Comparisons of Staphylococcus aureus grown in vitro or in vivo

In vitro pharmacokinetic/pharmacodynamic models in anti-infective drug development: focus on TB

For rapid anti-tuberculosis (TB) drug development in vitro pharmacokinetic/pharmacodynamic (PK/PD) models are useful in evaluating the direct interaction between the drug and the bacteria, thereby guiding the selection of candidate compounds and the optimization of their dosing regimens. Utilizing in vivo drug-clearance profiles from animal and/or human studies and simulating them in an in vitro PK/PD model allows the in-depth characterization of antibiotic activity of new and existing antibacterials by generating time–kill data. These data capture the dynamic interplay between mycobacterial growth and changing drug concentration as encountered during prolonged drug therapy. This review focuses on important PK/PD parameters relevant to anti-TB drug development, provides an overview of in vitro PK/PD models used to evaluate the efficacy of agents against mycobacteria and discusses the related mathematical modeling approaches of time–kill data. Overall, it provides an introduction to in vitro PK/PD models and their application as critical tools in evaluating anti-TB drugs.

Development of serological proteome analysis of mastitis by Staphylococcus aureus in ewes

Staphylococcus aureus is a major agent of mastitis in ruminants worldwide. So far, efficient measures for its prophylaxis (including vaccination) have proven to be unsuccessful and there is a need for a better understanding of the host response to udder infection by S. aureus. Serological proteome analysis (SERPA) is a promising technique that can be used to identify S. aureus immuno-dominant determinants providing that bacterial culture conditions used to grow S. aureus strains for protein sample preparation mimic the context of mastitis. A S. aureus strain was used in experimental mastitis to generate sheep serum used to determine the best growth conditions for SERPA. Sera collected in the field from different ewes suffering from mastitis by S. aureus were used to confirm experimental observations. Three different culture media (BHI, whey and iron-depleted RPMI) were tested. The influence of aeration and growth phase on protein production was also evaluated by immuno-detection of protein samples prepared from cultures grown in different conditions and obtained from different culture fractions (supernatant, cell wall, and total lysates). Our results showed that culturing in iron-depleted RPMI with (secreted proteins, prepared from stationary phase) or without aeration (cell wall proteins, prepared from early stationary phase, and total proteins, prepared from exponential phase) is the condition that best mimics growth in vivo during mastitis and this in vitro growth condition is to be used henceforth in experiments involving SERPA.

Pharmacodynamic modeling of ciprofloxacin resistance in Staphylococcus aureus

Three pharmacodynamic models of increasing complexity, designed for two subpopulations of bacteria with different susceptibilities, were developed to describe and predict the evolution of resistance to ciprofloxacin in Staphylococcus aureus by using pharmacokinetic, viable count, subpopulation, and resistance mechanism data obtained from in vitro system experiments. A two-population model with unique growth and killing rate constants for the ciprofloxacin-susceptible and -resistant subpopulations best described the initial killing and subsequent regrowth patterns observed. The model correctly described the enrichment of subpopulations with low-level resistance in the parent cultures but did not identify a relationship between the time ciprofloxacin concentrations were in the mutant selection window (the interval between the MIC and the mutant prevention concentration) and the enrichment of these subpopulations. The model confirmed the importance of resistant variants to the emergence of resistance by successfully predicting that resistant subpopulations would not emerge when a low-density culture, with a low probability of mutants, was exposed to a clinical dosing regimen or when a high-density culture, with a higher probability of mutants, was exposed to a transient high initial concentration designed to rapidly eradicate low-level resistant grlA mutants. The model, however, did not predict or explain the origin of variants with higher levels of resistance that appeared and became the predominant subpopulation during some experiments or the persistence of susceptible bacteria in other experiments where resistance did not emerge. Continued evaluation of the present two-population pharmacodynamic model and development of alternative models is warranted.

What in vitro models of infection can and cannot do

The science of pharmacodynamics analyzes the relationship between an antimicrobial's bactericidal effects and its pharmacokinetics. Ideally, randomized and well-controlled clinical trials are the best way to determine pharmacodynamic properties. However, in vitro models that recapitulate in vivo drug clearance profiles represent an increasingly important technology for carrying out pharmacodynamic studies in a more cost-effective, timely, and easily controlled fashion. Although in vitro pharmacodynamic models cannot incorporate all variables seen in vivo, they do provide valuable information for the drug development process and the determination of optimal dosing regimens.

A method for measuring specific antibodies in bovine lacteal secretions during the nonlactating period

A large portion of new IMI in dairy cattle occurs during the nonlactating period. Because antibiotic infusions at the beginning of the nonlactating period are only partially effective, attempts have been made to stimulate the production of protective antibodies in lacteal secretions during this period. However, measurement of antibodies in mammary secretions during the nonlactating period has been hampered by the complex, viscous nature of these secretions. This report describes the use of caprylic acid to clarify secretions from the bovine mammary gland during the nonlactating period to provide a more accurate measurement of specific antibody. Six healthy Jersey cows were injected in the area of the supramammary lymph node with an encapsulated strain of Staphylococcus aureus in dextran sulfate at the beginning of the nonlactating period and 15 and 30 d later. Seven healthy unimmunized Jersey cows served as controls. Lacteal secretions taken at the beginning of the nonlactating period; at 15, 30, and 45 d into the nonlactating period; and at calving were treated with caprylic acid prior to assay for specific antibodies using ELISA. Purified S. aureus capsule was used as the antigen in the ELISA. Caprylic acid lowered non-specific binding of IgG1 and IgM in secretions during the dry period from unimmunized control cows and lowered IgM from immunized cows. The most pronounced effect of caprylic acid was an increase in IgG2 binding in secretions from immunized cows. Treatment with caprylic acid more accurately measured specific activity of Ig in mammary secretions during the nonlactating period.

In vivo-like antigenic surface properties of Staphylococcus aureus from bovine mastitis induced upon growth in milk whey

Antigenic surface properties of Staphylococcus aureus strains grown in milk whey were compared with TSB-grown bacteria using immuno-gold electron microscopy. It is shown that colloidal gold (CG) particles coated with polyclonal antibody raised against Staphylococcus aureus surface antigen expressed in vivo bound to the surface of S. aureus strain F1440 grown in milk whey, but not to homologous bacteria grown in TSB. S. aureus strains grown in milk whey agglutinated in the presence of the polyclonal antibody, whereas the corresponding bacteria grown in TSB did not agglutinate. Immuno-gold particles did not bind to milk whey-grown bacteria treated with periodate. Periodate-treated milk whey-grown bacteria did not agglutinate in the presence of the polyclonal antibody, whereas periodate treatment had no effect on TSB-grown bacteria.

A field trial with an experimental vaccine against Staphylococcus aureus mastitis in cattle. 1. Clinical parameters

A total of 108 heifers were included in a placebo-controlled multicenter study on the use of an experimental Staphylococcus aureus mastitis vaccine containing whole, inactivated bacteria with pseudocapsule, alpha and beta toxoids, and a mineral oil as adjuvant. The heifers were injected in the area of the supramammary lymph nodes twice before calving and were observed and sampled throughout the first lactation. None of the vaccinated cows suffered from clinical Staph. aureus mastitis, and only 8.6% suffered from subclinical Staph. aureus mastitis, but a total of 16.0% of the control cows suffered from clinical or subclinical Staph. aureus mastitis. Mean SCC in vaccinated and control cows were the same throughout the lactation. Local swellings at the injection site were palpable in a substantial proportion of the vaccinated cows. In the statistical analyses, when cow was used as the unit of concern, no significant differences occurred between groups. However, when all parameters on udder health were considered together, the results indicated a potential protective effect of this vaccine during the entire lactation.

Recent advances in bovine vaccine technology

A description of new commercial and experimental vaccines for viral and bacterial diseases of cattle can be broadly divided into those used for both beef and dairy cows and those used predominantly in dairy cattle. For both types of cattle, newer and experimental vaccines are directed against several of the important viral (e.g., bovine herpesvirus 1, bovine viral diarrhea virus, bovine respiratory syncytial virus, parainfluenza type 3, and foot-and-mouth disease virus) and bacterial pathogens (e.g., Pasteurella spp., Haemophilus somnus). The viral vaccines include gene-deleted, modified live, subunit, and peptide antigens. Newer bacterial vaccines, particularly those for Pasteurella spp., are composed of either modified-live vaccines or bacterins supplemented with toxoid or surface antigens. Haemophilus somnus vaccine research has concentrated mainly on defining unique surface antigens. Novel dairy cow vaccines would include the lipopolysaccharide-core (J5) antigen approach, which has been used for successful immunization against coliform mastitis. Core antigen vaccines also have reduced calf mortality from Gram-negative pathogens. Staphylococcal mastitis vaccines that contain capsular antigens, toxoids, or the staphylococcal fibronectin receptor are of active research interest. Vaccines against mastitis induced by Streptococcus agalactiae and Streptococcus uberis also are areas of intensive research. Delivery of multiple subunit antigens with optimal immune response induction has led to the investigation of attenuated heterologous viral and bacterial expression vectors such as bovine herpesvirus 1, vaccinia, and Salmonella spp. This discussion also demonstrates that molecular biology is being used to advance bovine vaccine technology.

Effect of a Staphylococcus aureus bacterin on serum antibody, new infection, and mammary histology in nonlactating dairy cows

The influence of a Staphylococcus aureus mastitis vaccine on immunologic status and rate of new IMI was evaluated. At drying off, cows were vaccinated, either intramuscularly or subcutaneously in the area of the supramammary lymph node, or were left as unvaccinated controls; vaccinates received booster injections at 6 wk. Serum antibody concentrations, bacteriologic status, and SCC of quarter milk samples were determined. Four weeks after revaccination, cows were challenged by intramammary infusion of S. aureus and then killed 24 to 72 h later. Mean serum antistaphylococcal antibody titer of vaccinated cows during the trial was 4.7-fold that of controls. Challenge resulted in IMI rates of 92, 36, and 60% for control cows, cows vaccinated intramuscularly, and cows vaccinated in the area of the supramammary lymph node. Vaccination by either route had no influence on mammary parenchymal tissue components compared with controls; however, leukocyte infiltration was greater in quarters from cows vaccinated in the area of the supramammary lymph node than in quarters from unvaccinated controls. Plasma cell populations producing IgG1, IgG2, IgA, and IgM were greatest in quarters of cows vaccinated in the area of the supramammary lymph node followed by those in quarters of cows vaccinated intramuscularly and control cows.

Streptococcal products and leukocyte activities

Various streptococcal species are directly responsible for udder infections which should normally be countered by polymorphonuclear neutrophils (PMNs). In order to detect a putative inhibition of streptococcal products on the activities of bovine PMNs, we used a combination of four tests which permits an adequate evaluation of PMNs functions, e.g. PMN adherence on endothelial cells, chemotactic assay, phagocytosis of bacteria labelled with fluorescein isothiocyanate (FITC) and measurement of anion superoxide production. The conclusion is that neither of the two pathogenic streptococcal species isolated from mastitis appeared to produce in vitro factors affecting PMN activities.

Lysostaphin: use of a recombinant bactericidal enzyme as a mastitis therapeutic

A recombinant mucolytic protein, lysostaphin, was evaluated as a potential intramammary therapeutic for Staphylococcus aureus mastitis in dairy cattle. Lysostaphin, a product of Staphylococcus simulans, enzymatically degrades the cell wall of Staphylococcus aureus and is bactericidal. Thirty Holstein-Freisian dairy cattle in their first lactation were infected with Staphylococcus aureus (Newbould 305, ATCC 29740) in all quarters. Infections were established and monitored for somatic cell counts and Staphylococcus aureus colony-forming units 3 wk prior to subsequent treatment. Infected animals were injected through the teat canal with a single dose of recombinant lysostaphin (dose response 1 to 500 mg) or after three successive p.m. milkings with 100 mg of recombinant lysostaphin in 60 ml of sterile phosphate-buffered saline. Animals were considered cured if the milk remained free of Staphylococcus aureus for a total of 28 milkings after last treatment. Kinetic analysis of immunologically active recombinant lysostaphin demonstrated that a minimum bactericidal concentration was maintained in the milk for up to 36 to 48 h after a single infusion of 100 mg of recombinant lysostaphin. The cure rate of quarters receiving recombinant lysostaphin (100 mg in sterile phosphate-buffered saline, administered over three consecutive p.m. milkings) was 20% compared with 29% for sodium cephapirin in saline and 57% for a commercial antibiotic formulation, respectively. An improved formulation of recombinant lysostaphin may prove to be an effective alternative to antibiotic therapy for bovine mastitis.

Effect of anticapsular antibodies on neutrophil phagocytosis of Staphylococcus aureus

One of the major virulence factors of Staphylococcus aureus is development of an exopolysaccharide capsule in vivo, which inhibits recognition of antibodies to highly antigenic cell wall by neutrophils. To circumvent this inhibition, an attempt was made to produce anticapsular antibodies. Three cows per group were immunized in midlactation by injections in the area of the supramammary lymph node and intramuscularly and were boosted on d 14, 42, and 70 with three variants of Smith S. aureus: compact, unencapsulated; diffuse, rigid capsule; and diffuse large clearing, exceptionally large flaccid capsule using dextran sulfate as adjuvant. Serum agglutination and ELISA titers of cows immunized with diffuse and diffuse large clearing increased after immunization and after each boost and remained elevated to the end of the experiment at 112 d. Phagocytosis of diffuse and diffuse large clearing, measured by flow cytometry, was enhanced by immunization with either organism. No antibody response to capsule or enhanced phagocytosis of diffuse developed in cows immunized with compact. However, anticompact antibodies were opsonic for diffuse large clearing. These data show that bovine antibodies to S. aureus capsule are opsonic for bovine neutrophils and that capsule plays a role in inhibition of cell-wall opsonization of S. aureus.

Antibody response to Staphylococcus aureus surface proteins in rabbits with persistent osteomyelitis after treatment with demineralized bone implants

A rabbit model was used to study the effect of allogeneic demineralized bone powder (DBP) implants on the persistence of Staphylococcus aureus osteomyelitis. Thirty-one rabbits with chronic osteomyelitis of the tibia established by day 21, were started on systemic antibiotics followed by either no additional treatment or debridement plus either DBP (with or without supplemental antibiotics) or supplemental antibiotics only. On day 21, cultures showed a mean of 2 x 10(4) CFU/mg of debrided osseous material. By day 70, the treatment most effective in clearing infection was found in animals treated with supplemental antibiotics only (mean of 142 +/- 116 CFU/mg). In contrast, infection persisted at a 7- to 10-fold-higher level in animals receiving DBP with and without supplemental antibiotics; these results suggest that DBP contributed to persistence of infection. Longitudinal sera were tested again staphylococcal sonic extracts by immunoblot. Detection of numerous probe-positive bands indicated complex but remarkably similar antibody responses among infected animals. Antibodies attached directly to the cell surfaces of staphylococci as shown by immunogold and blocked the binding of organisms to HEp-2 and human fetal lung cells in a radioadherence assay. Antibodies could be absorbed out by intact organisms and were unreactive by immunoblot against antigens derived from cells pretreated with pronase, proteinase K, or lysostaphin. These results indicate that the major response was directed against staphylococcal cell surface proteins. Surprisingly, only one major band (molecular weight, approximately 12,000) was detected when a homologous in vivo antigen preparation was studied by immunoblot. Antibody reactive against this peptide did not appear to react with staphylococci grown in vitro.

Immunological cross-reactivity between pseudocapsular antigens of strains of Staphylococcus aureus isolated from cases of bovine mastitis

An assay was developed to measure the degree of immunological cross-reactivity between the pseudocapsules of a vaccine (reference) strain of Staphylococcus aureus and pseudocapsules of strains of S. aureus isolated from cases of bovine mastitis. The field strains were obtained from Australia, New Zealand, Norway, the U.K. and the U.S.A. There was large variation among strains in cross-reactivity of their pseudocapsules with those of the reference strain. For 104 Australian strains, the range of cross-reactivity indices (CRI) was 6.1-63.2% (on a scale of 0-100%, with 0% being complete identity and 100% being nil identity); for 61 overseas strains the range of CRI values was 25.7-72.1%. The data indicated that pseudocapsule antigens of Australian strains were antigenically more closely related to those of the reference strain than were pseudocapsular antigens of strains from the 4 other countries.

Colony morphology of Staphylococcus aureus in serum-soft agar following in vivo and in vitro growth

Staphylococcus aureus isolated from cases of mastitis in ruminants were grown in vitro and in vivo and subsequently examined for expression of diffuse colony morphology in serum-soft agar. Growth in the bovine mammary gland, but not in the ovine mammary gland or ovine peritoneal cavity, resulted in subsequent expression of diffuse colony morphology.

Synergism between antibody and neutrophils in the ruminant mammary gland

Immunological activities of the mammary gland are important both as a means of transferring immunity from mother to young and for defending the mammary gland itself against infection. The presence of immunoglobulins G1, G2 and A, and of neutrophils, macrophages and complement in the ruminant mammary gland is described, in particular the synergistic role of antibody and neutrophils is discussed and studies of immunization against staphylococcal mastitis are outlined.

Serological response of sheep to live and killed Staphylococcus aureus vaccines

Adult sheep were immunized intramuscularly with a killed, in vivo-grown Staphylococcus aureus vaccine combined either with dextran sulphate (group DD) or with Freund's incomplete adjuvant (group FF). Another group (LL) received an attenuated live S. aureus vaccine intracutaneously. The animals were given a primary vaccination followed two weeks later by a booster vaccination. A fourth group of sheep (group LD) was primed with the live vaccine and given a booster vaccination with the killed vaccine combined with dextran sulphate. ELISA was used to quantify blood serum levels of IgM, IgG1 and IgG2 antibody directed against the pseudocapsular antigens of S. aureus grown under in vivo conditions. Groups LD, DD, and FF had sharp increases in mean levels of IgM antibody in the first few weeks after vaccination with another large increase in mean values for group FF at 12 weeks after primary vaccination. Group LL showed virtually no increase in levels of IgG1 antibody; the other three groups had maximum mean values for IgG1 antibody at 5 weeks (FF) and 8 weeks (LD and DD) after primary vaccination. All groups had large IgG2 antibody responses (the largest for group LD), but the response for this isotype had waned by 14 weeks after primary vaccination. Examination of the ratios of IgG2 antibody to IgG1 antibody suggested that dextran sulphate may be a useful adjuvant for preferentially stimulating synthesis of IgG2 antibody against staphylococcal pseudocapsular antigens.

The immunophysiological basis for vaccinating ruminants against mastitis

The roles of humoral and cellular defence mechanisms in protection of the mammary gland against bacterial infection are reviewed. Effective protection depends on opsonisation of pathogens and subsequent phagocytosis by neutrophils. A concomitant requirement for protection is the rapid infiltration of neutrophils into the infected gland. Immunological studies have shown the need to prime animals against antigens expressed by bacteria when they grow in vivo. Vaccination procedures which promote these mammary defence mechanisms are discussed.

Evaluation of attenuated, live staphylococcal mastitis vaccine in lactating heifers

Four heifers were immunized in late pregnancy with two doses of attenuated, live Staphylococcus aureus and challenged during early lactation by intramammary infusion into one quarter of approximately 100 organisms of the same attenuated strain. Three unvaccinated control heifers were challenged similarly. At challenge immunoglobulins G1 and G2 antibodies against Staphylococcus aureus surface antigens were significantly greater in blood serum of vaccinated heifers than in controls. Also at challenge, serum from vaccinated heifers had a significantly greater opsonizing capacity for Staphylococcus aureus than did that of controls. The challenge dose of Staphylococcus aureus did not produce prolonged clinical signs of acute mastitis in any of the heifers; however, once of the control animals remained chronically infected. There was a decrease of milk production following challenge for controls but no such decrease for the immunized heifers. Taken together, results of clinical assessments, bacteriology, and measurements of milk production suggested that vaccinated heifers had higher resistance to the challenge dose than did controls.

Novel immunofluorescent technique for identification of ovine immunoglobulins and other potential opsonins binding to live Staphylococcus aureus

A qualitative in vitro technique has been developed for identification of potential opsonins (immunoglobulins G1, G2, A, M; the third component of ovine complement C3, and fibronectin) of sheep origin, binding to cell walls of viable Staphylococcus aureus. The technique uses monospecific antisera to these ovine proteins. The antisera are conjugated to FITC-protein A such that the protein A-binding sites in the Fc region of the immunoglobulin molecules are occupied with FITC-protein A complexes and are prevented, therefore, from binding to protein A in the staphylococcal cell wall. The technique is highly specific and sensitive, and once conjugated monospecific antisera are prepared, many tests can be done in a short time. Staphylococcus aureus incubated in samples of milk whey showed variable binding of immunoglobulins G1, G2, and M; immunoglobulin A was bound from only one sample of milk whey, and the third component of ovine complement C3- and fibronectin binding were not detected. Most samples of blood serum and colostral whey produced binding of immunoglobulins G1, G2, A, and M, and the third component of ovine complement C3 to the Staphylococcus aureus cell surface; fibronectin binding was found in serum but not in colostral whey. Washings from involuted mammary glands invariably produced binding of all immunoglobulins but not of the third component of ovine complement C3 or fibronectin.

Immunisation against experimental staphylococcal mastitis in sheep - effect of challenge with a heterologous strain of Staphylococcus aureus

Ewes were immunised in late pregnancy with killed Staphylococcus aureus vaccines prepared from organisms grown either under in vitro (vaccine T) or in vivo (Vaccine V) cultural conditions; other ewes were immunised with a live S. aureus vaccine and a further group remained non-vaccinated controls. The animals given either of the killed vaccines developed highest titres of agglutinating antibody in serum; there were only trivial levels of agglutinating antibody in milk from ewes in each treatment group. Ewes immunised with the live vaccine developed significantly greater levels of opsonins in serum than did those immunised with the killed vaccines or non-immunised controls. At 30 to 35 days post-partum the ewes were challenged by intramammary infusion of one million S. aureus of a strain different to the vaccination strain. In 4 of the 5 control ewes this resulted in the development of acute mastitis and a precipitous decline in milk production, whereas there was a considerable degree of resistance recorded in animals in each of the vaccinated groups. On criteria of milk production data, bacteriological status of milk and clinical signs of acute mastitis it was apparent that animals which had been immunised with the live vaccine were better protected from challenge than those immunised with either killed vaccines T or V.