6 - Staphylococcal pneumonia

Summary

Introduction

Staphylococcal pneumonia is a cause of substantial morbidity and mortality. Previously, Staphylococcus aureus was thought to be an uncommon aetiological agent in both community and hospital-acquired pneumonia, possibly because relatively little attention had been focused on it relative to other causes of pneumonia, for example, nosocomial Gram-negative pneumonia. Evidence is accruing, however, that places a greater emphasis on this pathogen. Recent literature suggests its contribution to pneumonia is increasingly recognised, emergence of methicillin resistant S. aureus in both the community and hospital is presenting therapeutic and management challenges, it may be under-diagnosed in HIV positive patients and there are significant shifts from previously perceived epidemiological-clinical features. S. aureus is the organism responsible for staphylococcal pneumonia and there is no definitive evidence that Staphylococcus epidermidis causes pneumonia.

Microbiology, pathology and immunology

S. aureus is a 0.9 mμ coccus, and member of the family micrococcacae whose triple axes division leads to a clustered appearance; occasionally five-membered chains occur. It is a Gram-positive organism but may appear Gram-negative within phagocytes or if aged. Facultatively anaerobic and growing on non-selective media/blood agar S. aureus prefers aerobic conditions under which well-demarcated, smooth, convexed colonies have enhanced production of golden carotenoid pigment. Characteristics permitting differentiation from S. epidermidis and Staphylococcus saprophyticus include plasma coagulation (coagulase positivity), mannitol fermentation and positive deoxyribonuclease reactivity.

The important ultrastructural antigenic components (Table 6.1) are cell wall surface polysaccharide and protein binding peptidoglycan, teichoic acid and protein A.