Antibodies to iron-regulated outer membrane proteins of coliform bacteria isolated from bovine intramammary infections

Detection of immunoreactive proteins of Escherichia coli, Streptococcus uberis, and Streptococcus agalactiae isolated from cows with diagnosed mastitis

Introduction

Mastitis is a widespread mammary gland disease of dairy cows that causes severe economic losses to dairy farms. Mastitis can be caused by bacteria, fungi, and algae. The most common species isolated from infected milk are, among others, Streptococcus spp., and Escherichia coli. The aim of our study was protein detection based on both in silico and in vitro methods, which allowed the identification of immunoreactive proteins representative of the following species: Streptococcus uberis, Streptococcus agalactiae, and Escherichia coli.

Methods

The study group included 22 milk samples and 13 serum samples obtained from cows with diagnosed mastitis, whereas the control group constituted 12 milk samples and 12 serum samples isolated from healthy animals. Detection of immunoreactive proteins was done by immunoblotting, while amino acid sequences from investigated proteins were determined by MALDI-TOF. Then, bioinformatic analyses were performed on detected species specific proteins in order to investigate their immunoreactivity.

Results

As a result, we identified 13 proteins: 3 (molybdenum cofactor biosynthesis protein B, aldehyde reductase YahK, outer membrane protein A) for E. coli, 4 (elongation factor Tu, tRNA uridine 5-carboxymethylaminomethyl modification enzyme MnmG, GTPase Obg, glyceraldehyde-3-phosphate dehydrogenase) for S. uberis, and 6 (aspartate carbamoyltransferase, elongation factor Tu, 60 kDa chaperonin, elongation factor G, galactose-6-phosphate isomerase subunit LacA, adenosine deaminase) for S. agalactiae, which demonstrated immunoreactivity to antibodies present in serum from cows with diagnosed mastitis.

Discussion

Due to the confirmed immunoreactivity, specificity and localization in the bacterial cell, these proteins can be considered considered potential targets in innovative rapid immunodiagnostic assays for bovine mastitis, however due to the limited number of examined samples, further examination is needed.

Iron uptake by Escherichia coli cultured with antibodies from cows immunized with high-affinity ferric receptors

The synergistic effects of immunoglobulin G (IgG) from cows vaccinated with ferric citrate receptor (FecA) and IgG from cows vaccinated with ferric enterobactin receptor (FepA) were measured in an in vitro iron uptake assay. Serum was isolated and pooled within treatment from five cows each vaccinated with FepA or FecA or not vaccinated. Immunoglobulin G was isolated by ammonium sulfate precipitation and protein G affinity chromatography. Six Escherichia coli isolates from bovine intramammary infections were cultured in an iron-depleted medium to induce high-affinity iron acquisition systems and, in iron-depleted conditions, to specifically induce the expression of FecA. The bacterial cells were mixed with either 3 or 6 mg/mL of purified IgG and 55Fe. The radioactivity of 55Fe taken up by the bacterial cells was measured by a liquid scintillation counter after 5-, 10-, and 15-min incubations at 37 degrees C. The combination of anti-FecA IgG and anti-FepA IgG reduced 55Fe uptake compared with either anti-FecA or anti-FepA alone. Iron uptake was reduced more by anti-FecA IgG than by anti-FepA IgG when the ferric citrate system was induced. Reduction of iron uptake did not differ between anti-FepA alone and anti-FecA alone when citrate was absent from the medium.

Growth responses of Escherichia coli to immunoglobulin G from cows immunized with ferric citrate receptor, FecA

Effects of purified immunoglobulin (Ig) G from cows immunized with ferric citrate receptor, FecA, on the in vitro growth of Escherichia coli were investigated. Twenty-one cows were assigned to one of 3 treatments: 1) FecA immunization, 2) E. coli J5 bacterin immunization, and 3) unimmunized control. FecA was derived from E. coli UT5600/pSV66. Immunoglobulin G was purified from pooled colostral whey for each treatment group. The IgG from FecA immunized cows had higher titers against FecA compared with other treatment groups. Bacterial isolates tested were 14 E. coli from intramammary infections and E. coli UT5600/pSV66. Iron depletion decreased the growth of E. coli compared with growth in Fe-replete medium. The presence of IgG further decreased the growth compared with the growth under iron restriction alone. Bacterial growth did not differ among IgG sources nor between IgG concentrations. Replenishing media with exogenous iron overrode the inhibitory effects of the Fe-depletion and IgG. Vaccinating cows with FecA had little effect on the growth inhibitory properties of IgG toward E. coli mastitis isolates cultured in Fe-deplete media.

Opsonic activity of serum and whey from cows immunized with the ferric citrate receptor

The effects of immunizing dairy cows with the ferric citrate receptor, FecA, on the opsonic activity of serum and whey were measured in a phagocytosis assay. Fifteen cows were assigned to five blocks of three cows based on date of expected parturition. Cows within a block were randomly assigned to one of three treatments: 1) FecA immunization, 2) immunization with a commercially available Escherichia coli J5 bacterin, and 3) unimmunized controls. Cows were challenged at approximately 21 DIM by intramammary infusion of E. coli 727 into one mammary quarter. Escherichia coli 727 were opsonized for the phagocytosis assay with either 10% heat-inactivated serum or 50% heat-inactivated whey collected from each cow at calving, immediately before challenge and 7 d after challenge. Cows immunized with FecA or the E. coli J5 bacterin had increased IgG titers against FecA and E. coli 727 compared with unimmunized control cows. However, sera and whey collected from cows immunized with FecA did not enhance opsonization of E. coli 727 compared with sera and whey from control cows. Immunization with the E. coli J5 bacterin increased opsonization of sera greater than immunization with FecA. Immunoglobulin M antibody titer against E. coli 727 in whey and phagocytic indexes were positively correlated. The phagocytic index of whey immediately before challenge and 7 d after challenge were negatively associated with peak bacterial counts in mammary quarters challenged with E. coli 727. Results of the current trial suggest that the immune response resulting from immunization with FecA did not enhance opsonization and in vitro phagocytosis of E. coli 727.

Effect of immunoglobulin G from cows immunized with ferric citrate receptor (FecA) on iron uptake by Escherichia coli

The effects of immunoglobulin (Ig) G from cows immunized with the ferric citrate receptor (FecA) on iron uptake by Escherichia coli were investigated. Receptor FecA was purified from E. coli UT5600/pSV66. Cows were immunized with 400 microg purified FecA three times at 21 d intervals during late lactation and the nonlactating period. Immunoglobulin G was purified by protein G affinity chromatography from colostral whey from cows immunized with FecA and from unimmunized control cows. The purified IgG from FecA immunized cows had higher IgG titers against FecA compared with control IgG. Fifteen E. coli isolated from intramammary infections and E. coli UT5600/pSV66 were grown in an iron-depleted medium containing 1 mM citrate to induce FecA. The bacterial cells were mixed with 0, 2, and 4 mg/ml purified IgG, and 55Fe was added to the assay. After 5, 10, and 15 min incubations at 37 degrees C, samples were passed through 0.45-pm pore size filters. Filters were washed with saline three times, and the radioactivity of 55Fe taken up by the bacterial cells on the filters was measured by a liquid scintillation counter. The measurements were expressed as numbers of 55Fe atoms per colony-forming unit and transformed to log10. The assay was repeated three times for each isolate in a partially balanced incomplete block design. The presence of IgG decreased 55Fe uptake by E. coli mastitis isolates and E. coli UT5600/pSV66. Anti-FecA IgG reduced 55Fe uptake by E. coli greater than IgG from unimmunized cows.

Phagocytosis and serum susceptibility of Escherichia coil cultured in iron-deplete and iron-replete media

The susceptibility of Escherichia coli cultured in either iron-deplete or iron-replete media to phagocytosis by bovine neutrophils and the bactericidal activity of bovine serum was tested in vitro. Fourteen E. coli isolates from naturally occurring intramammary infections (IMI) were cultured overnight at 37 degrees C in iron-replete media and iron-deplete media. The iron-replete media were trypticase soy broth or a chemically defined medium. The iron-deplete media were either trypticase soy broth plus 0.2 mM alpha, alpha' dipyridyl and 1mM citrate, or the chemically defined medium plus 0.2 mM alpha, alpha' dipyridyl, and 1 mM citrate. Iron-replenished medium was the chemically defined iron-deplete medium plus 40 mM ferric citrate. Bacteria grown in iron-deplete media were less susceptible to phagocytosis compared with bacteria grown in iron-replete media. Replenishing the chemically defined iron-deplete medium with ferric citrate obliterated the decreased susceptibility to phagocytosis observed in iron-deplete media. The iron availability in media used to culture E. coli before assay did not affect the bactericidal action of either the classical pathway of complement or the antibody independent alternative pathway of complement in serum. The growth of bacteria in iron-deplete medium did not alter the expression of capsule compared with growth in iron-replete medium. Iron availability during culture of E. coli altered the susceptibility of isolates to phagocytosis by neutrophils, but had no effect on the susceptibility of isolates to the bactericidal activity of serum.

Outer membrane proteins: key players for bacterial adaptation in host niches

Outer membrane proteins (OMPs) of Gram-negative bacteria have diverse functions and are directly involved in the interaction with various environments encountered by pathogenic organisms. Thus, OMPs represent important virulence factors and play essential roles in bacterial adaptation to host niches, which are usually hostile to invading pathogens. Understanding the structure and functions of bacterial OMPs will facilitate the design of antimicrobial drugs and vaccines. In this paper, we will present a brief review on OMPs that contribute to bacterial adaptive responses including iron uptake, antimicrobial peptide resistance, serum resistance, and drug/bile resistance.

Antigenic homology of the inducible ferric citrate receptor (FecA) of coliform bacteria isolated from herds with naturally occurring bovine intramammary infections

Expression of ferric citrate receptor FecA by Escherichia coli and Klebsiella pneumoniae isolated from bovine mastitis was investigated. Transformant E. coli UT5600/pSV66, which produces large quantities of FecA in the presence of citrate, was constructed. The FecA of E. coli UT5600/pSV66 was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used to prepare polyclonal antiserum in rabbits. All coliform isolates of E. coli (n = 18) and K. pneumoniae (n = 17) from naturally occurring bovine intramammary infections in five herds induced iron-regulated outer membrane proteins when grown in Trypticase soy broth containing 200 microM alpha-alpha'-dipyridyl and 1 mM citrate. Polyclonal antiserum against FecA was used in conjunction with an immunoblot technique to determine the degree of antigenic homology of FecA among isolates. In the presence of citrate, each isolate expressed FecA that reacted with the anti-FecA polyclonal antiserum. The molecular mass of FecA ( approximately 80.5 kDa) was also highly conserved among isolates. Therefore, the ferric citrate iron transport may be induced in coliform bacteria and utilized to acquire iron in milk for survival and growth. The FecA is an attractive vaccine component for controlling coliform mastitis during the lactation period.

Immunization of cows with ferric enterobactin receptor from coliform bacteria

The serum and milk immunoglobulin (Ig) G responses of lactating dairy cows were determined following immunization with ferric enterobactin receptor FepA. Escherichia coli 471 was cultured in iron-depleted medium, and outer membrane proteins were extracted by 2% N-lauroylsarcosine sodium salt and 2% Triton X-100. The FepA was isolated from the outer membrane proteins by ion-exchange chromatography. Twenty cows were assigned to four treatment groups of 5 cows blocked by breed and days in milk. Treatment groups were vaccinated with 100 micrograms of FepA, 500 micrograms of FepA, Escherichia coli J5 bacterin, or sterile phosphate-buffered saline. Primary immunization was at approximately 200 d in milk, and booster immunizations were given 14 and 28 d later. Serum and whey IgG titers to FepA in cows vaccinated with FepA were significantly higher than those from cows vaccinated with either E. coli J5 bacterin or phosphate-buffered saline. Serum and whey IgG titers to FepA were elevated by 14 d in cows vaccinated with FepA. Significant differences were not observed between doses of FepA. The degree of cross-reactivity of purified IgG from cows vaccinated with FepA to E. coli and Klebsiella pneumoniae isolates was significantly higher than that to a control isolate that lacked FepA production. Immunization with FepA elicited an immunological response in serum and milk.

Inhibition of in vitro growth of coliform bacteria by a monoclonal antibody directed against ferric enterobactin receptor FepA

The ability of a murine monoclonal antibody that blocks the enterobactin ligand-binding site of the ferric enterobactin receptor FepA to inhibit the growth of coliform bacteria derived from a bovine intramammary infection (IMI) was determined in an iron-restricted medium. Bacterial isolates from bovine IMI in five herds were tested by the chrome azurol sulfonate assay to detect siderophore production. Each of the isolates of Escherichia coli (n = 25) and Klebsiella pneumoniae (n = 25) were positive for siderophore production. Each isolate expressed iron-regulated outer membrane proteins when grown in trypticase soy broth plus the iron chelator alpha-alpha'-dipyridyl. Immunoblots revealed that the monoclonal antibody recognized FepA that was expressed by each of the E. coli isolates (n = 25). Only 4 of 25 K. pneumoniae isolates produced FepA that reacted with the monoclonal antibody. This result coincided with the results of an in vitro growth assay. Growth of all E. coli isolates was significantly inhibited by the addition of monoclonal antibody to synthetic medium containing apolactoferrin. Antigenic variation in the enterobactin-binding site resulted in a low percentage of K. pneumoniae isolates that were inhibited by the monoclonal antibody. Inhibition of bacterial growth by the monoclonal antibody was dose-dependent. As little as 50 micrograms/ml of purified antibody had an inhibitory effect on bacterial growth in the synthetic iron-restricted medium. Monoclonal antibody specific for the enterobactin ligand-binding site of FepA inhibited the growth of E. coli that was isolated from bovine IMI.

Immunization and immunotherapy for mastitis

Immunization and immunotherapy for mastitis are active areas of investigation. The past decade has seen development of effective and economical R-mutant vaccines for gram-negative mastitis. These vaccines doubtless will prove beneficial on well managed dairies that have eradicated contagious mastitis pathogens. Development of vaccines for other mastitis pathogens has been noticeably slower. A commercially available Staphylococcus aureus vaccine appears to reduce the frequency and severity of clinical episodes, but probably has minimal impact on the incidence or prevalence of infection. This product has not been extensively studied. The recent recognition of virulence factors produced in vivo by Staphylococcus aureus may provide a breakthrough in the development and production of Staphylococcus aureus vaccines. Bacterins employing this principle presently are not commercially available, however. In the case of all contagious mastitis pathogens (Streptococcus agalactiae, Staphylococcus aureus, and Mycoplasma spp.), traditional control and eradication efforts (teat dip, dry cow therapy, culling programs) likely will prove preferable to long-term immunization. Ongoing research may provide more efficacious vaccines for these mastitis syndromes. Immunostimulants are an active area of research. Although leukopoietic factors appear promising as immunostimulants, no compound has clearly demonstrated efficacy in either the prevention or treatment of bovine mastitis.