This study is part of an ongoing prospective observational study on severe pneumonia in critically ill adult (>16 years of age) patients at Asan Medical Center, a 2,700-bed tertiary referral center in Seoul, South Korea. Since March 2010, we have prospectively identified all adult patients admitted to the 28-bed medical intensive care unit (ICU) who were clinically suspected of having severe pneumonia and monitored them until hospital discharge (7–10). We collected data on patient demographics; underlying diseases or conditions; category of pneumonia; initial clinical manifestations; laboratory, microbiologic, and radiologic findings; treatment; complications; and mortality rates. For this study, we investigated patients with severe C. striatum pneumonia who were admitted to the medical ICU during January 2014–December 2019. This study was approved by the institutional review board of Asan Medical Center (IRB no. 2010–0079), which waived the need for informed consent due to the observational nature of the study.

We defined and categorized pneumonia as previously stated (11–13). We defined severe pneumonia as the necessity for mechanical ventilation or having septic shock at ICU admission (12). We defined sepsis and septic shock according to Sepsis-3 criteria (14). We defined immunocompromised state as described previously (15).

We cultured sputum specimens on a 5% sheep blood plate and MacConkey agar (Synergy Innovation, http://www.synergyinno.com). When coryneform gram-positive bacilli were isolated, we identified and performed antimicrobial susceptibility testing for specimens that were urea positive or from the ICU (16). We quantitatively cultured bronchoalveolar lavage specimens on chocolate agar and identified and performed susceptibility testing when coryneform gram-positive bacilli exclusively grew at >10⁴ CFU/mL (16). Until August 2015, our facility used the triple sugar iron, motility, API Coryne (bioMérieux-Vitek, https://www.biomerieux.com) system to identify coryneform gram-positive rods. In September 2015, our facility began using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (Bruker Daltonik, https://www.bruker.com). We determined antimicrobial susceptibility profiles by ETEST (bioMérieux-Vitek) with MHF medium (Mueller-Hinton agar with 5% horse blood + 20 mg/L β-NAD; bioMérieux-Vitek). We used the Clinical and Laboratory Standards Institute M45 guideline for interpreting susceptibility test results (17) and defined multidrug resistance as resistance to >3 antimicrobial drug families.

We compared patient demographics, underlying diseases and conditions, and clinical and laboratory parameters between the C. striatum group and the MRSA group. We used χ² or Fisher exact test to compare categorical variables and Student t-test or Mann-Whitney U test to compare continuous variables. We analyzed changes in the proportions of pneumonic pathogens over time by using a χ² test for trend. We performed all analyses in SPSS Statistics 24.0 (IBM Corp., https://www.ibm.com) and considered p<0.05 statistically significant.

During the study period, we identified a total of 1,740 patients with severe pneumonia. Among them, 27 had severe C. striatum pneumonia and 103 had severe MRSA pneumonia (Table 1). The median patient age in the C. striatum group was 72.0 years and in the MRSA group was 71.0 years. Solid cancer, diabetes mellitus, and structural lung diseases were the most common underlying conditions in both groups. More patients in the C. striatum group were immunocompromised (51.9% vs. 26.2%; p = 0.01). Most (70.4%) patients in the C. striatum group had HAP, 14.8% had healthcare-associated pneumonia (HCAP), 11.1% had ventilator-associated pneumonia, and 3.7% had CAP. HAP was significantly more common in the C. striatum group than the MRSA group (70.4% vs. 42.7%; p = 0.01); HCAP was more common in the MRSA group (32.0% vs. 14.8%; p = 0.08), albeit without statistical significance.

We identified bacterial pathogens in 565 patients who had severe HAP during 2014–2019 (Table 2). The proportion of severe MRSA HAP decreased significantly, from 12.0% (24/200) in 2014–2015 to 2.7% (5/185) in 2018–2019 (p<0.01), whereas the proportion of severe C. striatum HAP increased significantly, from 1.0% (2/200) in 2014–2015 to 5.4% (10/185) in 2018–2019 (p<0.001). Among 75 HAP cases from which bacterial pathogens were identified in 2018–2019, C. striatum was responsible for 13.3% (10/75) of cases, which was the fourth most common pathogen, after Acinetobacter baumannii (30.7%), Klebsiella pneumoniae (21.3%), and Pseudomonas aeruginosa (14.7%).

We identified co-infection pathogens in 13 (48.1%) patients in the C. striatum group and 37 (35.9%) patients in the MRSA group (p = 0.25) (Table 3). Co-infection with other bacteria was more common in the MRSA group (25.2% vs. 7.4%; p = 0.045), whereas viral co-infection was more common in the C. striatum group (33.3% vs. 14.6%; p = 0.047). Fungal co-infection, which included 4 Aspergillus species and 1 Pneumocystis jirovecii, was only found in the C. striatum group (14.8% vs. 0%; p<0.01).

Dyspnea, fever, sputum, and cough were the most common signs and symptoms in both groups (Table 4). Fever tended to be less common in the C. striatum group (66.7% vs. 82.5%; p = 0.07). The proportion of patients with septic shock at the time of ICU admission was significantly higher in the MRSA group (67.0% vs. 44.4%; p = 0.03). However, the proportion of mechanical ventilation, acute physiology and chronic health evaluation (APACHE II) score, and sequential organ failure assessment (SOFA) score at the time of ICU admission were similar between the 2 groups. Peripheral leukocyte counts, platelet counts, and serum C-reactive protein levels also were similar between the 2 groups, but serum procalcitonin level was significantly higher in the MRSA group than the C. striatum group (median 0.3 ng/mL vs. 1.8 ng/mL; p<0.01).

On microscopic examination of Gram stain specimens, gram-positive rods were identified in 69.2% (18/26) of specimens. Among 27 cases, 10 were quantitative cultures and 17 were semiquantitative cultures. Bacterial counts were >10⁵ CFU/mL in 8/10 quantitative cultures. Of the 17 semiquantitative culture specimens, 12 specimens were grade many (4+), 1 was grade moderate (3+), 1 grade few (2+), and 3 were grade rare (1+) (Appendix Table). All 27 C. striatum isolates underwent antimicrobial susceptibility testing. All isolates were resistant to penicillin, ceftriaxone, erythromycin, and ciprofloxacin, and susceptible to vancomycin, and all isolates were multidrug resistant.

The mortality rates between the C. striatum and MRSA group showed no statistically significant differences: 30-day mortality (40.7% vs. 29.1%; p = 0.25), 60-day (48.1% vs. 42.7%; p = 0.61), and 90-day (59.3% vs. 50.5%; p = 0.42) (Table 5). In-hospital mortality rates were higher (70.4%) in the C. striatum group than in the MRSA group (52.4%), albeit without statistical significance (p = 0.09). Mortality rates were similar for C. striatum and MRSA in subgroups regardless of the patient’s immune status. We noted no statistically significant differences in the median length of ICU stay between the C. striatum and MRSA group, both 14 days (p = 0.33), nor in the length of hospital stay after ICU admission, 30 days for the C. striatum versus 29 days for the MRSA group (p = 0.48).