Lactoferrin-binding proteins in Shigella flexneri

Antimicrobial Functions of Lactoferrin Promote Genetic Conflicts in Ancient Primates and Modern Humans

Lactoferrin is a multifunctional mammalian immunity protein that limits microbial growth through sequestration of nutrient iron. Additionally, lactoferrin possesses cationic protein domains that directly bind and inhibit diverse microbes. The implications for these dual functions on lactoferrin evolution and genetic conflicts with microbes remain unclear. Here we show that lactoferrin has been subject to recurrent episodes of positive selection during primate divergence predominately at antimicrobial peptide surfaces consistent with long-term antagonism by bacteria. An abundant lactoferrin polymorphism in human populations and Neanderthals also exhibits signatures of positive selection across primates, linking ancient host-microbe conflicts to modern human genetic variation. Rapidly evolving sites in lactoferrin further correspond to molecular interfaces with opportunistic bacterial pathogens causing meningitis, pneumonia, and sepsis. Because microbes actively target lactoferrin to acquire iron, we propose that the emergence of antimicrobial activity provided a pivotal mechanism of adaptation sparking evolutionary conflicts via acquisition of new protein functions.

Immunity genes can evolve rapidly in response to antagonism by microbial pathogens, but how the emergence of new protein functions impacts such evolutionary conflicts remains unclear. Here we have traced the evolutionary history of the lactoferrin gene in primates, which in addition to an ancient iron-binding function, acquired antimicrobial peptide activity in mammals. We show that, in contrast to the related gene transferrin, lactoferrin has rapidly evolved at protein domains that mediate iron-independent antimicrobial functions. We also pinpoint signatures of natural selection acting on lactoferrin in human populations, suggesting that lactoferrin genetic diversity has impacted the evolutionary success of both ancient primates and humans. Our work demonstrates how the emergence of new host immune protein functions can drastically alter evolutionary and molecular interactions with microbes.

Behavioral effects of bovine lactoferrin administration during postnatal development of rats

We tested the hypothesis that rats consuming bovine lactoferrin (bLf) during postnatal development would show better performance of stressful tasks during adolescence. In the first study, we orally administered bLf (750 mg/kg) once daily between postnatal days 16-34. Rats then underwent a battery of behavioral tests: open field (forced exploration of risky environment), light-dark emergence (voluntary exploration of risky environment), baited holeboard (working and reference memory), food neophobia (preference for familiar versus novel food), forced swim (test for antidepressant efficacy), and shuttle-box escape (learning to escape footshock). bLf-supplemented rats showed less exploration of the risky environment, greater preference for the familiar food odor, and faster escape responses. The effect of bLf on forced-swim behavior depended on sex: immobility increased for males and decreased for females. In the next study, we replaced the forced-swim test with an escape-swim test in which rats learned to use a visual cue to locate an escape platform, and we tested the dose response of bLf on this and the shuttle-box escape test, with subjects receiving vehicle or bLf at 500, 1,000, or 2,000 mg/kg. Under this modified testing battery, improvement of escape from footshock was not observed at any dose. However, males, but not females, showed a significant dose-dependent effect of bLf on acquisition of the water-escape task. On average, males receiving a higher dose mastered the task 20-25 % sooner than rats receiving a lower dose or vehicle. These results offer preliminary evidence that bLf supplementation during development can improve subsequent cognitive performance during stress.

Effect of bovine lactoferrin on the minimum inhibitory concentrations of ampicillin and trimethoprim–sulfamethoxazole for clinical Shigella spp. strains1This article is part of a Special Issue entitled Lactoferrin and has undergone the Journal’s usual peer review process

Here, we determined the effect of bovine lactoferrin (bLF) on the minimum inhibitory concentration (MIC) of ampicillin and trimethoprim–sulfamethoxazole in Shigella . Using a microdilution method, the MIC was determined in the presence or absence of bovine lactoferrin (10 mg/mL) on 88 Shigella strains (56 Shigella flexneri , 15 Shigella boydii , 13 Shigella sonnei , and 4 Shigella dysenteriae ) previously isolated from peruvian children <2 years old. A fold change of 2 or more in MIC values was considered significant. For ampicillin, 67 (76%) strains were highly resistant; one-third of the strains (32%) showed a decrease in ampicillin MIC in the presence of LF. This was more typical of MIC values in less resistant strains. For 7 (8%) ampicillin-resistant strains, the decrease in the MIC resulted in the strains reaching the cutoff for susceptible in the presence of bLF. For trimethoprim–sulfamethoxazole, 93% of the isolates (n = 82) were highly resistant and only 4 isolates (5%) decreased their MIC in the presence of bLF. None of the trimethoprim–sulfamethoxazole resistant strains became susceptible in the presence of LF. The decrease in the MIC in the presence of bLF seems to depend on the mechanisms of action of each antibiotic. In vivo studies are needed to further evaluate bLF as a coadjuvant to antibiotic treatment of resistant Shigella.

Transgenic milk containing recombinant human lactoferrin modulates the intestinal flora in piglets1This article is part of a Special Issue entitled Lactoferrin and has undergone the Journal’s usual peer review process

Lactoferrin (LF) is a beneficial multifunctional protein in milk. The objective of this study was to determine whether bovine transgenic milk containing recombinant human lactoferrin (rhLF) can modulate intestinal flora in the neonatal pig as an animal model for the human infant. We fed 7-day-old piglets (i) ordinary whole milk (OM), (ii) a 1:1 mixture of OM and rhLF milk (MM), or (iii) rhLF milk (LFM). LFM provided better average daily mass gain than OM (P = 0.007). PCR–denaturing gradient gel electrophoresis and 16S rDNA sequencing analysis revealed that the LFM piglets exhibited more diversity of the intestinal flora than the OM group. Except for the colon in the LFM group, an increasing trend in microbial diversity occurred from the duodenum to the colon. Fecal flora was not different across different ages or different treatment groups, but a cluster analysis showed that the fecal flora of OM- and MM-fed piglets had a higher degree of similarity than that of LFM-fed piglets. Based on culture-based bacterial counts of intestinal content samples, concentrations of Salmonella spp. in the colon and of Escherichia coli throughout the intestine were reduced with LFM (P < 0.01). Concentrations of Bifidobacterium spp. in the ileum and of Lactobacillus spp. throughout the intestine were also increased with LFM (P ≤ 0.01). We suggest that rhLF can modulate the intestinal flora in piglets.

Effect of lactoferrin on enteric pathogens

Much has been learned in recent years about the mechanisms by which breastfeeding improves child health and survival. However, there has been little progress in using these insights to improve pediatric care. Factors that are important for protecting the breast fed infant might be expected to decrease the adverse effects of weaning on diarrhea, growth, and development. Lactoferrin, an iron-binding protein with multiple physiological functions (anti-microbial, anti-inflammatory, and immunomodulatory), is one of the most important proteins present in mammalian milk. Protection against gastroenteritis is the most likely biologically relevant activity of lactoferrin. Multiple in vitro and animal studies have shown a protective effect of lactoferrin on infections with enteric microorganisms, including rotavirus, Giardia, Shigella, Salmonella and the diarrheagenic Escherichia coli. Lactoferrin has two major effects on enteric pathogens: it inhibits growth and it impairs function of surface expressed virulence factors thereby decreasing their ability to adhere or to invade mammalian cells. Thus, lactoferrin may protect infants from gastrointestinal infection by preventing the attachment by enteropathogens in the gut. Recently several clinical trials in children have started to address this issue. Whether lactoferrin can prevent a significant portion of diarrheal disease remains to be determined.

In vitro adhesion and invasion inhibition of Shigella dysenteriae, Shigella flexneri and Shigella sonnei clinical strains by human milk proteins

BACKGROUND

Shigella is the etiological agent of shigellosis, a disease responsible for more than 500,000 deaths of children per year, in developing countries. These pathogens colonize the intestinal colon, invade, spreading to the other enterocytes. Breastfeeding plays a very important role in protecting infants from intestinal infections. Amongst milk compounds, glycosylated proteins prevent the adhesion of many enteropathogens in vitro. The aim of this work was to determine the effect of human milk proteins on the colonization potential of Shigella dysenteriae, S. flexneri and S. sonnei. To fulfill this purpose, pooled milk samples from five donors, were fractionated by gel filtration and affinity chromatography. Using tissue culture, the milk fractions obtained were tested in Shigella adhesion and invasion assays.

RESULTS

Our revealed showed that both adhesion and invasion of Shigella species were inhibited by low concentration of secretory immunoglobulin A, lactoferrin and free secretory component. This work also showed that, these proteins bind to superficial and whole-cell Shigella proteins.

CONCLUSIONS

Our findings suggest that human milk may act inhibiting adhesion and, consequently, invasion of Shigella, thereafter preventing shigellosis in infants.

Lactoferrin protects rabbits from Shigella flexneri-induced inflammatory enteritis

Shigella species cause bacillary dysentery in humans by invasion, intracellular multiplication, spread to adjacent cells, and induction of brisk inflammatory responses in the intestinal epithelium. In vitro data suggest that lactoferrin, a glycoprotein present in human mucosal secretions, has a role in protection from bacterial enteric infections. We sought to determine the activity of lactoferrin in vivo, using the concentration present in human colostrum, to investigate its effect on the development of clinical and pathological evidence of inflammation in a rabbit model of enteritis. Lactoferrin protected rabbits infected with Shigella flexneri from developing inflammatory intestinal disease. Typical histological changes in ill animals included villous blunting with sloughing of epithelial cells, submucosal edema, infiltration of leukocytes, venous congestion, and hemorrhage. Lactoferrin at a concentration normally found in human colostrum blocks development of S. flexneri-induced inflammatory enteritis.

Lactoferrin and free secretory component of human milk inhibit the adhesion of enteropathogenic Escherichia coli to HeLa cells

BACKGROUND

Diarrhoea caused by Escherichia coli is an important cause of infant morbidity and mortality in developing countries. Enteropathogenic Escherichia coli (EPEC) is considered one of the major causes of diarrhoea in children living in developing countries. The ability of diarrhoeagenic strains of E. coli to adhere to and colonize the intestine is the first step towards developing the disease. EPEC strains adhere to enterocytes and HeLa cells in a characteristic pattern known as localized adherence. Many epidemiological studies of diarrhoea have shown that breast-feeding protects infants from intestinal infections. Both immunoglobulin and non-immunoglobulin elements of human milk are thought to contribute to the protection from diarrhoeal agents.

RESULTS

The effects of human milk and its protein components on the localized adherence of EPEC were investigated. Non-immunoglobulin components of human milk responsible for the inhibition of EPEC adhesion to HeLa cells were isolated by chromatographic fractionation of human whey proteins. Besides secretory immunoglobulin A, which has been previously reported to affect the adhesion of EPEC, free secretory component (fSC) and lactoferrin (Lf) were isolated. Even in concentrations lower than those usually found in whole milk, fSC and Lf were able to inhibit the adhesion of EPEC. alpha-lactalbumin was also isolated, but showed no activity on EPEC adhesion.

CONCLUSIONS

This study demonstrated that the immunoglobulin fraction, the free secretory component and lactoferrin of human milk inhibit EPEC adhesion to HeLa cells. These results indicate that fSC and Lf may be important non-specific defence factors against EPEC infections.

Porins OmpC and PhoE of Escherichia coli as specific cell-surface targets of human lactoferrin. Binding characteristics and biological effects

The binding of lactoferrin, an iron-binding glycoprotein found in secretions and leukocytes, to the outer membrane of Gram-negative bacteria is a prerequisite to exert its bactericidal activity. It was proposed that porins, in addition to lipopolysaccharides, are responsible for this binding. We studied the interactions of human lactoferrin with the three major porins of Escherichia coli OmpC, OmpF, and PhoE. Binding experiments were performed on both purified porins and porin-deficient E. coli K12 isogenic mutants. We determined that lactoferrin binds to the purified native OmpC or PhoE trimer with molar ratios of 1.9 +/- 0.4 and 1.8 +/- 0.3 and Kd values of 39 +/- 18 and 103 +/- 15 nM, respectively, but not to OmpF. Furthermore, preferential binding of lactoferrin was observed on strains that express either OmpC or PhoE. It was also demonstrated that residues 1-5, 28-34, and 39-42 of lactoferrin interact with porins. Based on sequence comparisons, the involvement of lactoferrin amino acid residues and porin loops in the interactions is discussed. The relationships between binding and antibacterial activity of the protein were studied using E. coli mutants and planar lipid bilayers. Electrophysiological studies revealed that lactoferrin can act as a blocking agent for OmpC but not for PhoE or OmpF. However, a total inhibition of the growth was only observed for the PhoE-expressing strain (minimal inhibitory concentration of lactoferrin was 2.4 mg/ml). These data support the proposal that the antibacterial activity of lactoferrin may depend, at least in part, on its ability to bind to porins, thus modifying the stability and/or the permeability of the bacterial outer membrane.

The Yfe system of Yersinia pestis transports iron and manganese and is required for full virulence of plague

Iron acquisition in Yersinia pestis is fundamental to the success of plague pathogenesis. We have previously identified an approximately 5.6 kb region (yfe) of Y. pestis genomic DNA, capable of restoring iron-deficient growth but not siderophore production to an Escherichia coli mutant (SAB11) incapable of synthesizing the siderophore, enterobactin. The yfe locus of Y. pestis, found in both pigmented (Pgm+) and nonpigmented (Pgm-) strains, comprises five genes arranged in two distinct operons (yfeA-D and yfeE ). The larger of these, yfeABCD, encodes an ABC transport system, whose expression is iron and Fur regulated and is repressed in cells grown in the presence of manganese. Cells from a Pgm-, Yfe- (DeltayfeAB ) mutant strain of Y. pestis exhibited reduced transport of both 55Fe and 54Mn. Furthermore, cells containing an intact yfe locus showed reduced 55Fe uptake when competing amounts of MnCl2 or ZnCl2 were present, whereas 54Mn uptake was inhibited by FeCl3 but not by ZnCl2. Similarly, yfe mutants of Y. pestis exhibited growth defects on media supplemented with the iron chelators 2,2'-dipyridyl or conalbumin. These growth defects were not relieved by supplementation with MnCl2. A ybt-, DeltayfeAB mutant of Y. pestis was completely avirulent in mice infected intravenously (LD50 > 1.7 x 107 cfu) compared with its parental ybt-, yfe+ strain, which had an LD50 of < 12. In addition, compared with its ybt+, yfe+ parent, a ybt+, DeltayfeAB mutant of Y. pestis had an approximately 100-fold increase in the LD50 from a subcutaneous route of infection. These data suggest that the Yfe and Ybt systems may function effectively to accumulate iron during different stages of the infectious process of bubonic plague.

An ABC transporter system of Yersinia pestis allows utilization of chelated iron by Escherichia coli SAB11

The acquisition of iron is an essential component in the pathogenesis of Yersinia pestis, the agent of bubonic and pneumonic plague. A cosmid library derived from the genomic DNA of Y. pestis KIM6+ was used for transduction of an Escherichia coli mutant (SAB11) defective in the biosynthesis of the siderophore enterobactin. Recombinant plasmids which had a common 13-kb BamHI fragment were isolated from SAB11 transductants in which growth but not enterobactin synthesis was restored on media containing the iron chelator EDDA [ethylenediamine-di(o-hydroxyphenyl acetic acid)]. Subcloning and transposon mutagenesis revealed a 5.6-kb region, designated yfe, essential for SAB11 growth stimulation. In vitro transcription-translation analysis identified polypeptides of 18, 29.5, 32, and 33 kDa encoded by the yfe locus. Sequence analysis shows this locus to be comprised of five genes in two separate operons which have potential Fur-binding sequences in both promoters. A putative polycistronic operon, yfeABCD, is Fur regulated and responds to iron and manganese. A functional Fur protein is required for the observed manganese repression of this operon. This operon encodes polypeptides which have strong similarity to the ATP-binding cassette (ABC) family of transporters and include a periplasmic binding protein (YfeA), an ATP-binding protein (YfeB), and two integral membrane proteins (YfeC and -D), which likely function in the acquisition of inorganic iron and possibly other ions. The approximately 21-kDa protein encoded by the separately transcribed yfeE gene may be located in the cell envelope, since a yfeE::TnphoA fusion is PhoA+. Mutations in this gene abrogate growth of SAB11 on iron-chelated media.

Identification of a lactoferrin-binding protein in Prevotella nigrescens

A 40-kDa lactoferrin-binding protein was identified in a strain of Prevotella nigrescens isolated from a patient with periodontitis. The protein was purified by affinity column chromatography using a Sepharose-lactoferrin column and detergent-solubilized membranes. The N-terminal sequence revealed no apparent similarities with any other sequenced bacterial protein. The native conformation of the 40-kDa protein was a condition to bind either iron-free or iron-saturated lactoferrin. A possible function of this Lf-binding protein could be related with an iron acquisition mechanism in P. nigrescens.

Cryptic domains of a 60 kDa heat shock protein of Helicobacter pylori bound to bovine lactoferrin

Bovine lactoferrin binds to a 60 kDa heat shock protein of Helicobacter pylori. Binding ability was related to human immunoglobulin G because bovine lactoferrin binding proteins were isolated by extraction of cell surface associated proteins with distilled water, applied on IgG-Sepharose and nickel sulphate chelate affinity chromatography. Binding was demonstrated by Western blot after purified protein was digested with alpha-chymotrypsin and incubated with peroxidase-labeled bovine lactoferrin. Binding was inhibited by bovine lactoferrin, lactose, rhamnose, galactose, and two iron-containing proteins, ferritin and haptoglobin. Helicobacter pylori binds ferritin and haptoglobin via charge or hydrophobic interactions because this binding was not inhibited by specific and various glycoproteins or carbohydrates. Carbohydrate moieties of bovine lactoferrin molecules seem to be involved in binding because glycoproteins with similar carbohydrate structures strongly inhibited binding. Scatchard plot analysis of the binding of peroxidase-labeled bovine lactoferrin to H. pylori cells yielded a kd 2.88 x 10(-6) M. In addition, binding of H. pylori cells to bovine lactoferrin was enhanced when bacteria treated with pepsin or alpha-chymotrypsin after isolation from iron-restricted and iron-containing media.

Lactoferrin interaction with Actinobacillus actinomycetemcomitans

The interaction of lactoferrin with Actinobacillus actinomycetemcomitans was examined in a 125I-labeled protein binding assay. The binding of human and bovine lactoferrins reached maximum within 1 h. Lactoferrin binding to the bacterium was pH-dependent and reversible. Scatchard analysis indicated the existence of two different types of binding sites on the bacterium, one with a high affinity constant k alpha approximately 8.8 x 10(-7) M) and the other with a low one (k alpha approximately 1.8 x 10(-6) M). Bacteria in the exponential phase of growth showed higher binding than cells in the stationary phase. Bacteria grown in medium containing serum and/or lysed erythrocytes bound lactoferrin to a lesser extent. Heat-inactivated serum, lysed erythrocytes and other proteins such as mucin and laminin inhibited lactoferrin binding to A. actinomycetemcomitans in a competitive binding assay. Sodium dodecyl sulfate polyacrylamide-gel electrophoresis and Western blot analysis of the cell envelope as well as the outer membrane of A. actinomycetemcomitans revealed lactoferrin-reactive protein bands at 29 kDa and 16.5 kDa. The 29-kDa band displayed a heat-modifiable lactoferrin-reactive form with a molecular weight of 34 kDa. Neither proteinase K-treated cell envelope nor lipopolysaccharide of this bacterium showed reactivity with lactoferrin. These data suggests a specific interaction of lactoferrin with outer membrane proteins of A. actinomycetemcomitans.

Lactoferrin interaction with salmonellae potentiates antibiotic susceptibility in vitro

Interaction of lactoferrin (Lf) with the cell envelope (CE) and outer membrane (OM) of Salmonella typhimurium-type strain ATCC13311 was tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western-blot analyses. The peroxidase-labeled bovine Lf (BLf) and human Lf both recognized a heat-modifiable protein with an estimated molecular mass of 38 kD in the OM. Simultaneous immunoblotting with an antiporin monoclonal antibody specific for a conserved porin domain in members of enterobacteriaceae confirmed that the Lf-binding protein is a porin. Such Lf-binding porin proteins (37-39 kD range) were readily detected in nine other common Salmonella species: S. dublin, S. panama, S. rostock, S. abony, S. hartford, S, kentucky, S. pullorum, S. thompson, and S. virchow. The latter six species also demonstrated one to three weak Lf-reactive bands of low molecular weight in their CE. The antibiotic susceptibility of Salmonella in the presence of Lf was examined. A mixture containing sub-minimum inhibitory concentration (MIC) levels of Lf (MIC/4) and cefuroxime (MIC/2) inhibited the bacterial growth. Lf strongly potentiated the action of erythromycin (eightfold), whereas it increased the activity only by two-fold for ampicillin, ciprofloxacin, chloramphenicol, and rifampicin; similarly, these antibiotics also reduced the MIC of BLf by twofold in S. typhimurium. Such antimicrobial potentiation was not observed with BLf mixtures containing cefalexin, gentamycin, or polymyxin B against strain ATCC13311. BLf and cefuroxime also demonstrated potentiation of varying degrees (two to 16-fold) with nine other Salmonella species. These data established the binding of Lf to porins in salmonellae and a potentiation effect of Lf with certain antibiotics.

Effect of lactoferrin on interaction of Prevotella intermedia with plasma and subepithelial matrix proteins

A lactoferrin-binding protein with an estimated molecular mass of 57 kDa was identified in the cell envelope of Prevotella intermedia by gel electrophoresis and Western-blot analysis. Peroxidase-labeled bovine lactoferrin and human lactoferrin showed similar specific binding to this protein. Whole cells of P. intermedia were also examined for interactions with 5 125I-labeled plasma and subepithelial matrix proteins. A high degree of binding was found with fibronectin, collagen type I and type IV and laminin, whereas a moderate interaction was detected with fibrinogen. The ability of bovine lactoferrin to affect the interactions of the above proteins with P. intermedia was examined. In the presence of unlabeled bovine lactoferrin, a dose-dependent inhibition of binding was observed with all 5 proteins tested. Unlabeled bovine lactoferrin also dissociated the bacterial complexes with these proteins. The complexes with laminin or collagen type I were more effectively dissociated than fibronectin or fibrinogen, whereas the interaction with collagen type IV was affected to a lesser extent. A strain-dependent variation in the effect of bovine lactoferrin was observed. These data establish the presence of a specific lactoferrin-binding protein in the cell envelope of P. intermedia. The ability of lactoferrin to inhibit the binding of some plasma and subepithelial matrix proteins to P. intermedia could be a protective mechanism against the establishment of this pathogen in the periodontal pocket.

Lactoferrin binds to porins OmpF and OmpC in Escherichia coli

Lactoferrin (Lf) is an iron-binding antimicrobial protein present in milk and on mucosal surfaces, with a suggested role in preimmune host defense. Certain strains of Escherichia coli (bacterial whole cells) demonstrate specific interaction with 125I-labeled Lf. A band with a mass of approximately 37 kDa, which was reactive with horseradish peroxidase-labeled Lf, was identified in the boiled cell envelope and outer membrane preparations of an Lf-binding E. coli strain, E34663, and a non-Lf-binding strain, HH45, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting (immunoblotting). Such a band was not detected in the unboiled native cell envelope and outer membrane preparations. The molecular mass and the property of heat modifiability suggested that the Lf-binding proteins were porins. The native trimeric form of porin OmpF isolated from strain B6 and its dissociated monomeric form both reacted with horseradish peroxidase-labeled Lf and with monoclonal antibodies specific for OmpF. Furthermore, by using E. coli constructs with defined porin phenotypes, OmpF and OmpC were identified as the Lf-binding proteins by urea-SDS-PAGE and Western blotting and by 125I-Lf binding studies with intact bacteria. These data establish that Lf binds to porins, a class of well-conserved molecules common in E. coli and many other gram-negative bacteria. However, in certain strains of E. coli these pore-forming proteins are shielded from Lf interaction.

The effects of lactoferrin on gram-negative bacteria

Lactoferrin is an iron-binding protein found in human mucosal secretions as well as the specific granules of polymorphonuclear leukocytes. A variety of functions have been ascribed to the protein, and it appears to contribute to antimicrobial host defense. In particular, it has been shown to have direct effects on pathogenic microorganisms including bacteriostasis and the induction of microbial iron uptake systems. Still its overall physiologic role remains to be defined. It has appeared logical that antimicrobial activity of the protein arises from sequestration of environmental iron thereby causing nutritional deprivation in susceptible organisms. This argument is buttressed by the finding that selected highly virulent pathogens have evolved techniques to subvert this effect and use the protein as an iron source. However, recent observations indicate that the protein has additional properties that contribute to host defense. Work by several groups has shown that the protein synergistically interacts with immunoglobins, complement, and neutrophil cationic proteins against Gram-negative bacteria. Further, both the whole protein and a cationic N-terminus peptide fragment directly damage the outer membrane of Gram-negative bacteria suggesting a mechanism for the supplemental effects. This review will summarize these diverse observations with a consideration of how the in vitro work relates to the physiological role of the protein.

Thermal behavior of bovine lactoferrin in water and its relation to bacterial interaction and antibacterial activity

Heat-induced enthalpy changes in different forms of bovine lactoferrin in water were examined by differential scanning calorimetry. Two thermal transitions with varying enthalpies were observed, depending on the iron-binding status of the protein. Iron-saturated lactoferrin was more resistant to heat-induced changes than was the apolactoferrin. Native lactoferrin had two transitional peaks, and pasteurization affected only the low temperature transition. Iron-saturated lactoferrin revealed a single transitional peak that was resistant to pasteurization. However, both protein forms were completely denatured by UHT. The effect of pasteurization and UHT on the protein interaction capacity with bacteria was examined in a 125I-labeled lactoferrin binding-inhibition assay. The ability of native and iron-saturated lactoferrins to bind various bacterial species was unaffected by pasteurization. However, UHT treatment decreased this interaction capacity. Native lactoferrins, both unheated and pasteurized, showed similar antibacterial properties and moderately inhibited Escherichia coli. However, this inhibitory capacity was lost after UHT treatment. Finally, iron-saturated lactoferrin did not inhibit bacterial growth; neither pasteurization nor UHT could change this property. Thus, UHT seems to affect structural as well as certain biological properties of both native and iron-saturated bovine lactoferrins, and pasteurization seems to be a treatment of choice for products containing this protein.

Relationship between antibacterial activity and porin binding of lactoferrin in Escherichia coli and Salmonella typhimurium

The effect of lactoferrin (Lf) on bacterial growth was tested by measuring conductance changes in the cultivation media by using a Malthus-AT system and was compared with the magnitude of 125I-labeled Lf binding in 15 clinical isolates of Escherichia coli. The binding property was inversely related to the change in bacterial metabolic rate (r = 0.91) and was directly related to the degree of bacteriostasis (r = 0.79). The magnitude of Lf-bacterium interaction showed no correlation with the MIC of Lf. In certain strains, Lf at supraoptimal levels reduced the bacteriostatic effect. Thus, the Lf concentration in the growth media was critical for the antibacterial effect. The cell envelopes of Salmonella typhimurium 395MS with smooth lipopolysaccharide (LPS) and its five isogenic rough mutants revealed 38-kDa porin proteins as peroxidase-labeled-Lf-reactive components in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (ligand blot) analysis. However, in the whole cell binding assay, parent strain 395MS demonstrated a very low interaction with 125I-Lf. On the other hand, Lf interaction gradually increased in correspondence with the decrease in LPS polysaccharide moiety in the isogenic rough mutants. Conductance measurement studies revealed that the low-level-Lf-binding (low-Lf-binding) strain 395MS with smooth LPS was relatively insusceptible to Lf, while the high-Lf-binding mutant Rd was more susceptible to Lf. These data suggested a correlation between Lf binding to porins and the Lf-mediated antimicrobial effect. The polysaccharide moiety of LPS shielded porins from the Lf interaction and concomitantly decreased the antibacterial effect.