Abstract
Traditionally, researchers have viewed different species and strains of bacteria as pathogens or as commensals. Different bacterial species and strains may be placed on a spectrum of pathogenicity or virulence, with highly pathogenic bacteria at one end, and minimally pathogenic bacteria at the other. It has also been recognized that a species or strain of bacteria can behave sometimes as a pathogen, and at other times as a commensal. Consequently, researchers have proposed that a spectrum of less pathogenic to more pathogenic bacteriological behavior be produced, in contrast to the spectrum of less pathogenic to more pathogenic bacteriological species. I propose that a third spectrum of pathogenicity should be developed: a spectrum of pathogenic situations. This is illustrated using the example of Staphylococcus lugdunensis. Some researchers consider S. lugdunensis to be a commensal that might be used to eradicate the dangerous pathogen S. aureus from a patient's nose, whereas other researchers consider S. lugdunensis to be a dangerous pathogen in its own right. Inoculating S. lugdunensis into the nose of a patient might be understood as inoculating them with a dangerous pathogen. However, thinking in terms of pathogenic situations, this same intervention could be understood as reducing the pathogenicity of the situation. As knowledge of the situations in which S. lugdunensis is dangerous is incomplete, this review of concepts of pathogenicity should not be used to justify challenge experiments, but rather to illustrate a different way of thinking about pathogenicity.