Human Versus Staphylococcus: The Fight for Iron
By: Carol A. Rouzer, VICB Communications
Published: Jaunary 5, 2011
To get the iron it needs to live, Staphylococcus aureus has evolved a system to bind and degrade hemoglobin - particularly human hemoglobin.
One of the most fundamental defenses that vertebrate animals use in the perpetual war against invading microorganisms is sequestration of nutrients. By denying a pathogenic bacterium the essential building blocks of life, a potentially dangerous infection may be aborted. A clear example of this defense mechanism is seen in the competition between host and pathogen for iron. In vertebrate animals, nearly 100% of iron is bound to proteins, rendering it unavailable as a nutrient for bacteria.
Figure 1. Electron micrograph of S.aureus (50,000 x). Image courtesy of Wikimedia Commons under the GNU Free Documentation License.
Of course, bacteria are highly adaptable, and many have evolved systems to obtain iron from protein-bound sources. For example, Staphylococcus aureus (Figure 1, above) has developed a highly efficient mechanism to acquire iron from hemoglobin. With four iron-binding heme prosthetic groups per molecule, hemoglobin is the most abundant source of iron in the vertebrate host. To get access to the protein, S. aureus secretes a hemolytic toxin that lyses hemoglobin-containing red blood cells. Then, the bacteria use the iron-regulated surface determinant (Isd) group of proteins to acquire iron from the released hemoglobin. At the bacterial cell wall, IsdB binds intact hemoglobin directly, while IsdH and IsdA bind hemoglobin and heme that are carried by haptoglobin and hemopexin, respectively. These proteins, deliver heme to the interior of the cell through the combined action of IsdC, IsdD, IsdE, and IsdF. Once inside, IsdG and IsdI degrade the heme, releasing the iron (Figure 2).
Figure 2. The Isd group of proteins. IsdB, IsdH, and IsdA bind heme in various forms at the bacterial cell wall. The heme is transferred into the cell through the interaction of IsdC, IsdD, IsdE, and IsdF. IsdG and IsdI work intracellularly to degrade the heme, releasing the iron. Image courtesy of Eric Skaar.
Although S. aureus is primarily a human pathogen, most studies of S. aureus infection are carried out in mice. This led VICB member Eric Skaar and his laboratory [G. Pishchany et al. (2010) Cell Host & Microbe, 8, 544] to investigate how well the Isd system works for mouse hemoglobin (mHb) versus human hemoglobin (hHb). Their surprising result revealed that S. aureus’s IsdB is highly specific for hHb, providing an explanation for it’s human host preference.
The Skaar group cultured S. aureus in low iron medium to induce high levels of expression of Isd proteins. They then exposed the bacteria to mHb and hHb and measured the amount of protein bound to the bacterial surface. They found greater quantities of bound hHb than mHb. In contrast, no difference in binding was observed in the ΔIsdB strain of S. aureus, which does not express a functional IsdB protein, suggesting that it was the interaction of hemoglobin with IsdB that caused the difference in binding. Further confirmation for this conclusion came from a direct measurement of the binding of mHb or hHb to purified IsdB protein. The results showed that hHb binds about 18-fold more strongly to IsdB than does mHb. The higher affinity of IsdB for hHb than mHb correlated with the bacteria’s ability to utilize the two proteins as an iron source. S. aureus grown on iron-deficient medium supplemented with hHb grew much faster than bacteria grown with mHb supplementation.
These in vitro and cell culture results further translated into S. aureus infectivity in vivo. The Skaar group discovered that transgenic mice that expressed hHb were more readily colonized by S. aureus than wild-type mice indicating that the higher efficiency of binding hHb than mHb enabled the bacteria to better survive other host defense systems and thrive. Since the transgenic mice expressed a 50:50 mixture of mHb and hHb, one can imagine that even higher levels of infection would be observed in animals expressing 100% hHb.
These observations were found to apply to multiple strains of S. aureus, and S. aureus was found to bind hHb more strongly than that of many species, including rat, rabbit, cow, pig, horse, and baboon. Furthermore, other human-preferring bacteria, including Staphylococcus lugdunensis, Corynebacterium diphtheriae, and Staphylococcus simulans, demonstrated a higher affinity for hHb than mHb. In contrast, bacteria that show no strong species preference, such as Acinetobacter baumannii, Pseudomonas aeruginosa, Bacillus anthracis, and Bacillus cereus, exhibited similar affinities for both hemoglobins.
The results strongly suggest that a selective affinity for hemoglobin contributes to host preference in bacteria. They also suggest that mice are not an ideal model for human S. aureus infection. The Skaar group proposes using their transgenic hHb-expressing mouse strain as a substitute. This would be one of the first examples of a “humanized” mouse model that is founded on the basis of bacterial nutrient requirements.