrobial resistance

Semiquantitative culture of endotracheal aspirates (ETA) is the recommended microbiological procedure to diagnose VAP, since it is more sensitive and can be done more rapidly. Other biological specimens have been used, including the ones obtained by invasive sampling, such as: bronchoalveolar lavage (BAL), blind bronchial sampling (mini-BAL), and protected specimen brush (PSB). Blood cultures should also be performed for all patients with suspected VAP. In all cases, samples should be obtained before the patients initiate antibiotic

The main problem with the semiquantitative culture of ETA is that its high sensitivity promotes the unnecessary prescription of antibiotics to some patients. In the case of quantitative cultures of lower respiratory tract secretions, the following threshold cut-offs are usually

–106

may lead to false-negative results and worse clinical outcomes in some patients [61].

, BAL 104

, and PSB 103

CFU/mL. This strategy

resistance on health systems is a major concern in hospitals worldwide.

**6. Laboratory diagnosis**

controversial.

genes.

**6.1. Culture**

therapy [61].

applied to diagnosis true infection: ETA 105

Antibiotic options for Gram-negative coverage are more varied and must contain two antipseudomonal antibiotics from different classes in the presence of risk factors for MDR pathogens for the initial treatment of suspected VAP. If the patient does not present risk factors for MDR pathogens, only one anti-pseudomonas drug should be prescribed [61].

The frequency of infections caused by *P. aeruginosa* has increased in combination with the morbidity and mortality among hospitalized patients, all of which are exacerbated by antimicrobial resistance. Studies have demonstrated that resistance to carbapenems, aminoglycosides, and fluoroquinolones has increased gradually over the past few years, as well as episodes caused by MDR strains. Many *P. aeruginosa* isolates display an intrinsic reduced susceptibility to several antibacterial agents, as well as a tendency to develop resistance during therapy, especially in carbapenem-resistant strains. The most common mechanism of imipenem resistance in *P. aeruginosa* is a combination of chromosomal AmpC production and porin alterations. It also produces extended-spectrum β-lactamases (ESBLs) and can harbor other antibiotic resistance enzymes such as *K. pneumoniae* carbapenemases (KPC) and imipenem metallo-β-lactamases. β-Lactamase production, especially ESBLs, remains the main factor to acquired β-lactam resistance [52, 64, 65].

*K. pneumoniae* may present two major types of antibiotic resistance: (1) expression of ESBLs, which make them resistant to cephalosporins and monobactams and (2) the expression of carbapenemases that make *K. pneumoniae* resistant to almost all available β-lactams, including carbapenems. The first reported of carbapenemase by *K. pneumoniae* was in the USA, in 1996, which was designated KPC. Currently four classes of carbapenemases (classes A–D) have already been described and KPCs are classified into class A. To date, 16 KPC class A variants have already been identified. In addition to KPCs, *K. pneumoniae* strains may carry other forms of carbapenemases, such as class B metallo-β-lactamases (such as New Delhi's metalloβ-lactamase NDM-1 enzymes) and OXA class. In addition to β-lactamases, mutations in outer membrane proteins (OMPs) may also make the bacterium more resistant to β-lactams, particularly if it was in combination with the expression of a carbapenemase [66].

*A. baumannii* is also considered an emerging cause of nosocomial outbreaks, especially by MDR strains in ICUs. The most significant mechanism of carbapenem resistance in *A. baumannii* is the production of carbapenemases, which can be either intrinsic or acquired. Carbapenems have been considered the agents of choice for infections caused by susceptible pathogens, but the rapid increase in carbapenem resistance rates has complicated this issue. Other mechanisms include: changes in OMPs, penicillin-binding proteins, and efflux pumps; resistance to aminoglycosides, mediated by aminoglycoside phosphotransferases, acetyltransferases, and adenyltransferases; resistance to quinolones, polymyxins, tetracyclines, among others [28].

A recent cohort study of bacteremia associated with pneumonia found that inappropriate initial antibiotic treatment seems to be the most important independent determinant of mortality and is the only identified mortality predictor amenable to intervention [67]. These Gramnegative bacteria are responsible for increasing numbers of infections encountered in hospitals, particularly among immunocompromised patients, and community-acquired infections are also increasing in prevalence. Furthermore, the impact of *P. aeruginosa* and *A. baumannii* resistance on health systems is a major concern in hospitals worldwide.
