Anti-Bacterial Agents/pharmacology; Cross Infection/diagnosis; Gram-Negative Bacterial Infections/diagnosis; Pneumonia, Bacterial/diagnosis; Staphylococcal Infections/diagnosis; Acinetobacter baumannii/drug effects; Acinetobacter baumannii/growth & development; Bacterial Typing Techniques; Bronchoalveolar Lavage Fluid/microbiology; Cross Infection/epidemiology; Cross Infection/microbiology; Drug Resistance, Multiple, Bacterial; Gram-Negative Bacterial Infections/epidemiology; Gram-Negative Bacterial Infections/microbiology; Humans; Intubation, Intratracheal; Klebsiella pneumoniae/drug effects; Klebsiella pneumoniae/growth & development; Microbial Sensitivity Tests; Pneumonia, Bacterial/epidemiology; Pneumonia, Bacterial/microbiology; Poland/epidemiology; Pseudomonas aeruginosa/drug effects; Pseudomonas aeruginosa/growth & development; Retrospective Studies; Severity of Illness Index; Staphylococcal Infections/epidemiology; Staphylococcal Infections/microbiology; Staphylococcus aureus/drug effects; Staphylococcus aureus/growth & development