13 - Pneumonia due to small bacterial organisms

Summary

Introduction

Mycoplasma pneumoniae, Chlamydia psittaci and Chlamydia pneumoniae and Coxiella burnetti are now widely recognised as causes of lower respiratory tract infection and pneumonia. They are all relatively small bacterial organisms which do not have effective cell walls and are resistant to the action of β-lactam antibiotics. They are, therefore, grouped together in this chapter under the heading of ‘small bacterial organisms’.

Mycoplasma pneumoniae

Mycoplasmas are ubiquitous pathogens in the plant and animal kingdoms. They represent the smallest known free-living forms being prokaryotes that lack a cell wall. They are bounded by a cell membrane containing sterols, substances not found in either other bacteria or viruses. Because of their small size (150–250 nm) when first discovered they were thought to be viruses. However, their ability to grow in cell-free media and the fact that they contain both RNA and DNA clearly sets them apart from viruses. In addition, DNA homology studies have failed to demonstrate any significant relationship between mycoplasmas and known bacteria. As a result, mycoplasmas have now been assigned taxonomically to their own class, Mollicutes, within which the family, Mycoplasmataceae, contains the genus subgroup, M. pneumoniae, which can cause respiratory tract infection and pneumonia. This organism is primarily an extracelluar parasite which attaches to the surface of ciliated and non-ciliated epithelial cells and causes cell damage by elaboration of substances such as hydrogen peroxide and by initiation of an inflammatory response through chemotaxis of mononuclear cells.