 The study investigates the levels and processing of mid-chi, MK, in relation to Staphylococcus aureus infection in the airways during acute exacerbations of chronic obstructive pulmonary disease, copped, and ventilator-associated pneumonia, VAP. MK was detected in copped lung tissue but less so in normal lung tissue and killed S aureus at below micromolar concentrations in vitro. Proteolytic processing of MK by the Staphylococcal metalloprotease aureolicin, AL, resulted in impaired bactericidal activity, while cysticine protease staphopane A did not affect it. MK was detected in sputum from VAP patients caused by S aureus but less so in copped exacerbations associated with the same bacterium. Recombinant MK was degraded more rapidly in copped sputum than in VAP sputum and a greater proteolytic activity in copped sputum was confirmed by zymography. The study suggests that proteases of both bacteria and the host contribute to the degradation of MK, resulting in an impaired defense of the airways, particularly in copped where chronic inflammation could be important. This article was authored by Helena N. Lynch, to see Yer Anderson, Sauer El-Nordin and others. We are article.tv, links in the description below.