Bacterial Infections of the Lung: Factors that Predispose to Development of Pneumonia

bacterial pneumoniaIt has been estimated that greater than 1,200,000 cases of bacterial pneumonia (with 55,000 deaths) occur annually in the United States. The purpose of this article is to review bacterial pneumonia in adults.

Pulmonary defense mechanisms are normally effective in preventing pneumonia. These defenses include humidification of inspired air, mucus secretion and ciliary action of airway epithelium, cough, lymphoid tissue, immunoglobulins and complement, pulmonary macrophages and leukocytes, and leukocyte chemotactic factors.

Community-acquired pneumonia typically involves organisms such as Streptococcus pneumoniae, Haemophilus influenzae and, in the case of gross aspiration of oropharyngeal contents, anaerobic bacteria. Hospital-acquired pneumonia, on the other hand, is more likely to involve Staphylococcus aureus and gram-negative aerobic or facultative bacilli, such as Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Proteus species, Enterobac-ter spp, Serratia marcescens, and Acinetobacter calcoaceticus. There is, however, substantial overlap between the flora of community- and hospital-acquired pneumonias.

The oropharynx is the major source of pulmonary bacterial pathogens. Underlying conditions associated with oropharyngeal colonization by gram-negative aerobic or facultative bacilli are shown in Table 1; colonization of healthy individuals is uncommon. Recent data suggest that adherence, possibly mediated by pili, of gram-negative bacilli to buccal epithelial cells is an important factor in oropharyngeal colonization. Serveral species of anaerobes that normally reside in the oral cavity, particularly Bacteroides melaninogenicus, have been shown to inhibit the growth of a number of potential pulmonary pathogens; this bacterial interference may be an important defense against oropharyngeal colonization.bacterial interference

Although the term aspiration pneumonia is used to refer to patients with anaerobic pleuropulmo-nary infection, it is important to realize that aspiration of oropharyngeal contents is a common occurrence in both healthy and ill persons during sleep and is undoubtedly important in the pathophysiology of most cases of bacterial pneumonia regardless of the pathogen. Factors, in addition to sleep, that predispose to aspiration include drug-induced depression of the sensorium, epilepsy, head trauma, cerebrovascular accident, and bulbar or pseudobulbar palsy. Community-acquired aspiration pneumonia usually involves anaerobic with or without facultative bacteria, whereas about two thirds of hospital-acquired aspiration pneumonia involve only facultative or aerobic bacteria. Because bacteria that proliferate in diseased periodontal tissues are largely anaerobic and are present in extremely high counts, the presence of gingivitis or periodontal disease is an important predisposition to anaerobic pulmonary infection.

Once bacteria have gained access to the normally sterile subglottic region, pneumonia may be averted if the pulmonary clearing mechanisms are intact. Endotracheal intubation, tracheostomy, chronic obstructive pulmonary disease, and neurodepressants may render the cough reflex ineffective. Mucociliary clearance of die airways may be depressed by ethanol, narcotics, chronic airway disease, or recent viral infection. Continuous assisted ventilation may cause laige numbers of bacteria to be aerosolized into the respiratory tract. Obstruction of a bronchus (by a foreign body, endobronchial tumor, or extrinsic compression) may also severely impair pulmonary clearing of aspirated bacteria. In addition, conditions such as hypoxia, acidosis, uremia, and therapy with cytotoxic or glucocorticoid agents may impair pulmonary defenses.

Bacteremia with secondary seeding of the lung or spread of contiguous infection into the chest cavity are occasional causes of pneumonia.

Table 1—Factors Associated with Oropharyngeal Colonisation by Gram-negative Bacilli

Hospitalization in an acute care medical facility
Inability to ambulate without assistance
Deteriorating clinical status
Underlying respiratory disease
Antimicrobial therapy
Long-term ethanol abuse
Diabetes mellitus
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