Bacterial community-acquired respiratory infections are usually sustained by strains highly responsive to antibiotic therapy. Thus, the clinical approach is based on an empirical treatment and does not require the isolation of the causative pathogen and the determination of the bacterial susceptibility to antibiotics. On the other hand, Gram-negative bacteria, most commonly multidrug resistant, frequently affect immunocompromised and nosocomial patients and their identification in cultures is absolutely necessary for proper antibacterial treatment. To this aim, two conventional methods are used, i.e. the blood culture, which is positive only in 20% of pneumonia cases, and the sputum culture, which is not invasive but easily contaminated by oropharyngeal flora. Consequently, invasive techniques for sampling the pathologic specimen, such as the BAL and the PSB, performed with the help of fiberoptic bronchoscope, are needed. The diagnostic power and the limits of both these techniques are analyzed. Moreover, the opportunity to obtain quantitative cultures, which may discriminate between contamination and infection is considered.