J. J. Bullen

Iron-binding Proteins in Milk and Resistance to Escherichia coli Infection in Infants

Human milk contains large quantities of iron-binding protein, of which the greater proportion is lactoferrin, though small amounts of transferrin are also present. Three samples of human milk with unsaturated iron-binding capacities of between 56 and 89% had a powerful bacteriostatic effect on Escherichia coli O111/B4. The bacteriostatic properties of milk were abolished if the iron-binding proteins were saturated with iron. Purified human lactoferrin, in combination with specific E. coli antibody, strongly inhibited the growth of E. coli, and this effect was also abolished by saturating the lactoferrin with iron. Guinea-pig milk also contains lactoferrin and transferrin. Newly born guinea-pigs fed on an artificial diet and dosed with E. coli O111 had higher counts of E. coli O111 in the intestine than suckled animals. The apparent suppressive effect of guinea-pig milk on E. coli in the intestine could be reversed by feeding the iron compound haematin. It seems that iron-binding proteins in milk may play an important part in resistance to infantile enteritis caused by E. coli.

Role of Iron in Bacterial Infection

Iron is essential for most living things. The importance of the metal lies in its remarkable capacity to engage in electron transport reactions in biological systems (Neilands, 1974). From the point of view of infection, a clear distinction must be made between the quantity of iron present in body fluids and its availability to bacteria. In the living body, iron is not freely available. The bulk of the metal is locked up in ferritin, hemosiderin, myoglobin, and in the hemoglobin in red cells (Lanzkowsky, 1976). The iron-binding proteins, transferrin and lactoferrin, which possess only a minute fraction of the total body iron, are normally only partly saturated with Fe and have an exceptionally high association constant of about 1036 for the metal. This means that the amount of free iron in equilibrium with these proteins is only about 10−8 M, which is far too low for normal bacterial growth. To obtain Fe from normal tissue, bacteria must therefore possess iron chelating agents with association constants similar to those of transferrin and lactoferrin. In injured or dead tissue the situtation may be very different. For example, the lysis of red cells can provide large amounts of Fe for those bacteria that can assimilate heme compounds.

Iron and infection : New developments and their implications

Unsaturated transferrin in plasma ensures that the amount of free ferric iron available to bacteria is about 10(-18) mol/L. This low iron environment is essential for the bacteriostatic and bactericidal systems in blood, lymph, and exudates. Antibacterial systems are abolished when iron becomes freely available. This results in rapid extracellular bacterial growth and greatly increased bacterial virulence. In human plasma, a fall in Eh (oxidation-reduction potential) or pH results in the abolition or marked reduction of its bactericidal properties. This is highly relevant to infection after trauma, where a fall in Eh and pH frequently accompanies tissue damage. Bacterial resistance to antibiotics has put the treatment of serious infections in jeopardy. Reinforcement of natural means of resistance needs to be explored, as well as examining new antibacterials that interfere with bacterial iron metabolism.

Sepsis: the critical role of iron

Sepsis is a global problem which is exacerbated by increasing bacterial resistance to antibiotics. However, mechanisms of natural resistance can be extremely effective, and need to be exploited, but the availability of iron is critical for controlling bacterial growth. The diagnosis of sepsis and possible strategies for limiting iron availability are discussed.

The critical role of iron in some clinical infections

The role of iron in certain clinical infections is reviewed. In normal persons the antibacterial and antifungal properties of blood and other tissue fluids cannot be maintained unless there are exceptionally low levels of available iron. This is controlled by the presence of the unsaturated iron-binding proteins, transferrin and lactoferrin. In several clinical conditions an abnormal availability of iron is responsible for fatal septicaemia. This is because the phagocytic system is overwhelmed by rapidly growing organisms when iron is freely available.

BACTERIAL IRON METABOLISM IN INFECTION AND IMMUNITY

Publisher Summary This chapter discusses bacterial iron metabolism in infection and immunity. The injection of iron compounds can enhance the virulence of certain bacteria and, in some cases, can abolish passive immunity. The characteristic feature of the effect of iron is the stimulation of rapid bacterial multiplication in circumstances where it previously did not occur. Death is invariably caused by an overwhelming infection. The injection of iron compounds does not interfere with the action of complement or the neutralizing power of specific antitoxin. It also does not interfere with phagocytosis by leukocytes or the production of antibody. With some infections, iron compounds have little or no effect. Experiments in vitro have shown that the bacteriostatic or bactericidal effects of serum and milk can be abolished by saturating the iron-binding proteins—transferrin and lactoferrin—with iron. In the case of E. coli , the bacteriostatic effect appears to be because of the iron-binding protein and specific antibody acting together; with some organisms, such as P. septica , the presence of complement is also required. The iron-binding proteins do not bind heme compounds, and if hemoglobin is released from red cells in vivo , the iron in heme can be taken up by pathogenic bacteria if they have a binding site for the heme molecule. Thus, in some infections, such as those caused by E. coli or salmonellae, the presence of hemoglobin is responsible for rapid bacterial growth and a considerable enhancement of virulence.

The role of Eh, pH and iron in the bactericidal power of human plasma

The bactericidal power of fresh human plasma against Klebsiella pneumoniae and Escherichia coli was extremely sensitive to changes in Eh and pH. At a high Eh (approx. +200 mV) the bacteria were destroyed, but rapid regrowth occurred when the Eh was lowered to approx. -400 mV. Abolition of the bactericidal effect was also produced by adding ferric iron at a high Eh (approx. +200 mV). Lowering the pH to 6.50 reduced or prevented the bactericidal effect. These results are probably related to the availability of iron for bacterial growth, and could be important for understanding the development of infection in injured or diseased tissue.

Abolition of passive immunity to bacterial infections by iron.

COMPARATIVELY little is known of the mechanisms of non-specific resistance to bacterial infection, although it is clear that many different factors are involved1. Experimentally, it is difficult to assess the importance of any one factor in the tissue itself because the interaction between parasite and host is a dynamic process which is not only complicated but subject to rapid change. Individual factors can sometimes be investigated in isolation in vitro, but this technique has the obvious disadvantage that the effect observed may have no real relevance to the situation in vivo. These difficulties may largely disappear if circumstances arise where a single factor can be seen to operate in a similar way in both situations.

Iron, infection, and the role of bicarbonate.

Ferric iron will not saturate transferrin in Tris buffer, and its use in experimental infections has been criticized. However, in the presence of bicarbonate, as in plasma, saturation occurs rapidly. This is comparable to natural iron overload, and infections.

The bactericidal power of the blood and plasma of patients with burns.

Patients with burns are unusually susceptible to bacterial infections, but so far there is no satisfactory explanation for this lack of resistance. Since resistance to infection involves many different mechanisms, examination of individual components of the immune system may not sufficiently explain the underlying reasons for increased susceptibility. The use of whole blood for antibacterial tests has the advantage that all the immune systems present in that fluid compartment can take part in the bactericidal effect. Tests with Klebsiella pneumoniae and Staphylococcus aureus showed no evidence that the bactericidal power of the blood and plasma of patients with burns was less than that of normal control plasma. This suggests that the solution to the problem of increased susceptibility to infection in patients with burns does not lie with the blood but must be looked for elsewhere.