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Eur Respir J 2006; 27:795-800
Copyright ©ERS Journals Ltd 2006

Mixed community-acquired pneumonia in hospitalised patients

A. de Roux1, S. Ewig2, E. García3, M. A. Marcos4, J. Mensa2, H. Lode5 and A. Torres1

1 Servei de Pneumologia I Allergia respiratoria, Institut Clínic del Torax, Institut d'Investigacions Biomèdiques August Pi i Sunyer, Facultad de Medicina, Universitat de Barcelona, and 3 Servei de Malalties Infeccioses, and 4 Servei de Microbiologia, Hospital Clínic i Provincial de Barcelona, Barcelona, Spain. 2 Augusta Kranken-Anstalt Bochum, Klinik für Pneumologie, Beatmungsmedizin und Infektiologie, Bochum, and 5 Lungenklinik Heckeshorn, Abteilung für Pneumologie I, Berlin, Germany.

CORRESPONDENCE: A. Torres, Pulmonology Dept, Hospital Clinic, Villarroel 170, Barcelona, E-08036, Spain. Fax: 34 932275454. E-mail: atorres{at}ub.edu

Keywords: Community-acquired pneumonia, hospitalisation, mixed pneumonia

Received: May 18, 2005
Accepted November 11, 2005


   ABSTRACT
TOP
 ABSTRACT
MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
The role of mixed community-acquired pneumonia (CAP) is controversial. The aim of the present study was to determine the incidence, principal microbial patterns, clinical predictors and course of mixed CAP.

The current study included 1,511 consecutive hospitalised patients with CAP. Of these, 610 (40%) patients had an established aetiology. One pathogen was demonstrated in 528 patients and 82 (13%) patients had mixed pneumonia. Cases including CAP, by a pyogenic bacteria and a complete paired serology for "atypicals", revealed that 82 (13%) patients had definite single pyogenic pneumonia and 28 patients (5%) had mixed pyogenic pneumonia.

In patients with mixed CAP, Streptococcus pneumoniae was the most prevalent microorganism (44 out of 82; 54%). The most frequent combination was S. pneumoniae with Haemophilus influenzae (17 out of 82; 21%). Influenza virus A and S. pneumoniae (five out of 28; 18%) was the most frequent association in the mixed pyogenic pneumonia group. No clinical predictors for mixed pneumonias could be identified. Patients with mixed pyogenic pneumonia more frequently developed shock when compared with patients with single pyogenic pneumonia (18 versus 4%).

In conclusion, mixed pneumonia occurs in >10% of cases with community-acquired pneumonia requiring hospitalisation.

The aetiology of community-acquired pneumonia (CAP) has been under constant study during recent years. It has been estimated that 1.62–3.79 cases per 1,000 inhabitants occur each year in Spain 1, 2. Recent studies have revealed that more than one causative microorganism could be detected in a considerable amount of cases. However, the reported rates for mixed aetiologies differ in the range of 2.5–38% 3, 4. The relevance of these mixed aetiologies in terms of outcome and selection of initial empiric antimicrobial treatment remains largely unsettled. In theory, the presence of multiple pathogens may represent a serious challenge for any potential future approach tailored towards a more pathogen-directed narrow spectrum antimicrobial treatment, e.g. using bedside antigen testing or molecular techniques. There is increasing evidence suggesting that in patients with severe pneumococcal pneumonia, monotherapy with ß-lactams might be suboptimal 5–7. This phenomenon may hint at a currently unrecognised bearing of mixed aetiologies on outcome.

Therefore, the rate of mixed infections and types, as well as combinations, of microorganisms causing mixed aetiological CAP in the cohort of patients was studied. The current authors also sought to determine clinical predictors of mixed CAP. Although many series on CAP specifically addressing the prevalence of selected microorganism are available, few data are available about the microbial aetiology and clinical characteristics of mixed CAP. Many types of aetiological combinations are possible, but the main aim of the study was to identify mixed CAP cases including a "classic" pneumonia causing microorganism, such as Streptococcus pneumoniae. In the present study those microorganisms were defined as "pyogenic bacteria". The current authors think this approach is viable as CAP caused by "pyogenic" bacteria represents the predominant entity in all CAP series. In view of the intriguing data on adverse outcomes using ß-lactam monotherapy in CAP, particularly in severe bacteremic pneumococcal pneumonia, special attention was given to the comparison of single pyogenic and mixed pneumonias (MPs).


   MATERIAL AND METHODS
TOP
 ABSTRACT
 MATERIAL AND METHODS
RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Patients and study design
The present cohort included 1,511 consecutive patients hospitalised with CAP at the authors' 1,000-bed teaching hospital (Hospital Clinic of Barcelona, Barcelona, Spain) between October 1996 and November 2001. Clinical, microbiological, radiographical and laboratory data were recorded as described in detail elsewhere 8–10. The empirical antibiotic treatment was given according to hospital clinic guidelines. Specifically, for hospitalised patients, it includes the association of a ß-lactam plus a macrolide or a respiratory quinolone in monotherapy. For patients admitted to the intensive care unit (ICU) the guidelines recommend the association of a ß-lactam plus a macrolide or with a respiratory quinolone. The ethical committee of the authors' hospital approved the study.

Definition of CAP and exclusion criteria
CAP was defined in the presence of a new infiltrate on the chest radiographs, together with suggestive symptoms of a lower respiratory tract infection and no alternative diagnosis during follow-up. Exclusion criteria were: 1) severe immunosuppression (e.g. HIV infection); 2) immunosuppressive therapy, defined as daily doses >20 mg prednisolone equivalent for >2 weeks; or 3) any dose of an immunosuppressive combination regimen, including azathioprine, cyclosporin and/or cyclophosphamide, or neutropenia <1,000 neutrophils·µL–1. Cases with lung abscess (cavity on the chest radiograph), aspiration pneumonia and tuberculosis were also excluded.

Microbiological evaluation
Samples considered valid for microbiological evaluation included: 1) sputum (considering >25 neutrophils and <10 epithelial cells as a valid specimen, evaluated by Gram-stain and culture); 2) blood culture; 3) pleural fluid culture; 4) urine antigen-detection for S. pneumoniae and Legionella pneumophila and quantitative culture for tracheobronchial aspirates (threshold 105 cfu·mL–1); 5) protected specimen brush (threshold 103 cfu·mL–1); and 6) bronchoalvelolar lavage fluid (threshold 104 cfu·mL–1). Diagnosis of the following microorganisms was performed by means of paired serology at admission and within the third and sixth week thereafter. 1) Respiratory viruses: influenza virus (A and B), parainfluenza virus (1, 2 and 3), respiratory syncitial virus (RSA) and adenovirus (diagnosis in the case of a four-fold increase of immunoglobulin (Ig)G-titres). 2) "Atypical" microorganisms including: Chlamydia pneumoniae, Mycoplasma pneumoniae (diagnosis in the case of four-fold increase of IgG-titres and/or initial single IgM-titre >1:32) as well as L. pneumophila serogroup 1 and Coxiella burnetii (diagnosis in the case of four-fold increase in IgG-titres). Urine samples were obtained for detection of S. pneumoniae antigen (Binax NOW Streptococcus pneumoniae urinary antigen Test; Binax Inc., Portland, ME, USA) and L. pneumophila serogroup 1 antigen (BiotestR Legionella Urine Antigen EIA; Biotest, Dreieich, Germany).

Data collection
The following parameters were collected. 1) Baseline characteristics: age, sex, nursing home residency. 2) Clinical symptoms: temperature, chills, cough, expectoration, chest pain, dyspnoea, crackles on auscultation, "flu-like" symptoms (headache, myalgia, fatigue), fever before admission. 3) Radiographical patterns as type of condensation (alveolar or interstitial), presence of cavitation, atelectasias or pleural effusion. 4) Presence and number of comorbid illnesses, i.e. history of chronic heart failure, structural pulmonary disease, hepatic disease, renal neurological disease, diabetes mellitus. 5) Toxic habits, such as alcohol abuse, defined as a daily consumption >80 g, tobacco abuse defined as current smoking habit of >10 cigarettes a day and/or >10 pack-yrs or i.v. drug abuse. 6) Prior ambulatory antimicrobial treatment (defined as any oral antimicrobial treatment administered during the evolution of symptoms attributable to the current pneumonia episode). 7) Parameters recorded at admission and related to disease severity: mental confusion, systolic and diastolic blood pressure, respiratory rate, leukocyte count, C-reactive protein, creatinine, blood urea nitrogen, sodium, blood glucose, and arterial oxygen tension (Pa,O2). The pneumonia severity index was calculated as described by Fine et al. 11.

Definition of groups for comparison
MP was defined as pneumonia due to more than one pathogen. Thus, the whole population of patients with mixed aetiologies represents the MP group. Since not all patients were evaluated by all diagnostic methods, the following minimal set of diagnostic tools was required for inclusion in the analysis: 1) at least one valid lower respiratory tract sample; 2) blood cultures; and 3) paired serology. In this group of MP patients, a subgroup was defined and called mixed pyogenic pneumonia (MPP). This is because of the presence of at least one pyogenic pathogen plus a definitive evidence of a second microbial aetiology.

Single pyogenic pneumonia (SPP) was defined as: 1) isolation of a pyogenic microorganism such as, S. pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae, Proteus vulgaris, Pseudomonas aeruginosa, Acinetobacter baumanii, Stenotrophomonas maltophilia; or 2) absence of another microorganism in culture or serology.

Statistical analysis
Results are expressed as mean±SD. Categorical variables were compared using the Chi-squared test or Fishers exact test when appropriate. Quantitative continuous variables were compared using unpaired t-tests or the Mann–Whitney U-test when appropriate. The level of significance (two-sided p-value) was set at <0.05.


   RESULTS
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Patient population
In total, 1,511 patients with CAP were admitted to the emergency dept between October 1996 and November 2001. They were prospectively evaluated and followed up. The mean±SD (range) age was 67±18 (17–101) yrs. Of these, 995 (66%) were male and 516 (34%) female.

Aetiology of CAP could be established in 610 out of 1,511 (40%) patients. Of these, 528 (87%) patients only had one aetiology. Accordingly, 82 patients had more than one microorganism (82 out of 610, 13%; MP group). Two microorganisms were isolated in 79 out of 82 (96%) patients and three out of 82 (4%) patients (fig. 1Go). The subgroup of MPP was formed by 28 (5%) cases.


Figure 1
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  		Fig. 1— Overview of patient populations studied with community-acquired pneumonia (CAP). Of the 130 cases with "atypical CAP" 110 were diagnosed by paired serology and 20 Legionella spp. cases were diagnosed by urinary antigen testing. blk14: groups included in the analysis. The total number of patients with aetiological diagnosis was as follows: 130+316+82+82 = 610. #: see table 1Go; : see table 2Go.

 
Out of the 610 patients, 110 (5.5%) had at least one pyogenic microorganism and a complete diagnostic workup, including paired serology for atypical and viral pathogens. Of these, 82 patients had definite SPP.

Mixed pneumonia
At least one pyogenic pathogen was involved in 58 out of 82 (70%) patients with MP. S. pneumoniae was the most prevalent microorganism involved in mixed infections (44 out of 82, 54%). Of all pneumococcal infections, 44 out of 272 (16%) were mixed. When only patients with paired serology were taken into account, this number increased to 25%. The combination of S. pneumoniae and H. influenzae (17 out of 82, 21%) was the most frequent. In 43% (35 out of 82) of all MPs, an "atypical" microorganism was found, with C. pneumoniae being the most frequent (24 out of 35, 69%).

A combination of two pyogenic bacterial pathogens was found in 36% (30 out of 82) of patients. Pyogenic bacteria and a respiratory virus were found in 20% (17 out of 82). An "atypical" microorganism and a respiratory virus were found in 18% (15 out of 82), pyogenic bacterial pathogen and an "atypical" microorganism in 15% (12 out of 82), and a combination of two "atypical" microorganisms in 13% (11 out of 82).

Table 1Go displays the associations of pathogens with defined groups. Of note is the fact that the most frequent combination of mixed infections within the pyogenic pathogens was that with another pyogenic pathogen. This was particularly evident for Gram-negative pathogens. Atypical bacterial pathogens were most frequently associated with other atypical pathogens. Viruses were more frequently associated with bacterial and atypical pathogens.


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  		Table 1— Number of isolates and distribution of defined pathogens within groups of pathogens#

 
In the MPP subgroup S. pneumoniae was responsible for 20 out of 28 (71%) cases. The most frequent combination was a co-infection of S. pneumoniae and Influenza A (five out of 28, 18%). The aetiological combinations found in this subgroup are shown in table 2Go.


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  		Table 2— Distribution of microorganisms with mixed pyogenic pneumonia(MPP) subgroup and a paired serology for "atypical" bacterial and viral pathogens

 
Single pyogenic pneumonia
In this group, S. pneumoniae was the most frequent microorganism (60 out of 82, 73%), followed by H. influenzae (12 out of 82, 15%) and P. aeruginosa (five out of 82, 6%).

Comparison of the SPP group and the MPP subgroup
Baseline characteristics
No differences with regard to sex distribution, toxic habits or previous antimicrobial treatment were found. While mean age was similar in both groups, patients with SPP were more frequently older than 65 (77% SPP versus 54% MPP; p = 0.04). Distribution into Fine classes revealed no significant differences. Most patients in both groups belonged to Fine classes IV and V (SPP 60%, MPP 54%). There were no differences found with regard to comorbid illness, such as cardiac, hepatic, pulmonary, renal or neoplastic illness. The type of empirical treatment administered was similar between the two groups.

Clinical presentation, radiographical findings and laboratory values
The current authors did not find any differences comparing the clinical presentation of CAP. The majority of patients had typical signs, such as cough, expectoration and crackles on auscultation (>80%, >75% and >70%, respectively, in both groups). Radiographical findings on admission were also similar. Also, no differences with regard to leukocyte count, C-reactive protein levels, haematocrite, creatinine and Pa,O2 on admission were found (table 3Go).


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  		Table 3— Characteristics of monobacterial infections, including a single pyogenic pneumonia(SPP) compared with community-acquired pneumonia cases with a mixed pyogenic pneumonia (MPP) subgroup

 
Severity and outcome
The amount of patients with mental confusion on admission, creatinine levels >1.5 mg·dL–1 and multilobar disease was <1?tlsb=-.005w?>similar in both groups. Patients with SPP had a respiratory frequency >30·min–1 more frequently (49% SPP versus 18% MPP; p = 0.008), but were less likely to have a lower systolic blood pressure rate than 90 mmHg at admission (5% SPP versus 18% MPP; p = 0.05). Accordingly, the development of shock was more frequent in the subgroup of patients with MPP (4% SPP versus 18% MPP; p = 0.03). Also, the requirement for admission to ICU was more frequent in the MPP group, being at borderline significance (9% SPP versus 21% MPP; p = 0.07; table 3Go). However, the overall duration of hospitalisation did not differ in both groups (10.1±10.08 SPP versus 10.1+10.7 MPP; p = 0.095).

In-hospital mortality of all patients was 10% (151 out of 1,511). The mortality rate was 13% (11 out of 82) in all cases with mixed CAP. Mortality was low in both the SPP and MPP groups (2% versus 0%), since only patients with paired serology were considered.


   DISCUSSION
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
ACKNOWLEDGEMENTS
 REFERENCES
 
The main findings of the present study were as follows: 1) a mixed aetiology was found in 13% of cases of hospitalised CAP; 2) S. pneumoniae was the pathogen most frequently involved in MP; 3) co-infection with two pyogenic bacteria was the most frequent mixed infection overall; 4) most mixed infections (71%) included at least one pyogenic pathogen; 5) no important differences were found comparing the clinical presentation of single versus MPs; and 6) patients in the MPP subgroup had hypotension and shock more frequently, which may hint at a potential adverse prognostic in this mixed infection group.

The investigation of mixed infections in CAP was hampered for several reasons. First, the incidence of mixed infections does not usually surpass 30% in most series, making it difficult to collect a relevant number of valid cases with MP for study, even in large databases. Secondly, most viral and "atypical" infections are usually diagnosed by paired serology. However, a complete serology is not only difficult to obtain in all patients leaving the hospital, but requires a patient surviving for at least 14 days after hospitalisation, which necessarily introduces a bias of survival on all outcome data. Even if diagnostic assessment was focused on antigen- and molecular-based techniques, paired serology, with its inherent limitations, would still have a place and challenge the analysis of patients with unavailable paired serology. Thirdly, it is probably important to address different types of combinations of mixed infections in any outcomes separately. Finally, aspiration pneumonia is probably a frequent mixed infectious syndrome, particularly in elderly patients 12. However, the microbial assessment of aspiration pneumonia requires a vigorous investigation of anaerobic pathogens that is virtually impossible to conduct in a large study population. Keeping these limitations in mind, it appears that the most rewarding way of studying mixed infections is to look for trends in differences of clinical presentation and outcomes that can form the basis of a more tailored future prospective study approach. The issue of aspiration pneumonia should be addressed separately in a small, but properly selected, subgroup of patients.

Most series of CAP describe mixed infections. The number varies in series originating from Israel, Finland, Switzerland and Wales (38, 27, 16 and 28% of cases, respectively) 4, 13–15. In Spain, figures of 10% for mixed pneumococcal CAP and 7% in general have been described 1, 16. The present lower rate of 13% in a very large population of patients with CAP is probably close to reality, since the selection of cases was very strict according to a systematic diagnostic approach. The exact reasons behind the large variations in mixed infections can be ignored; however, in view of the rigorous diagnostic approach it seems improbable that these variations can be explained exclusively through methodological reasons.

In the current study, S. pneumoniae was the most frequently involved pathogen in mixed CAP (54%). Corresponding rates in recent series range between 27 and 47% 1, 13, 15. Conversely, 16% of all pneumococcal infections were mixed. The number even increased to 25% when only those patients with a paired serology were taken into account. Thus, all currently available data indicate that S. pneumoniae is not only the leading pathogen in CAP, but also in those cases with mixed infections. The present data expand these observations by finding that pyogenic bacteria as a group are also the most frequent pathogens involved in mixed infections. It can be shown that in the presence of a complete paired serology, 65% of mixed pneumococcal infections were associated with a respiratory virus, mainly influenza. There is evidence from a large study that pneumococcus has a major role in the development of pneumonia associated with influenza virus A, RSV, parainfluenza virus and adenovirus 17. Further recent data suggest that influenza virus neuraminidase contributes to secondary bacterial pneumonia and subsequent excess mortality 18.

In the study by Blanquer et al. 3, which reported the highest incidence of mixed CAP (38%), the most frequent microbial combinations were S. pneumoniae with M. pneumoniae followed by C. pneumoniae, L. pneumophila and respiratory viruses. In the present study, the combination of S. pneumoniae with H. influenzae was the most prevalent combination in MP. This might be influenced by the high rate of patients with chronic obstructive pulmonary disease in the current population. "Atypical" microorganisms also played an important role in mixed infections. They were involved in 43% of all MPs with C. pneumoniae being the most important microorganism (29%). However, in the MPP group the combination of S. pneumoniae with Influenza virus A was found to be the most frequent.

The distribution of pathogens within defined groups revealed that pyogenic microorganisms tended to be the microorganisms most frequently associated with another pyogenic pathogen. This was particularly true for Gram-negative microorganisms. In contrast, there was an obvious trend for viral pathogens to be associated with bacterial or atypical microorganisms, whereas there was no clear trend for the group of atypical bacterial pathogens. These associations may hint at predominant patterns of mixed infections inherent to different groups of microorganisms. No clinically significant differences were found when comparing patients with SPP and MPP. In addition, comorbidities predisposing patients to mixed infections could not be identified. Although, due to the limited number of cases in the MPP group a type II error cannot be excluded, the current authors were not aware of any other data indicating that MP can be predicted on clinical grounds. Moreover, it seems highly unlikely that such predictors will be identified in the future since, even in single aetiological CAP, "typical" and "atypical" pathogens cannot be reliably predicted 10, 19, 20. Therefore, the approach of tailoring antimicrobial treatment to one single identified pathogen carries the risk of ignoring additional pathogens.

The importance of mixed infections in terms of outcome is particularly difficult to assess for the reasons outlined above. In particular, MPs represent an extremely heterogeneous group. Very large numbers of patients would be necessary to detect any meaningful difference between SPP and pneumonia due to mixed aetiologies. In a study from Finland, mixed infections were associated with a more severe, longer hospital stay and an increased rate of antibiotic treatment failures 21. At least the current data, comparing well-defined and more homogeneous groups (SPP and MPP), indicate that whereas SPP patients more often presented with an increased respiratory frequency, MPP patients tended to develop shock and required ICU admission more often. This observation could hint at a higher risk of patients with MP to reach systemic compromise, which would fit well into the hypothesis that mixed infections could be the reason behind an excess mortality of ß-lactam monotherapy in CAP or severe pneumococcal pneumonia. Nevertheless, this observation must be interpreted very cautiously and should form the basis for further investigation.

As a limitation the authors recognise that microbiological investigations were not uniformly performed. The design of the study left part of the diagnostic procedures taken at admission to the discretion of the attending physician. An unknown number of mixed infections might have been missed. However, the current authors think that the presented results underestimate the rate of mixed infections. This underlines the relevance of mixed infections in CAP. Furthermore, it is recognised that all mixed infections were diagnosed retrospectively due to the characteristics of serological testing. This has led to a rather retrospective subgroup analysis of prospectively recorded data.

In conclusion, the current data show that mixed pneumonia may occur in 13% of cases with an established aetiology. Streptococcus pneumoniae and other pyogenic bacteria are the most frequently encountered infections. If searched after, respiratory viruses are frequently part of mixed infections. Rather than recommending antiviral agents the current authors would emphasise the use of influenza vaccine in risk groups. It was not possible to define clinical predictors for mixed pneumonia. The comparison of markers of severity indicates that patients with mixed infections might be more prone to severe sepsis or septic shock. In view of these observations, further studies evaluating the role of mixed aetiologies, particularly in these patients, seem mandatory.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
REFERENCES
 
The authors would like to thank J. Angrill for statistical assistance and the valuable support of A. Rañó, M. Ruíz, I. Aldabò and M. Badia.


   REFERENCES
TOP
 ABSTRACT
 MATERIAL AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 

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