P. aeruginosa is a frequent pathogen causing hospital-acquired pneumonia and healthcare-associated pneumonia, but is a rare cause of Community Acquired Pneumonia (CAP) in an immune competent host [1] . However, P. aeruginosa CAP is prone to develop septicemia, is often rapidly progressive, and may be fatal [2] . Appropriate initial empiric antibiotic treatment is crucial in improving outcomes. We herein report a case of P. aeruginosa CAP with septicemia in a previously healthy woman who was successfully treated by appropriate antibiotic treatment.

A 53-year-old woman was brought by ambulance to the emergency department of our hospital due to fever and right shoulder pain for a few hours. She had no pertinent past medical history, family history, or recent history of overseas travel. However, she had been a smoker of 20 cigarettes per day for 30 years. On examination, her body temperature was 36.3˚C, blood pressure was 90/40 mmHg, pulse rate was 100 beats/min, respiratory rate was 22 breaths/min, and oxygen saturation was 97% at an oxygen support of 10 L/min by face mask. Auscultation of the chest revealed coarse crackles on the right upper lobe. Initial laboratory tests revealed white blood cell (WBC) count of 6740 cells/µL, and elevations of C-reactive protein (CRP; 2.94 mg/dl), procalcitonin (PCT; 37.73 ng/ml). And arterial blood gas revealed lactic acidosis (pH; 7.363, base excess; -0.5 mmol/L, lactate; 22.6 mg/dl). Chest radiograph showed right upper lobe consolidation and computed tomography (CT) scan showed air space consolidation in the posterior segment of the right upper lobe (Figure 1).

Figure 2 summarizes her clinical course in the hospital. She was diagnosed as septic shock due to lobar pneumonia. Treatment with bolus infusion of lactated Ringer’s solution and norepinephrine (0.3 µg/kg/min) was started at the emergency room. She did not require emergency intubation, but she was transferred to the intensive care unit. Sputum Gramstain on admission showed Gram-negative rods with phagocytosis. At the emergency room, empiric antibiotic treatment with intravenous meropenem (MEPM) at 3 g/day was immediately started. The results of blood and sputum cultures were positive for P. aeruginosa. An acid-fast stain of the sputum was negative. After the 4th day of hospitalization, cardiovascular stabilization was achieved and norepinephrine drip was discontinued. Subsequently, her clinical symptoms and the level of CRP improved and her body temperature normalized. On the 7thday of hospitalization, antibiotic treatment was de-escalated to intravenous ciprofloxacin (CPFX) at 0.6 g/day.

A repeat chest CT scan on the 11th hospital day showed reduction of air space consolidation, but anew cavity formation was observed (Figure 3). Although not detected in any of the specimens, co-infection with an anaerobe was suspected and intravenous clindamycin (CLDM) at 2.4 g/day was added and continued for 10 days. Follow-up chest radiograph and CT scan on the 24th hospital day revealed further improvement of the consolidation and resolution of the cavity (Figure 4). Intravenous CPFX was continued until discharge on the 28th hospital day and oral CPFX was prescribed for one more week after discharge. Six months after discharge, there were no clinical symptoms of recurrence and follow-up chest radiograph revealed resolution of the infiltrates (Figure 5).

P. aeruginosa is a frequent pathogen causing hospital-acquired pneumonia and healthcare-associated pneumonia [1] [3] . However, CAP secondary to P. aeruginosa is uncommon, with an incidence of only 0.6%, as shown in Sibila’s cohort study on 62,689 pneumonia patients aged ≥65 years old [4] . It is often rapidly progressive and may be fatal [4] , with a 30-day mortality rate of 17.4% [4] .

There have been only 12 case reports found in PubMed on P. aeruginosa CAP with septicemia in patients without comorbidities (Table 1) [3] [5] - [13] . The clinical characteristics of these cases were pleuritic chest pain and right upper lobe predilection; in

addition, mortality rate was high. The outcome was shown to be dependent on initiation of appropriate antibiotic therapy against P. aeruginosa within the first 48 hours of

admission [14] , therefore, timely appropriate empiric antibiotic treatment is crucial [14] . For this patient, we chose MEPM because the main target of treatment was septicemia due to lobar pneumonia and based on the presence of gram-negative rods on sputum. Gramstain is the most commonly performed microbiology test that can immediately identify the cause of a bacterial infection. Henderson et al. [10] suggest that P. aeruginosa CAP should be suspected in any patient who has gram-negative bacilli seen on sputum sample. Gram-negative bacteria include P. aeruginosa, which is reported to be one of the risk factors for death in CAP patients [15] .

P. aeruginosa CAP may sometimes progressive to cavitary pneumonia [2] [3] or lung abscess; for the latter, anaerobes are the major pathogens involved and may co-infect