Serum Stimulates Growth of and Proteinase Secretion byAspergillus fumigatus

Abstract

Serum contains iron-binding proteins, which inhibit the growth of most pathogenic microorganisms, including fungi. The purpose of this research was to investigate the effect of serum on growth of the opportunistic fungal pathogen Aspergillus fumigatus. Supplementing minimal essential medium (MEM) with up to 80% human serum or up to 80% fetal bovine serum (FBS) stimulated growth and increased the amount of A. fumigatus dry biomass approximately fourfold. In addition, a 100-fold increase in proteinase secretion, as measured by azocasein hydrolysis, was observed when 10% human serum or 10% FBS was added to MEM. The fungal proteinases secreted in serum-containing media were shown to degrade 3 H-labeled basal lamina proteins. The factor in serum that stimulated proteinase secretion was larger than 10 kDa and was 85% inactivated when the serum was heated for 30 min at 66°C. The proportions of proteinases of each catalytic class secreted by A. fumigatus in the presence of serum were different from the proportions secreted in media containing single proteins. Proteinase secretion did not result from increased protein concentration in the medium per se because bovine serum albumin (BSA) at a concentration equivalent to the concentration of serum produced only 20% of the proteinase activity per milligram (dry weight) that was produced by FBS. Addition of BSA plus 100 M FeCl 3 to MEM resulted in the same level of growth as addition of serum, indicating that a combination of nutritional factors in serum may stimulate growth. However, the level of proteinase secretion was still only 30% of the level observed with FBS. These data indicate that serum does not inhibit the growth of A. fumigatus and that the nutrients in serum result in high levels of proteinase secretion, potentially increasing the invasiveness of this species.

Frequent severe liver iron overload after stem cell transplantation and its possible association with invasive aspergillosis

“…It seems A. fumigatus is able to use iron in serum and that a combination of available iron and protein in serum permits abundant growth of the pathogen. 21 Second, the mechanism by which A. fumigatus obtains iron in serum-containing media has recently been revealed. Five siderophores were purified from A. fumigatus culture medium, and the two major siderophores were identified as triacetylfusarinine C and ferricrocin.…”

Isolation, activity and immunological characterisation of a secreted aspartic protease, CtsD, from Aspergillus fumigatus

“…Interestingly, GiVord et al [5] have shown that induction of protease secretion occurs in A. fumigatus cultured in the presence of human or fetal bovine serum (FBS) and that it may, in part, contribute to the release of iron from ironbinding proteins by proteolysis. These authors noted that iron levels were undetectable in MEM and that FBS contained 20 M iron (equivalent to 1-4 M iron in 5-20% (v/ v) FCS).…”

“…The degradative properties of secreted proteases have attracted much attention as potential mediators of fungal invasion in infected tissue, conidophore development and iron acquisition [4,5]. For instance, the role of aspartic proteases in particular has been extensively investigated in the yeast Candida albicans and studies have shown that these key virulence factors are associated with hyphal formation, phenotypic switching, adhesion and evasion of the host immune response [6].…”

Multiplexed Activity-based Protein Profiling of the Human Pathogen Aspergillus fumigatus Reveals Large Functional Changes upon Exposure to Human Serum

“…A. fumigatus is one of the few pathogenic organisms that readily grows in the presence of serum (18) due to its ability to extract iron from human transferrin in this iron-limited environment (19). Furthermore, A. fumigatus can use serum proteins for biomass generation (20,21), but the full effect of serum on the cellular processes of A. fumigatus and its relevance to disease is not fully understood.…”