Objective    To discuss the etiological changes in old patients with severe community acquired pneumonia and re-infection caused by ventilator-associated pneumonia.     Methods    Thirty-four old patients with severe community acquired pneumonia were included. The sputum from lower respiratory tract was collected and quantitatively cultured. The antibiotic susceptibility was detected with disk diffusion method

and extended spectrum β lactamases (ESBLs) were measured by E test.     Results    On admission

gram-negative bacilli (GNB) was the primary pathogen. When ventilator-associated pneumonia（VAP） occurred

GNB increased from 75% to 87%. The detection rate of non-fermentative bacilli increased from 24% to 37%

and the rate of ESBLs also increased from 32% to 47%. The incidence of mixed infection and secondary infection respectively increased from 12% to 59% and 9% to 35%. On admission

GNB was sensitive to imipenem. The drug resistance rate of GNB to antibiotics (lactamase inhibitor

ceftazidime

amikacin and latamoxef) was 20% to 30%

and that to the other 3th cephalosporin

piperacillin and ciprofloxacin was 40% to 50%; When VAP occurred

the drug resistance isolates to imipenem appeared. The trend of the resistance rate was similar with admission but the rate increased by 12% to 20%. Except for the isolates to imipenem

antibiotics with lactamase inhibitor and latamoxef

the resistance rate of ESBLs producing strains was 30% to 50%

higher than that of non-ESBLs-producing strains. The mortality was 53%.     Conclusion    Severe CAP and VAP in old patients were characterized by gram-negative bacilli and high detection rate of ESBLs. The detection rate of non-fermentative bacilli

Candida albicans and ESBLs increased greatly when VAP occurred. The above characteristics of pathogens should be fully considered during anti-infection and antibiotics treatment in old patients with severe community-acquired pneumonia complicated by ventilator-associated pneumonia.