The Microbiology of Bacterial Lung Abscess: Time for a Reappraisal?

ed an overall mortality around a dismal 15–20% [5, 6] , the most recent studies dealing with primary lung abscesses (including the one published here below) report a substantial reduction of the mortality which, at present, does not seem to exceed a very encouraging 1–5% [7] . However, it must be emphasized that mortality is related not only to the underlining medical conditions, but also to certain predisposing factors, including the size and the topography of the lesion, as well as to the involvement of highly virulent pathogens (e.g. S. aureus, Klebsiella pneumoniae, P. aeruginosa ) [8] . Therefore, a potential increment of the implication of any of these (or other) particularly virulent pathogens could have a dramatic impact on actual mortality. New insight on the microbiology of lung abscess came in 2005 from the study by Wang et al. [9] . Quite surprisingly, the authors found that the bacteriology of this destructive infection (at least in Taiwan) has changed, as the contribution of the anaerobes was limited to 31% of the cases, while K. pneumoniae was the predominant bacterium in 33% of the patients with community-acquired lung abscess. Despite the fact that the number of the subjects was moderate (90), an advantage of the study was that microbiology specimens were obtained by the use of transthoracic aspiration. Of course, a direct consequence of these results (if they are confirmed and geographically generalized by similar prospective studies) would be a The microbial etiology of lung abscess is thought to be well documented and the organisms considered to be predominantly responsible consist of the (mostly anaerobic) normal flora in the mouth and, more specifically, gingival crevices [1] . A number of studies using sample collection techniques that avoid contamination with oral flora, in combination with efficient anaerobic culture methods, have shown that anaerobes participate in up to 90% of the lung abscesses [2] . Notably, these organisms are the only pathogens present in about half of the cases. Usually, lung abscesses contain multiple anaerobe species [3] . Monomicrobial abscesses are only occasionally identified and may be caused by bacteria such as Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella spp., Pasteurella multocida, Enterobacteriaceae , Haemophilus influenzae (types b and c) , Actinomyces spp. , Nocardia spp., Rhodococcus equi, Burkholderia pseudomallei, Streptococcus anginosus group, Streptococcus pneumoniae, Legionella , etc. Physicians should avoid misdiagnosing a tuberculous cavitary lesion as a lung abscess, as most signs and symptoms are nonspecific [4] . Anyhow, most of the publications containing data on the etiology of lung abscess are more than a quarter of a century old, with new information scarce and sketchy. It is beyond any doubt that both the incidence and the mortality of lung abscess are constantly declining. In fact, while studies published in the mid-1980s steadily reportPublished online: April 29, 2010