Mouse mastitis model of infection for antimicrobial compound efficacy studies against intracellular and extracellular forms of Staphylococcus aureus

Rationally Designed Pyrimidine Compounds: Promising Novel Antibiotics for the Treatment of Staphylococcus aureus-Associated Bovine Mastitis

Staphylococcus aureus is one of the major pathogens causing bovine mastitis, and antibiotic treatment is most often inefficient due to its virulence and antibiotic-resistance attributes. The development of new antibiotics for veterinary use should account for the One Health concept, in which humans, animals, and environmental wellbeing are all interconnected. S. aureus can infect cattle and humans alike and antibiotic resistance can impact both if the same classes of antibiotics are used. New effective antibiotic classes against S. aureus are thus needed in dairy farms. We previously described PC1 as a novel antibiotic, which binds the S. aureus guanine riboswitch and interrupts transcription of essential GMP synthesis genes. However, chemical instability of PC1 hindered its development, evaluation, and commercialization. Novel PC1 analogs with improved stability have now been rationally designed and synthesized, and their in vitro and in vivo activities have been evaluated. One of these novel compounds, PC206, remains stable in solution and demonstrates specific narrow-spectrum activity against S. aureus. It is active against biofilm-embedded S. aureus, its cytotoxicity profile is adequate, and in vivo tests in mice and cows show that it is effective and well tolerated. PC206 and structural analogs represent a promising new antibiotic class to treat S. aureus-induced bovine mastitis.

Effective Treatment of Staphylococcus aureus Intramammary Infection in a Murine Model Using the Bacteriophage Cocktail StaphLyse™

Staphylococcus aureus causes intramammary infections (IMIs), which are refractory to antibiotic treatment and frequently result in chronic mastitis. IMIs are the leading cause of conventional antibiotic use in dairy farms. Phage therapy represents an alternative to antibiotics to help better manage mastitis in cows, reducing the global spread of resistance. A mouse mastitis model of S. aureus IMI was used to study the efficacy of a new cocktail of five lytic S. aureus-specific phages (StaphLyse™), administered either via the intramammary (IMAM) route or intravenously (IV). The StaphLyse™ phage cocktail was stable in milk for up to one day at 37 °C and up to one week at 4 °C. The phage cocktail was bactericidal in vitro against S. aureus in a dose-dependent manner. A single IMAM injection of this cocktail given 8 h after infection reduced the bacterial load in the mammary glands of lactating mice infected with S. aureus, and as expected, a two-dose regimen was more effective. Prophylactic use (4 h pre-challenge) of the phage cocktail was also effective, reducing S. aureus levels by 4 log10 CFU per gram of mammary gland. These results suggest that phage therapy may be a viable alternative to traditional antibiotics for the control of S. aureus IMIs.

Novel Proteoliposome-Based Vaccine against E. coli: A Potential New Tool for the Control of Bovine Mastitis

Simple Summary

Mastitis is a highly prevalent disease in dairy cattle, affecting animal welfare and generating economic losses for the dairy industry. Control measures for coliform mastitis are limited, due to the constant exposure of the teat to bacteria and the emergence of antimicrobial-resistant bacteria, making vaccination an important strategy for control of mastitis. However, currently available vaccines show limited efficacy, which could be attributed to inactivation processes that alter the antigenic preservation of the vaccines. The aim of this study was to assess the efficacy of a novel vaccine against mastitis using proteoliposomes obtained from E. coli in a murine model of coliform mastitis. We demonstrated that the proteoliposome vaccine was safe, immunogenic and effective against an experimental model of E. coli mastitis, decreasing bacterial count and tissue damage. This proteoliposome vaccine is a potential new tool for prevention of mastitis.

Abstract

Escherichia coli is an important causative agent of clinical mastitis in cattle. Current available vaccines have shown limited protection. We evaluated the efficacy of a novel vaccine based on bacterial proteoliposomes derived from an E. coli field strain. Female BALB/c mice were immunized subcutaneously with two doses of the vaccine, 3 weeks apart. Between days 5 and 8 after the first inoculation, the females were mated. At 5–8 days postpartum, the mice were intramammary challenged with the same E. coli strain. Two days after bacterial infection, mice were euthanized, and the mammary glands were examined and removed to evaluate the efficacy and safety of the vaccine as well as the immune response generated by the new formulation. The vaccinated mice showed mild clinical symptoms and a lower mammary bacterial load as compared to non-vaccinated animals. The vaccination induced an increase in levels of IgG, IgG1 and IgG2a against E. coli in blood and mammary glands that showed less inflammatory infiltration and tissue damage, as compared to the control group. In summary, the vaccine based on bacterial proteoliposomes is safe, immunogenic, and effective against E. coli, constituting a new potential tool for mastitis control.

Ex Vivo Pharmacokinetics and Pharmacodynamics Modeling and Optimal Regimens Evaluation of Cefquinome Against Bovine Mastitis Caused by Staphylococcus aureus

Cefquinome, the fourth-generation cephalosporin applied solely for veterinary medicine, is commonly used for bovine mastitis caused by Staphylococcus aureus. The present study aims to establish an optimal dose and provide a PK/PD Cutoff value (COPD) for cefquinome against S. aureus based on ex vivo pharmacokinetics and pharmacodynamics (PK/PD) integration. This study investigated the pharmacokinetics (PK) of cefquinome when administered as three consecutive intramammary (IMM) doses of cefquinome in three healthy dairy cows at 75 mg/gland. Drug concentration was determined by HPLC-MS/MS assay. The ex vivo pharmacodynamics (PD) of cefquinome were evaluated by using a milk sample from a PK experiment. The relationship between the AUC/ MIC of cefquinome and bacterial loading reduction was simulated using a Sigmoid Emax model. The cefquinome concentration in milk attained a maximum level of 1.55 ± 0.21 mg/mL at 1.8 h after the third administration. The mean value of the area under the concentration-time curve (AUC0−24) was 26.12 ± 2.42 mg·h/mL after the third administration. The elimination half-life was 10.6 h. For PD profile, the MICs of cefquinome in milk were 2–4 times higher than those in the broth. In vitro time-killing curve shows that initial bacterial concentration has a huge impact on antibacterial effect on three strains. The antibacterial effect was weakened with the initial bacterial concentration increasing from 106 to 108 CFU/mL. The AUC0−24h/MIC index correlated well with ex vivo efficacy both for the initial inoculum of 106 CFU/mL and 108 CFU/mL (R2 > 0.84). According to the inhibitory sigmoid Emax model analysis, the PK/PD surrogate (AUC0−24/MIC) values were 8,638, 1,397, and 3,851 for bactericidal effect (E = −3) with an initial inoculum of 106 CFU/mL, while the corresponding values were 12,266, 2,295, and 5,337, respectively, with the initial inoculum of 108 CFU/mL. The ex vivo PK/PD based population dose prediction indicated a target attainment rate (TAR) of 90% of 55 mg/gland/12 h. The COPD for cefquinome against S. aureus was 2 μg/mL under the recommended dose of 55 mg/gland/12 h. However, it should be validated in clinical practice in future investigations. These results contribute to the rational use of cefquinome for mastitis treatment in clinical veterinary medicine.

Phenotypic PIA-Dependent Biofilm Production by Clinical Non-Typeable Staphylococcus aureus Is Not Associated with the Intensity of Inflammation in Mammary Gland: A Pilot Study Using Mouse Mastitis Model

Simple Summary

Staphylococcus aureus-associated human clinical infections are predominantly caused by the encapsulated strains, with non-typeable strains representing less than 25%. In contrast, 80% of the S. aureus from bovine mastitis cases are non-typeable as they do not possess the Capsular Types 1, 2, 5, and 8. In our previous studies, it was demonstrated that the extent of mammary tissue damage was associated with the strength of biofilms formed by encapsulated S. aureus strains. This study assesses the impact of biofilm formation, as a virulence factor of non-typeable Staphylococcus aureus, causing mammary tissue damage in a mouse mastitis model. The study demonstrates no association between the strength of biofilm production by non-typeable S. aureus and the mammary tissue damage. However, the mice infected with strong biofilm producing non-typeable S. aureus died 6h earlier than those infected with weak biofilm producing non-typeable S. aureus suggesting the role of biofilm in the advancement of the time of mice mortality.

Abstract

Non-typeable (NT) Staphylococcus aureus strains are associated with chronic bovine mastitis. This study investigates the impact of biofilm formation by clinical NT S. aureus on cytokine production and mammary tissue damage by using a mouse mastitis model. Mice infected with two different NT S. aureus strains with strong and weak biofilm forming potential demonstrated identical clinical symptoms (moderate), minimal inflammatory infiltrates, and tissue damage (level 1 histopathological changes) in the mammary glands. However, the S. aureus load in the mammary glands of mice and the level of pro-inflammatory cytokines (IL-1β, IL-6, IL-12, IL-17 and IFN-γ) in serum were significantly higher (p ≤ 0.05) in those infected with the strong biofilm forming NT S. aureus strain. The level of IL-6 in sera samples of these mice was extremely high (15,479.9 ± 532 Pg/mL). Furthermore, these mice died in 24h of post infection compared to 30 h in the weak biofilm forming NT S. aureus infected group. The study demonstrates no association between the strength of PIA (polysaccharide intercellular adhesion)-dependent biofilm production by clinical NT S. aureus and mammary gland pathology in a mouse mastitis model. However, the role of biofilm in the virulence of S. aureus advancing the time of mortality in mice warrants further investigation.

Engineering a genome-reduced bacterium to eliminate Staphylococcus aureus biofilms in vivo

Bacteria present a promising delivery system for treating human diseases. Here, we engineered the genome‐reduced human lung pathogen Mycoplasma pneumoniae as a live biotherapeutic to treat biofilm‐associated bacterial infections. This strain has a unique genetic code, which hinders gene transfer to most other bacterial genera, and it lacks a cell wall, which allows it to express proteins that target peptidoglycans of pathogenic bacteria. We first determined that removal of the pathogenic factors fully attenuated the chassis strain in vivo. We then designed synthetic promoters and identified an endogenous peptide signal sequence that, when fused to heterologous proteins, promotes efficient secretion. Based on this, we equipped the chassis strain with a genetic platform designed to secrete antibiofilm and bactericidal enzymes, resulting in a strain capable of dissolving Staphylococcus aureus biofilms preformed on catheters in vitro, ex vivo, and in vivo. To our knowledge, this is the first engineered genome‐reduced bacterium that can fight against clinically relevant biofilm‐associated bacterial infections.

Relative virulence of Staphylococcus aureus bovine mastitis strains representing the main Canadian spa types and clonal complexes as determined using in vitro and in vivo mastitis models

Staphylococcus aureus is one of the main pathogens leading to both clinical and subclinical bovine mastitis in dairy cattle. Prediction of disease evolution based on the characteristics of Staph. aureus isolates that cause intramammary infections and understanding the host-pathogen interactions may improve management of mastitis in dairy herds. For this study, several strains were selected from each of the 6 major Canadian spa types associated with mastitis (t267, t359, t529, t605, t2445, and t13401). Adherence to host cells and intracellular persistence of these strains were studied using a bovine mammary gland epithelial cell line (MAC-T). Additionally, relative virulence and host response (cytokines production) were also studied in vivo using a mouse model of mastitis. Whole-genome sequencing was performed on all strains and associations between clonal complex, sequence type, and presence of certain virulence factors were also investigated. Results show that spa type t2445 was correlated with persistence in MAC-T cells. Strains from spa t359 and t529 showed better ability to colonize mouse mammary glands. The exception was strain sa3154 (spa t529), which showed less colonization of glands compared with other t359 and t529 strains but possessed the highest number of superantigen genes including tst. All strains possessed hemolysins, but spa types t529 and t2445 showed the largest diameter of β-hemolysis on blood agar plates. Although several spa types possessed 2 or 3 serine-aspartate rich proteins (Sdr) believed to be involved in many pathogenic processes, most t529 strains expressed only an allelic variant of sdrE. The spa types t605 (positive for the biofilm associated protein gene; bap+) and t13401 (bap-), that produced the largest amounts of biofilm in vitro, were the least virulent in vivo. Finally, strains from spa type t529 (ST151) elicited a cytokine expression profile (TNF-α, IL-1β and IL-12) that suggests a potential for severe inflammation. This study suggests that determination of the spa type may help predict the severity of the disease and the ability of the immune system to eliminate intramammary infections caused by Staph. aureus.

Secondary Staphylococcus aureus intramammary colonization is reduced by non-aureus staphylococci exoproducts

Non-aureus staphylococci (NAS) and Staphylococcus aureus are pathogens that cause bovine mastitis, a costly disease for dairy farmers, however; many NAS are considered part of the normal udder microbiota. It has been suggested that through a mechanism that remains to be elucidated, NAS intramammary colonization can prevent subsequent infection with other bacterial pathogens. This study shows that in a murine mastitis model, secondary Staph. aureus intramammary colonization is reduced by exoproducts from Staph. chromogenes and Staph. simulans, both NAS, while Streptococcus spp. exoproducts have much less ability to affect the course of the infection caused by S. aureus.

Invited review: A critical appraisal of mastitis vaccines for dairy cows

Infections of the mammary gland remain a frequent disease of dairy ruminants that negatively affect animal welfare, milk quality, farmer serenity, and farming profitability and cause an increase in use of antimicrobials. There is a need for efficacious vaccines to alleviate the burden of mastitis in dairy farming, but this need has not been satisfactorily fulfilled despite decades of research. A careful appraisal of past and current research on mastitis vaccines reveals the peculiarities but also the commonalities among mammary gland infections associated with the major mastitis pathogens Escherichia coli, Staphylococcus aureus, Streptococcus uberis, Streptococcus agalactiae, or Streptococcus dysgalactiae. A major pitfall is that the immune mechanisms of effective protection have not been fully identified. Until now, vaccine development has been directed toward the generation of antibodies. In this review, we drew up an inventory of the main approaches used to design vaccines that aim at the major pathogens for the mammary gland, and we critically appraised the current and tentative vaccines. In particular, we sought to relate efficacy to vaccine-induced defense mechanisms to shed light on some possible reasons for current vaccine shortcomings. Based on the lessons learned from past attempts and the recent results of current research, the design of effective vaccines may take a new turn in the years to come.

Zebrafish: An Attractive Model to Study Staphylococcus aureus Infection and Its Use as a Drug Discovery Tool

Non-mammalian in vivo disease models are particularly popular in early drug discovery. Zebrafish (Danio rerio) is an attractive vertebrate model, the success of which is driven by several advantages, such as the optical transparency of larvae, the small and completely sequenced genome, the small size of embryos and larvae enabling high-throughput screening, and low costs. In this review, we highlight zebrafish models of Staphyloccoccus aureus infection, which are used in drug discovery and for studying disease pathogenesis and virulence. Further, these infection models are discussed in the context of other relevant zebrafish models for pharmacological and toxicological studies as part of early drug profiling. In addition, we examine key differences to commonly applied models of S.aureus infection based on invertebrate organisms, and we compare their frequency of use in academic research covering the period of January 2011 to January 2021.

PK/PD Modeling to Assess Rifaximin Clinical Dosage in a Mouse Model of Staphylococcus aureus-Induced Mastitis

Staphylococcus aureus (S. aureus) is a common pathogen that causes mastitis, an infection of the milk-secreting tissue of the udder, in dairy cows, and presents a huge economic problem for the dairy industry worldwide. Thus, control and treatment of mastitis in dairy cows is vital in order to reduce the costs associated with the disease. The main purpose of the current work was to examine the current dosage of rifaximin for the treatment mastitis in cows caused by S. aureus using pharmacokinetic/pharmacodynamic integration in a mouse mastitis model. The mouse mastitis model was established via injection of S. aureus Newbould 305 (400 CFU/gland) into the mouse mammary gland. A single dose of 50, 100, 200, or 400 μg/gland, administered via intramammary infusion, was used to study the pharmacokinetics of rifaximin. The pharmacokinetic parameters were analyzed by non-compartment and non-linear mixed-effect models using Phoenix software (version 8.1; Pharsight, USA). In vivo pharmacodynamics was used to examine 18 therapeutic regimens covering various doses ranging from 25 to 800 μg/gland and three dosing intervals of 8, 12, and 24 h per 24 h experiment cycle. The antibacterial effect of rifaximin was elevated with higher concentrations of rifaximin or shorter intervals of administration. The percentage of time that drug concentrations exceeded the MIC during a dose interval (%T > MIC) was generally 100% for rifaximin and was not better than AUC₂₄/MIC in the sigmoid E_max model of inhibitory effect. The optimal antibacterial effect was 2log₁₀CFU/gland when the magnitude of AUC₂₄/MIC reached 14,281.63 h. A total of 14,281.63 h of AUC₂₄/MIC was defined as a target value in the Monte Carlo simulation. The clinically recommended dosage regimen of 100 mg/gland every 8 h in 1 day achieved an 82.97% cure rate for the treatment of bovine mastitis caused by Staphylococcus aureus infection.

A natural oil increases specific anti-OVA IgG levels and induces a cellular immune response combined with aluminum hydroxide

The efficacy of certain vaccines is improved by the use of adjuvants. Nowadays, the development of new, effective, and safe adjuvants that stimulate the innate immune response is researched. In this context, medicinal plants appear as a suitable alternative. Minthostachys verticillata essential oil (EO) has demonstrated the ability to modulate mechanisms of the innate immune response. Thus, the present work aimed to evaluate the EO adjuvant effect on humoral and cellular immunity, coadministered with OVA as antigen. The chemical analysis of EO by gas chromatography-mass spectrometry revealed a predominant pulegone-menthone chemotype. EO (1.25, 2.5, or 5.0 mg/ml) did not alter the viability of murine fibroblasts (3T3 cell line) neither showed signs of toxicity in Balb/c mice inoculated subcutaneously. The serum of mice immunized with OVA + EO showed increased levels of anti-OVA-specific antibodies of IgG1 subclass compared with the mice immunized with OVA alone revealing an adjuvant effect of EO. The delayed type hypersensitivity showed that the combination OVA + Al(OH)₃  + EO was the best to induce a cellular immune response that extended until 48 h postinjection of OVA. M. verticillata EO appears as a new, safe, and effective adjuvant, which should continue to be studied for their possible future incorporation into vaccine formulations.

Characterization of an Enterococcus faecium strain in a murine mastitis model

AIM

The aim of the study was to characterize phenotypically and genotypically Enterococcus faecium strains collected from bovine mastitis milk and to evaluate one of them for its virulence in a murine mastitis model.

METHODS AND RESULTS

A total of five E. faecium isolates were collected from cows with subclinical mastitis. EF-7A showed resistance to antibiotics tested, it presented alpha haemolysin and did not present gelatinase activity. It yielded cyA, efafm and gelE1 genes and it could be characterized as a moderate biofilm producer. It was able to internalize in MAC-T cells and 1×10⁸ colony forming unit ml^-1 was able to establish an intramammary infection in mice. The strain could be recovered from liver, kidney and blood samples. RAPD profiles showed different bands with respect to the inoculated strain. Histopathology analyses showed different grades of polymorphonuclear neutrophils infiltration in mammary glands.

CONCLUSION

This is the first report that studied E. faecium strain in a lactating mouse model of mastitis and showed that the experimental inoculation was able to stimulate an inflammatory response resulting in mastitis. Results contribute to a better understanding of intramammary infections caused by E. faecium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This investigation shows that mice represent a valuable model for the study of the mastitis pathogenesis caused by E. faecium considering the high costs of using cows for mastitis research.

A Novel Peptide Nucleic Acid against the Cytidine Monophosphate Kinase of S. aureus Inhibits Staphylococcal Infection In Vivo

Here, we report a novel bactericidal peptide nucleic acid (PNA) that can induce the antisense effect on the cytidine monophosphate kinase (Cmk) of Staphylococcus aureus, a putative essential component for bacterial species. Based on the genome sequence of S. aureus N315, a set of PNA conjugates with a bacterial penetration peptide, (KFF)₃K, were synthesized to target the seven potentially essential genes (cmk, deoD, ligA, smpB, glmU, pyrH, and ftsA) and further evaluated for their antibacterial properties in vitro as well as in vivo. The results demonstrated that two peptide-conjugated PNAs (P-PNAs), antisense P-PNA (ASP)-cmk1 and ASP-deoD1, targeting either the cmk or the deoD genes, had the strongest inhibitory effects on the growth of S. aureus ATCC 29740 (a bovine mastitic milk isolate) in a dose-dependent manner. In vivo application of ASP-cmk1 resulted in a significant reduction of bacterial loads in mice intraperitoneally infected with a sublethal dose of S. aureus. Moreover, ASP-cmk1 significantly increased the survival rate of the breast-fed infant mice after intramammary infection of the lactating CD-1 mice. Taken together, our characterization of ASP-cmk1 demonstrated its bactericidal activity against S. aureus as well as its effectiveness in vivo.

Staphylococcal Enterotoxin C Is an Important Virulence Factor for Mastitis

Staphylococcus aureus is an important bacterial pathogen causing bovine mastitis, but little is known about the virulence factor and the inflammatory responses in the mammary infection. Staphylococcal enterotoxin C (SEC) is the most frequent toxin produced by S. aureus, isolated from bovine mastitis. To investigate the pathogenic activity of SEC in the inflammation of the mammary gland and the immune responses in an animal model, mouse mammary glands were injected with SEC, and the clinical signs, inflammatory cell infiltration, and proinflammatory cytokine production in the mammary glands were assessed. SEC induced significant inflammatory reactions in the mammary gland, in a dose-dependent manner. SEC-injected mammary glands showed a severe inflammation with inflammatory cell infiltration and tissue damage. In addition, interleukin (IL)-1β and IL-6 production in the SEC-injected mammary glands were significantly higher than those in the PBS control glands. Furthermore, the SEC-induced inflammation and tissue damage in the mammary gland were specifically inhibited by anti-SEC antibody. These results indicated, for the first time, that SEC can directly cause inflammation, proinflammatory cytokine production, and tissue damage in mammary glands, suggesting that SEC might play an important role in the development of mastitis associated with S. aureus infection. This finding offers an opportunity to develop novel treatment strategies for reduction of mammary tissue damage in mastitis.

Nanocurcumin ameliorates Staphylococcus aureus-induced mastitis in mouse by suppressing NF‑κB signaling and inflammation

Mastitis is the inflammation of the mammary glands caused by bacteria. It causes severe economic loss to dairy industry. Curcumin, a polyphenol obtained from turmeric, has considerable anti-inflammatory effect. Since it is rapidly eliminated from the body, its oral bioavailability is low. However, nanoformulation of curcumin significantly enhances its therapeutic efficiency by improving its oral bioavailability. We evaluated whether nanocurcumin could be more effective than normal curcumin against bovine Staphylococcus aureus mastitis in mouse model. Curcumin-loaded PLGA nanoparticles (CUR-NP) were prepared by solid-in-oil-in-water emulsion method. The mouse model of mastitis was induced by inoculation of a field strain of S. aureus (bovine mastitis isolate) on the 9th day of parturition through the duct of the mammary gland. CUR-NP and curcumin were given orally for 7 days (day 2 to day 8 of parturition) prior to S. aureus inoculation. We determined the levels of inflammatory cytokines and the mRNA expression of NF‑κB. S. aureus infection increased the levels of tumor necrosis factor‑α, interleukin‑1β and myeloperoxidase in mammary tissues and C-reactive protein in serum. Both CUR-NP and curcumin significantly attenuated the levels of these cytokines. However, comparatively, the ameliorative efficiency of CUR-NP was better than normal curcumin. S. aureus infection-induced NF‑κB mRNA expression was significantly reduced to the healthy control level by CUR-NP. Our study demonstrates that the nanoformulation of curcumin can reduce pro-inflammatory mediators in S. aureus-infected mammary tissues by improving NF‑κB signaling. Besides, compared to normal curcumin, this nanoformulation appears to be a better alternative against murine mastitis.

Staphylococcus aureus Extracellular Vesicles Elicit an Immunostimulatory Response in vivo on the Murine Mammary Gland

Staphylococcus aureus is a major pathogen responsible for bovine mastitis, the most common and costly disease affecting dairy cattle. S. aureus naturally releases extracellular vesicles (EVs) during its growth. EVs play an important role in the bacteria-bacteria and bacteria-host interactions and are notably considered as nanocarriers that deliver virulence factors to the host tissues. Whether EVs play a role in a mastitis context is still unknown. In this work, we showed that S. aureus Newbould 305 (N305), a bovine mastitis isolate, has the ability to generate EVs in vitro with a designated protein content. Purified S. aureus N305-secreted EVs were not cytotoxic when tested in vitro on MAC-T and PS, two bovine mammary epithelial cell lines. However, they induced the gene expression of inflammatory cytokines at levels similar to those induced by live S. aureus N305. The in vivo immune response to purified S. aureus N305-secreted EVs was tested in a mouse model for bovine mastitis and their immunogenic effect was compared to that of live S. aureus N305, heat-killed S. aureus N305 and to S. aureus lipoteichoic acid (LTA). Clinical and histopathological signs were evaluated and pro-inflammatory and chemotactic cytokine levels were measured in the mammary gland 24 h post-inoculation. Live S. aureus induced a significantly stronger inflammatory response than that of any other condition tested. Nevertheless, S. aureus N305-secreted EVs induced a dose-dependent neutrophil recruitment and the production of a selected set of pro-inflammatory mediators as well as chemokines. This immune response elicited by intramammary S. aureus N305-secreted EVs was comparable to that of heat-killed S. aureus N305 and, partly, by LTA. These results demonstrated that S. aureus N305-secreted EVs induce a mild inflammatory response distinct from the live pathogen after intramammary injection. Overall, our combined in vitro and in vivo data suggest that EVs are worth to be investigated to better understand the S. aureus pathogenesis and are relevant tools to develop strategies against bovine S. aureus mastitis.

Effective Antimicrobial Activity of Plectasin-Derived Antimicrobial Peptides against Staphylococcus aureus Infection in Mammary Glands

Staphylococcus aureus (S. aureus) is the causative agent for a wide variety of illnesses ranging from minor skin infections to life-threatening diseases. Development of antibiotic resistance by the bacteria has rendered many antibiotics ineffective. It has been known that plectasin-derived antimicrobial peptides (AMPs; NZ2114 and MP1102) are promising alternatives to antibiotics. However, their activities against S. aureus in mammary glands were unknown. Our objective was to assess the antimicrobial activities of NZ2114 and MP1102 against S. aureus in milk, in cultured mammary epithelial cells, and in a mouse model in order to evaluate their potentials as anti-mastitis agents. NZ2114 and MP1102 showed in vitro bactericidal effects against S. aureus in both the culture medium and the milk. NZ2114 and MP1102 at the concentration of 100 μg/mL reduced the number of S. aureus by almost 100% within 4 h in processed bovine milk. Similarly, both NZ2114 and MP1102 were efficient to reduce the number of internalized S. aureus in cultured mammary epithelial cells. Finally, both AMPs significantly reduced the S. aureus load and concentrations of TNF-α and IL-6 in mammary glands, compared to a buffer control in the mouse model. Our results suggest that NZ2114 and MP1102 may be used to treat S. aureus-induced mastitis.

Efficacy and Safety of a Bovine-Associated Staphylococcus aureus Phage Cocktail in a Murine Model of Mastitis

Overuse of antibiotics is a major problem in the treatment of bovine mastitis, and antibiotic treatment is frequently non-curative, thus alternative treatments are necessary. The primary aim of this study was to evaluate the efficacy of a purified phage cocktail for treatment of bovine Staphylococcus aureus mastitis in a well-defined mouse model. Candidate phages were selected based on their in vitro performance and subsequently processed into an optimally composed phage cocktail. The highest scoring phages were further tested for efficacy and resistance suppression in broth and raw milk, with and without supplemental IgG. As these in vitro results displayed significant decreases in CFU, the cocktail was purified for testing in vivo. Lactating mice were intramammarily inoculated with S. aureus N305 (ATCC 29740), a clinical bovine mastitis isolate commonly used for experimental infection of dairy cows. The phage cocktail was applied via the same route 4 h post-inoculation. Treated mammary glands were graded for gross pathological appearance and excised for bacterial and phage load quantification as well as histopathology. Observation of gross macroscopic and histopathological changes and CFU quantification demonstrated that the phage cocktail treatment significantly improved mastitis pathology and decreased bacterial counts. Phage PFU quantification indicated that the tested phage cocktail treatment was able to maintain high intramammary phage titers without spreading systemically. The in vivo results complement the in vitro data and support our concept of phage therapy as an innovative alternative or supplementation therapy to antibiotics for the treatment of bovine mastitis.

The Electron Transport Chain Sensitizes Staphylococcus aureus and Enterococcus faecalis to the Oxidative Burst

Small-colony variants (SCVs) of Staphylococcus aureus typically lack a functional electron transport chain and cannot produce virulence factors such as leukocidins, hemolysins, or the antioxidant staphyloxanthin. Despite this, SCVs are associated with persistent infections of the bloodstream, bones, and prosthetic devices. The survival of SCVs in the host has been ascribed to intracellular residency, biofilm formation, and resistance to antibiotics. However, the ability of SCVs to resist host defenses is largely uncharacterized. To address this, we measured the survival of wild-type and SCV S. aureus in whole human blood, which contains high numbers of neutrophils, the key defense against staphylococcal infection. Despite the loss of leukocidin production and staphyloxanthin biosynthesis, SCVs defective for heme or menaquinone biosynthesis were significantly more resistant to the oxidative burst than wild-type bacteria in human blood or the presence of purified neutrophils. Supplementation of the culture medium of the heme-auxotrophic SCV with heme, but not iron, restored growth, hemolysin and staphyloxanthin production, and sensitivity to the oxidative burst. Since Enterococcus faecalis is a natural heme auxotroph and cause of bloodstream infection, we explored whether restoration of the electron transport chain in this organism also affected survival in blood. Incubation of E. faecalis with heme increased growth and restored catalase activity but resulted in decreased survival in human blood via increased sensitivity to the oxidative burst. Therefore, the lack of functional electron transport chains in SCV S. aureus and wild-type E. faecalis results in reduced growth rate but provides resistance to a key immune defense mechanism.

A mouse mastitis model to study the effects of the intramammary infusion of a food-grade Lactococcus lactis strain

Lactococcus lactis is one of the most important microorganisms in the dairy industry and has “generally recognized as safe” (GRAS) status. L. lactis belongs to the group of lactic acid bacteria (LAB) and is encountered in a wide range of environments. Recently, the use of the intramammary infusion of a live culture of LAB has been investigated as a new antibiotic alternative for treating mastitis in dairy ruminants. Controversial results are described in literature regarding its efficacy and safety. In this study we conducted in-depth investigation of the mammary gland immune response induced by intramammary inoculum of a live culture of L. lactis LMG 7930 using the mouse mastitis model. Overnight cultures either of L. lactis (≈ 10⁷ CFU) or of the mastitis pathogens Staphylococcus chromogenes (≈ 10⁵ CFU) or S. aureus (≈ 10² CFU/ml) were injected into the mouse inguinal glands. A double injection, consisting of S. chromogenes first and then L. lactis, was also investigated. Bacterial recovery from the gland and inflammatory cell infiltration were assessed. L. lactis-treated and control glands were analysed for proinflammatory cytokine production. Microbiological results showed that L. lactis was able to survive in the mammary gland 24 h post infection, as were the mastitis pathogens S. chromogenes and S. aureus. L. lactis reduced S. chromogenes survival in the glands and increased its own survival ability by coexisting with the pathogen. Histology showed that L. lactis-treated glands presented variable histological features, ranging from undamaged tissue with no inflammatory cell infiltrate to severe PMN infiltrate with focal areas of tissue damage. S. aureus-treated glands showed the most severe histological grade of inflammation despite the fact that the inoculum size was the smallest. In contrast, most S. chromogenes-treated glands showed normal structures with no infiltration or lesions. Significant increases in IL-1β and TNF-α levels were also found in L. lactis-inoculated glands. The above findings seem to suggest that food-grade L. lactis at a high-inoculum dose such as an overnight culture may elicit a suppurative inflammatory response in the mammary gland, thus becoming a potential mastitis-causing pathogen. Because of the unpredictable potential of L. lactis in acting as a potential mastitis pathogen, this organism cannot be considered a safe treatment for bovine mastitis.

Antibacterial, anti‐inflammatory and antioxidant effects of acetyl‐11‐α‐keto‐β‐boswellic acid mediated silver nanoparticles in experimental murine mastitis

Mastitis is an important economic disease causing production losses in dairy industry. Antibiotics are becoming ineffective in controlling mastitis due to the emergence of resistant strains requiring the development of novel therapeutic agents. In this study, the authors present the phytochemical synthesis of silver nanoparticles (AgNPs) with acetyl‐11‐α‐keto‐β‐boswellic acid and evaluation of their activity in Staphylococcus aureus induced murine mastitis. Boswellic acid mediated AgNP (BANS) were oval, polydispersed (99.8 nm) with an minimum inhibitory concentration of 0.033 µg ml−1 against S. aureus , inhibitory concentration (IC50) of 30.04 µg ml−1 on mouse splenocytes and safe at an in vivo acute oral dose of 3.5 mg kg−1 in mice. Mastitis was induced in lactating mice by inoculating S. aureus (log10 5.60 cfu) and treated 6 h post‐inoculation with BANS (0.12 mg kg−1, intramammary and intraperitoneal), and cefepime (1 mg kg−1, intraperitoneal). S. aureus inoculated mice showed increased bacterial load, neutrophil infiltration in mammary glands and elevated C‐reactive protein (CRP) in serum. Oxidative stress was also observed with elevated malondialdehyde level, superoxide dismutase (SOD) and catalase (CAT) activities. BANS treatment significantly (P  < 0.05) reduced bacterial load, CRP, SOD, CAT activities and neutrophil infiltration in affected mammary glands. BANS could be a potential therapeutic agent for managing bovine mastitis.

Antibiofilm and antibacterial effects of specific chitosan molecules on Staphylococcus aureus isolates associated with bovine mastitis

Staphylococcus aureus is one of the major pathogens causing bovine intramammary infections (IMIs) and mastitis. Mastitis is the primary cause for the use of antibiotics in dairy farms but therapeutic failure is often observed. One of the reasons for the lack of effectiveness of antibiotic therapy despite the observed susceptibility of bacterial isolates in vitro are bacterial biofilms. In this study, we used chitosan of well-defined molecular weight (0.4–0.6, 1.3, 2.6 and 4.0 kDa) and investigated their antibiofilm and antibacterial activities in in vitro and in vivo models related to S. aureus IMIs. A chitosan of at least 6 units of glucosamine was necessary for maximum antibacterial activity. The 2.6 and 4.0 kDa forms were able to prevent biofilm production by the biofilm hyperproducer strain S. aureus 2117 and a bovine MRSA (methicillin-resistant S. aureus). The intramammary administration of the 2.6 kDa chitosan showed no adverse effects in mice or in cows, as opposed to the slight inflammatory effect observed in mammary glands with the 4.0 kDa derivative. The 2.6 kDa chitosan killed bacteria embedded in pre-established biofilms in a dose-dependent manner with a >3 log10 reduction in CFU at 4 mg/ml. Also, the 2.6 kDa chitosan could prevent the persistence of the internalized MRSA into the mammary epithelial cell line MAC-T. An in vitro checkerboard assay showed that the 2.6 kDa chitosan produced a synergy with the macrolide class of antibiotics (e.g., tilmicosin) and reduced the MIC of both molecules by 2–8 times. Finally, the intramammary administration of the 2.6 kDa chitosan alone (P<0.01) or in combination with tilmicosin (P<0.0001) reduced the colonization of mammary glands in a murine IMI model. Our results suggest that the use of chitosan alone or in combination with a low dose of a macrolide could help reduce antibiotic use in dairy farms.

Mammary Gland Pathology Subsequent to Acute Infection with Strong versus Weak Biofilm Forming Staphylococcus aureus Bovine Mastitis Isolates: A Pilot Study Using Non-Invasive Mouse Mastitis Model

Background

Biofilm formation by Staphylococcus aureus is an important virulence attribute because of its potential to induce persistent antibiotic resistance, retard phagocytosis and either attenuate or promote inflammation, depending upon the disease syndrome, in vivo. This study was undertaken to evaluate the potential significance of strength of biofilm formation by clinical bovine mastitis-associated S. aureus in mammary tissue damage by using a mouse mastitis model.

Methods

Two S. aureus strains of the same capsular phenotype with different biofilm forming strengths were used to non-invasively infect mammary glands of lactating mice. Biofilm forming potential of these strains were determined by tissue culture plate method, ica typing and virulence gene profile per detection by PCR. Delivery of the infectious dose of S. aureus was directly through the teat lactiferous duct without invasive scraping of the teat surface. Both bacteriological and histological methods were used for analysis of mammary gland pathology of mice post-infection.

Results

Histopathological analysis of the infected mammary glands revealed that mice inoculated with the strong biofilm forming S. aureus strain produced marked acute mastitic lesions, showing profuse infiltration predominantly with neutrophils, with evidence of necrosis in the affected mammary glands. In contrast, the damage was significantly less severe in mammary glands of mice infected with the weak biofilm-forming S. aureus strain. Although both IL-1β and TNF-α inflammatory biomarkers were produced in infected mice, level of TNF-α produced was significantly higher (p<0.05) in mice inoculated with strong biofilm forming S. aureus than the weak biofilm forming strain.

Conclusion

This finding suggests an important role of TNF-α in mammary gland pathology post-infection with strong biofilm-forming S. aureus in the acute mouse mastitis model, and offers an opportunity for the development of novel strategies for reduction of mammary tissue damage, with or without use of antimicrobials and/or anti-inflammatory compounds for the treatment of bovine mastitis.

In Vivo Pharmacokinetics/Pharmacodynamics of Cefquinome in an Experimental Mouse Model of Staphylococcus Aureus Mastitis following Intramammary Infusion

Staphylococcus aureus remains the major cause of morbidity of bovine mastitis worldwide leading to massive economic losses. Cefquinome is a fourth generation cephalosporin, which preserves susceptibility and antibacterial activity against S. aureus. This work aims to study the pharmacokinetic (PK) and pharmacodynamic (PD) modeling following intramammary administration of cefquinome against S. aureus mastitis. The mouse model of S. aureus mastitis was developed for the PK/PD experiments. The plasma PK characteristics after intramammary injection of cefquinome at various single doses of 25, 50, 100, 200, 400 μg per gland (both fourth pairs of glands: L4 and R4) were calculated using one-compartment and first-order absorption model. PD study was investigated based on twenty-one intermittent dosing regimens, of which total daily dose ranged from 25 to 4800 μg per mouse and dosage intervals included 8, 12 or 24 h. The sigmoid Emax model of inhibitory effect was employed for PK/PD modeling. The results of PK/PD integration of cefquinome against S. aureus suggested that the percentage of duration that drug concentration exceeded the minimal inhibitory concentration (%T>MIC) and the ratio of area under time-concentration curve over MIC (AUC/MIC) are important indexes to evaluate the antibacterial activity. The PK/PD parameters of %T>MIC and AUC0-24/MIC were 35.98% and 137.43 h to obtain a 1.8 logCFU/gland reduction of bacterial colony counts in vivo, against S. aureus strains with cefquinome MIC of 0.5μg/ml.

Synergistic streptococcal phage λSA2 and B30 endolysins kill streptococci in cow milk and in a mouse model of mastitis

Bovine mastitis results in billion dollar losses annually in the USA alone. Streptococci are among the most relevant causative agents of this disease. Conventional antibiotic therapy is often unsuccessful and contributes to development of antibiotic resistance. Bacteriophage endolysins represent a new class of antimicrobials against these bacteria. In this work, we characterized the endolysins (lysins) of the streptococcal phages λSA2 and B30 and evaluated their potential as anti-mastitis agents. When tested in vitro against live streptococci, both enzymes exhibited near-optimum lytic activities at ionic strengths, pH, and Ca(2+) concentrations consistent with cow milk. When tested in combination in a checkerboard assay, the lysins were found to exhibit strong synergy. The λSA2 lysin displayed high activity in milk against Streptococcus dysgalactiae (reduction of CFU/ml by 3.5 log units at 100 μg/ml), Streptococcus agalactiae (2 log), and Streptococcus uberis (4 log), whereas the B30 lysin was less effective. In a mouse model of bovine mastitis, both enzymes significantly reduced intramammary concentrations of all three streptococcal species (except for B30 vs. S. dysgalactiae), and the effects on mammary gland wet weights and TNFα concentrations were consistent with these findings. Unexpectedly, the synergistic effect determined for the two enzymes in vitro was not observed in the mouse model. Overall, our results illustrate the potential of endolysins for treatment of Streptococcus-induced bovine mastitis.

Staphylococcus aureus adapts to oxidative stress by producing H2O2-resistant small-colony variants via the SOS response

The development of chronic and recurrent Staphylococcus aureus infections is associated with the emergence of slow-growing mutants known as small-colony variants (SCVs), which are highly tolerant of antibiotics and can survive inside host cells. However, the host and bacterial factors which underpin SCV emergence during infection are poorly understood. Here, we demonstrate that exposure of S. aureus to sublethal concentrations of H₂O₂ leads to a specific, dose-dependent increase in the population frequency of gentamicin-resistant SCVs. Time course analyses revealed that H₂O₂ exposure caused bacteriostasis in wild-type cells during which time SCVs appeared spontaneously within the S. aureus population. This occurred via a mutagenic DNA repair pathway that included DNA double-strand break repair proteins RexAB, recombinase A, and polymerase V. In addition to triggering SCV emergence by increasing the mutation rate, H₂O₂ also selected for the SCV phenotype, leading to increased phenotypic stability and further enhancing the size of the SCV subpopulation by reducing the rate of SCV reversion to the wild type. Subsequent analyses revealed that SCVs were significantly more resistant to the toxic effects of H₂O₂ than wild-type bacteria. With the exception of heme auxotrophs, gentamicin-resistant SCVs displayed greater catalase activity than wild-type bacteria, which contributed to their resistance to H₂O₂. Taken together, these data reveal a mechanism by which S. aureus adapts to oxidative stress via the production of a subpopulation of H₂O₂-resistant SCVs with enhanced catalase production.

Expression of acute phase proteins and inflammatory cytokines in mouse mammary gland followingStaphylococcus aureuschallenge and in response to milk accumulation

We used a mouse model of pathogenic (Staphylococcus aureus) and non-pathogenic (teat sealing) mammary inflammation to investigate mRNA expression of several inflammatory cytokines and acute phase proteins (APP) in mammary tissue and liver, and the appearance of some of these factors in plasma and milk. The expression levels of IL1β and TNFα were markedly up-regulated inStaph. aureus-inoculated mammary tissue at 72 h, whilst IL6 was up-regulated to a lesser extent in a way which was not confined to the inoculated glands. APP expression was up-regulated at 48 and 72 h in bothStaph. aureus-inoculated and teat-sealedmammaryglands. These differences between cytokine and APP expression provide additional support for the contention that APPs are produced within the mammary tissue itself during inflammation, rather than in associated immune cells. We propose that measurement of cytokines and APP in combination might provide a tool for diagnostic discrimination between mastitis caused by pathogenic invasion and milk accumulation, and hence allow for better targeting of antibiotic therapy. In comparison with mammary expression, expression of cytokines in liver tissue was up-regulated to a similar or lesser extent, whilst expression of APP was up-regulated to a much greater extent. The first appearance of increased cytokine and APP concentrations in plasma and of milk amyloid A (MAA) in milk occurred in advance of the measurable up-regulation of expression, hence their origin cannot be stated with certainty.

Role of sortase A in the pathogenesis of Staphylococcus aureus-induced mastitis in mice

Sortase A (SrtA), a transpeptidase, anchors surface proteins with an LPXTG-motif sorting signal to the cell envelope. To determine the role of SrtA in the pathogenesis of Staphylococcus aureus, we constructed a mutant strain, ∆SrtA, by genetic techniques and identified its functions in a S. aureus-induced mastitis mouse model. The histological and myeloperoxidase (MPO) level results showed that the ∆SrtA strain attenuated the inflammatory reaction in the mammary tissue of mice compared with wild-type S. aureus challenge. Additionally, the ELISA results showed that the ∆SrtA strain impaired the induction of pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6), and the Western blot results showed that the mutant strain blocked the activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPKs) by attenuating the degradation and phosphorylation of signaling pathway molecules such as IκBα, p65 and p38. These results suggest that SrtA is a key virulence factor in the pathogenesis of S. aureus-induced mastitis in mice. It appears that the srtA mutant affected the attachment of S. aureus to host cells, thus attenuating the activation of the NF-κB and MAPK signaling pathways, which regulated the expression of pro-inflammatory cytokines and decreased the susceptibility to mastitis.

Short communication: Antimicrobial efficacy of intramammary treatment with a novel biphenomycin compound against Staphylococcus aureus, Streptococcus uberis, and Escherichia coli-induced mouse mastitis

Bovine mastitis undermines udder health, jeopardizes milk production, and entails prohibitive costs, estimated at $2 billion per year in the dairy industry of the United States. Despite intensive research, the dairy industry has not managed to eradicate the 3 major bovine mastitis-inducing pathogens: Staphylococcus aureus, Streptococcus uberis, and Escherichia coli. In this study, the antimicrobial efficacy of a newly formulated biphenomycin compound (AIC102827) was assessed against intramammary Staph. aureus, Strep. uberis, and E. coli infections, using an experimental mouse mastitis model. Based on its effective and protective doses, AIC102827 applied into the mammary gland was most efficient to treat Staph. aureus, but also adequately reduced growth of Strep. uberis or E. coli, indicating its potential as a broad-spectrum candidate to treat staphylococcal, streptococcal, and coliform mastitis in dairy cattle.

In vitro and in vivo antibacterial activities of cranberry press cake extracts alone or in combination with β-lactams against Staphylococcus aureus

BACKGROUND

Cranberry fruits possess many biological activities partly due to their various phenolic compounds; however the underlying modes of action are poorly understood. We studied the effect of cranberry fruit extracts on the gene expression of Staphylococcus aureus to identify specific cellular processes involved in the antibacterial action.

METHODS

Transcriptional profiles of four S. aureus strains grown in broth supplemented or not with 2 mg/ml of a commercial cranberry preparation (Nutricran®90) were compared using DNA arrays to reveal gene modulations serving as markers for biological activity. Ethanol extracted pressed cakes from fresh fruits also produced various fractions and their effects on marker genes were demonstrated by qPCR. Minimal inhibitory concentrations (MICs) of the most effective cranberry fraction (FC111) were determined against multiple S. aureus strains and drug interactions with β-lactam antibiotics were also evaluated. Incorporation assays with [(3)H]-radiolabeled precursors were performed to evaluate the effect of FC111 on DNA, RNA, peptidoglycan (PG) and protein biosynthesis.

RESULTS

Treatment of S. aureus with Nutricran®90 or FC111 revealed a transcriptional signature typical of PG-acting antibiotics (up-regulation of genes vraR/S, murZ, lytM, pbp2, sgtB, fmt). The effect of FC111 on PG was confirmed by the marked inhibition of incorporation of D-[(3)H]alanine. The combination of β-lactams and FC111 in checkerboard assays revealed a synergistic activity against S. aureus including strain MRSA COL, which showed a 512-fold drop of amoxicillin MIC in the presence of FC111 at MIC/8. Finally, a therapeutic proof of concept was established in a mouse mastitis model of infection. S. aureus-infected mammary glands were treated with amoxicillin, FC111 or a combination of both; only the combination significantly reduced bacterial counts from infected glands (P<0.05) compared to the untreated mice.

CONCLUSIONS

The cranberry fraction FC111 affects PG synthesis of S. aureus and acts in synergy with β-lactam antibiotics. Such a fraction easily obtained from poorly exploited press-cake residues, may find interesting applications in the agri-food sector and help reduce antibiotic usage in animal food production.

The intramammary efficacy of first generation cephalosporins against Staphylococcus aureus mastitis in mice

Staphylococcus aureus-induced mastitis in cattle causes important financial losses in the dairy industry due to lower yield and bad milk quality. Although S. aureus is susceptible to many antimicrobials in vitro, treatment often fails to cure the infected udder. Hence, comprehensive evaluation of antimicrobials against S. aureus mastitis is desirable to direct treatment strategies. The mouse mastitis model is an elegant tool to evaluate antimicrobials in vivo while circumventing the high costs associated with bovine experiments. An evaluation of the antimicrobial efficacy of the intramammary (imam) applied first generation cephalosporins cefalexin, cefalonium, cefapirin and cefazolin, was performed using the S. aureus mouse mastitis model. In vivo determination of the effective dose 2log(10) (ED(2log10)), ED(4log10), protective dose 50 (PD(50)) and PD(100) in mouse mastitis studies, support that in vitro MIC data of the cephalosporins did not fully concur with the in vivo clinical outcome. Cefazolin was shown to be the most efficacious first generation cephalosporin to treat S. aureus mastitis whereas the MIC data indicate that cefalonium and cefapirin were more active in vitro. Changing the excipient for imam application from mineral oil to miglyol 812 further improved the antimicrobial efficacy of cefazolin, confirming that the excipient can influence the in vivo efficacy. Additionally, statistical analysis of the variation of S. aureus-infected, excipient-treated mice from fourteen studies emphasizes the strength of the mouse mastitis model as a fast, cost-effective and highly reproducible screening tool to assess the efficacy of antimicrobial compounds against intramammary S. aureus infection.

Chimeric phage lysins act synergistically with lysostaphin to kill mastitis-causing Staphylococcus aureus in murine mammary glands

Staphylococci cause bovine mastitis, with Staphylococcus aureus being responsible for the majority of the mastitis-based losses to the dairy industry (up to $2 billion/annum). Treatment is primarily with antibiotics, which are often ineffective and potentially contribute to resistance development. Bacteriophage endolysins (peptidoglycan hydrolases) present a promising source of alternative antimicrobials. Here we evaluated two fusion proteins consisting of the streptococcal λSA2 endolysin endopeptidase domain fused to staphylococcal cell wall binding domains from either lysostaphin (λSA2-E-Lyso-SH3b) or the staphylococcal phage K endolysin, LysK (λSA2-E-LysK-SH3b). We demonstrate killing of 16 different S. aureus mastitis isolates, including penicillin-resistant strains, by both constructs. At 100 μg/ml in processed cow milk, λSA2-E-Lyso-SH3b and λSA2-E-LysK-SH3b reduced the S. aureus bacterial load by 3 and 1 log units within 3 h, respectively, compared to a buffer control. In contrast to λSA2-E-Lyso-SH3b, however, λSA2-E-LysK-SH3b permitted regrowth of the pathogen after 1 h. In a mouse model of mastitis, infusion of 25 μg of λSA2-E-Lyso-SH3b or λSA2-E-LysK-SH3b into mammary glands reduced S. aureus CFU by 0.63 or 0.81 log units, compared to >2 log for lysostaphin. Both chimeras were synergistic with lysostaphin against S. aureus in plate lysis checkerboard assays. When tested in combination in mice, λSA2-E-LysK-SH3b and lysostaphin (12.5 μg each/gland) caused a 3.36-log decrease in CFU. Furthermore, most protein treatments reduced gland wet weights and intramammary tumor necrosis factor alpha (TNF-α) concentrations, which serve as indicators of inflammation. Overall, our animal model results demonstrate the potential of fusion peptidoglycan hydrolases as antimicrobials for the treatment of S. aureus-induced mastitis.

Enhancement of antibacterial activity of tilmicosin against Staphylococcus aureus by solid lipid nanoparticles in vitro and in vivo

This study aimed to enhance the antibacterial activity of tilmicosin by solid lipid nanoparticles (SLN). Tilmicosin-loaded hydrogenated castor oil (HCO)-SLN was prepared using a hot homogenisation and ultrasonication method. The physicochemical characteristics of SLN were investigated by scanning electron microscopy (SEM) and photon correlation spectroscopy (PCS). The antibacterial activity of tilmicosin-SLN against Staphylococcus aureus was evaluated by growth inhibition and colony-counting method. A therapeutic study of tilmicosin-SLN was conducted by subcutaneous injection in a mouse mastitis model infected with S. aureus by teat canal infusion. Therapeutic efficacy was assessed by physical appearance of the mammary gland and measurement of colony-forming units (CFU) per gland. The results showed that the diameter, polydispersivity index, zeta potential, encapsulation efficiency and loading capacity of the nanoparticles were 343±26 nm, 0.33±0.08, -7.9±0.4 mV, 60.4±3.3% and 11.2±0.47%, respectively. Tilmicosin-SLN showed a sustained-release effect and sustained and enhanced antibacterial activity in vitro. SLN significantly enhanced the therapeutic efficacy of tilmicosin determined by lower CFU counts and a decreased degree of inflammation. These results demonstrated that the HCO-SLN is an effective carrier to enhance the antibacterial activity of tilmicosin.

Leukocyte populations and cytokine expression in the mammary gland in a mouse model of Streptococcus agalactiae mastitis

Streptococcus agalactiae is a contagious, mastitis-causing pathogen that is highly adapted to survive in the bovine mammary gland. This study used a BALB/c mouse model of Streptococcus agalactiae mastitis to evaluate leukocyte populations in regional lymph nodes and cytokine expression in the mammary gland involved in the immune response against Streptococcus agalactiae. It was found that the bacteria replicated efficiently in the mammary gland, peaking after 24 h and increasing by 100-fold. Dissemination of bacteria to systemic organs was observed 6 h after infection. At the same time, a massive infiltration of polymorphonuclear cells and an increase in the inflammatory cytokines interleukin (IL)-1beta, IL-6 and tumour necrosis factor-alpha were detected in mammary glands, indicating an early inflammatory response. A decrease in the levels of inflammatory cytokines in mammary glands was observed 72 h after infection, accompanied by an increase in the levels of IL-12 and IL-10, which were related to a gradual decrease in bacterial load. An increase in the number of macrophages and B220(+) lymphocytes and similar increases in both CD4(+) and CD8(+) T cells in regional lymph nodes were observed, being most pronounced 5 days after infection. Moreover, increased levels of anti-Streptococcus agalactiae antibodies in the mammary gland were observed 10 days after infection. Overall, these data suggest that the host exhibits both innate and acquired immune responses in response to Streptococcus agalactiae mastitis.

A global view of Staphylococcus aureus whole genome expression upon internalization in human epithelial cells

Background

Staphylococcus aureus, a leading cause of chronic or acute infections, is traditionally considered an extracellular pathogen despite repeated reports of S. aureus internalization by a variety of non-myeloid cells in vitro. This property potentially contributes to bacterial persistence, protection from antibiotics and evasion of immune defenses. Mechanisms contributing to internalization have been partly elucidated, but bacterial processes triggered intracellularly are largely unknown.

Results

We have developed an in vitro model using human lung epithelial cells that shows intracellular bacterial persistence for up to 2 weeks. Using an original approach we successfully collected and amplified low amounts of bacterial RNA recovered from infected eukaryotic cells. Transcriptomic analysis using an oligoarray covering the whole S. aureus genome was performed at two post-internalization times and compared to gene expression of non-internalized bacteria. No signs of cellular death were observed after prolonged internalization of Staphylococcus aureus 6850 in epithelial cells. Following internalization, extensive alterations of bacterial gene expression were observed. Whereas major metabolic pathways including cell division, nutrient transport and regulatory processes were drastically down-regulated, numerous genes involved in iron scavenging and virulence were up-regulated. This initial adaptation was followed by a transcriptional increase in several metabolic functions. However, expression of several toxin genes known to affect host cell integrity appeared strictly limited.

Conclusion

These molecular insights correlated with phenotypic observations and demonstrated that S. aureus modulates gene expression at early times post infection to promote survival. Staphylococcus aureus appears adapted to intracellular survival in non-phagocytic cells.

Invasive potential of bacterial isolates associated with subclinical bovine mastitis

This work describes differences in the invasive ability of bacterial isolates associated with mastitis. Invasion ability was determined by the uptake and survival in a primary culture of bovine mammary epithelial cells (BMEC). BMEC were isolated from a healthy lactating cow and characterized by their morphology, immunostaining for cytokeratin and the detection of beta- and kappa-casein mRNAs. Ten bacterial isolates comprising the staphylococcal species Staphylococcus aureus (3), Staphylococcus epidermidis (1), Staphylococcus haemolyticus (1), Staphylococcus equorum (2), Staphylococcus xylosus (1) and Brevibacterium stationis (2) obtained from raw milk of cows with mastitis from backyard farms were assayed for their ability to invade BMEC. Only two S. aureus and one S. epidermidis isolates were able to invade BMEC, at similar levels to the S. aureus control strain ATCC 27543. In conclusion, using the in vitro model of infection used in this study, differences in bacterial invasion capability may be detected.

Helenalin reduces Staphylococcus aureus infection in vitro and in vivo

Staphylococcus (S.) aureus is a major udder pathogen causing bovine mastitis. Some pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha), enhance extracellular and intracellular growth of S. aureus, indicating that the inflammatory process favors S. aureus infection. Helenalin is a sesquiterpene lactone with potent anti-inflammatory properties. This study was designed to evaluate the effects of helenalin on S. aureus infection. First, in vitro experiments were conducted. These studies revealed that proliferation of S. aureus in bovine mammary epithelial MAC-T cells treated in the presence or absence of TNF-alpha was markedly reduced in the presence of helenalin. Secondly, in vivo effects of helenalin were investigated. Lactating mice treated in the presence or absence of helenalin were challenged by the intramammary route with S. aureus and the bacteria in the mammary glands were counted 12 h after infection. Significantly less numbers of bacteria were recovered from the infected glands of helenalin-treated mice compared with untreated mice. Moreover, histological examination of mammary tissue from helenalin-treated mice that were challenged with S. aureus indicated that helenalin is able to significantly reduce leukocyte infiltration in the mammary gland following S. aureus inoculation. Our results show that helenalin reduces S. aureus intracellular growth and experimental S. aureus infection. We conclude that helenalin may be of potential interest in the treatment of S. aureus-induced mastitis in the bovine species.