c. Home infusion therapy (including antibiotics)
d. Chronic dialysis in the previous 30 days
e. Home wound care
f. Family member with an MDR infection- Immunosuppressive disease and/or therapy
Pneumonia due toS. aureusis more common in patients with diabetes mellitus and head
trauma and in ICU patients (4,53–55).P. aeruginosais a frequent pathogen in patients with
severe chronic obstructive pulmonary disease (COPD) and in those with prior hospitalization,
prolonged intubation (more than eight days), and prior exposure to antibiotics (56). Infection
with Acinetobacter baumannii has been related to specific risk factors (57), including
neurosurgery, acute respiratory distress syndrome (ARDS), head trauma, and large-volume
pulmonary aspiration. Moreover, in some hospitals,Acinetobacterspp. are starting to account
for a significant number of cases of nosocomial pneumonia (58–60).
Rates of polymicrobial infection are highly variable, although they seem to be on the rise
and are particularly high in patients with ARDS (26,27,29,53,61–66).
The detection of an increased load of oropharyngeal commensals (viridans group strepto-
cocci, coagulase-negative staphylococci, andCorynebacteriumspp.) in distal bronchial specimens is
difficult to interpret, but it is not generally considered that they could cause pneumonia.
The role of anaerobic bacteria is still under investigation (67). In one report, anaerobes
were isolated from 23% of patients with VAP diagnosed by quantitative culture methods (61).
The authors of this study highlighted that the anaerobes recovered mirrored the bacteriology
of the oropharynx and that only in four patients were they the only microorganisms isolated.
No anaerobic bacterium was found in the blood or associated with necrotizing disease. In a
more recent study, however, no pathogenic anaerobes could be recovered using the same
culture methods in 143 patients strictly followed during 185 episodes of VAP (68). Collectively,
these and other findings point to an unlikely role of anaerobes in VAP or late-onset HAP. Their
role in patients with poor dentition could, however, be more significant.
Early-onset and late-onset disease can be distinguished using quantitative culture
methods of diagnosis. When pneumonia develops within four or five days of admission (or
intubation), microorganisms associated with community-acquired pneumonia are isolated
with some frequency. In contrast, when disease develops after five days, few pathogens
associated with community-acquired pneumonia are recovered, and gram-negative bacilli and
S. aureusare the main agents detected. Although indicators of late-onset disease, these bacteria
can also cause early-onset pneumonia, especially in patients with severe comorbidities under
recent antimicrobial treatment, making it more difficult to distinguish between early-onset and
late-onset disease. As mentioned above, a longer period of mechanical ventilation and
antimicrobial therapy will increase the risk of infection by MDR pathogens.
Fungal or viral pathogens are rarely the causative agents in immunocompetent patients.
NosocomialAspergillusspp. infection should warn of airborne transmission by spores related
to an environmental source, such as contaminated hospital air ducts. Recently, a high rate of
hospital-acquired Aspergillus pneumonia was observed in patients with COPD under therapy
with antibiotics and high-dose corticosteroids (69).Candida albicansor otherCandidaspecies are
often detected in endotracheal aspirates (EA), but usually indicate airway colonization rather
than pneumonia, and antifungal treatment is rarely necessary (70–74).
Outbreaks of HAP and VAP due to viruses, such as influenza, parainfluenza,
adenovirus, measles, and respiratory syncitial virus, are usually seasonal. Influenza,
parainfluenza, adenovirus, and respiratory syncitial virus account for 70% of all nosocomial
viral pneumonias. The diagnosis of these viral infections is often made by rapid antigen testing
and viral cultures or serological assays. Influenza A is probably the most common viral cause
of HAP in adult patients and predisposes the patient to secondary bacterial infection (2,75–79).
The role of herpes simplex virus (HSV) as a causative agent of VAP is presently under
discussion. In a prospective study performed at our center, HSV was isolated from respiratory
secretions in 6.4% of all patients not fulfilling VAP criteria and in 13.4% of those who did fulfill
these criteria (80). However, the role of HSV in pneumonia is yet far from clear.
Nosocomial Pneumonia in Critical Care 181