Leukotoxins of gram-negative bacteria

Detecting Forest Musk Deer Abscess Disease Pathogens Using 16S rRNA High-Throughput Sequencing Technology

Currently, researchers use bacterial culture and targeted PCR methods to classify, culture, and identify the pathogens causing abscess diseases. However, this method is limited by factors such as the type of culture medium and culture conditions, making it challenging to screen and proliferate many bacteria effectively. Fortunately, with the development of high-throughput sequencing technology, pathogen identification at the genetic level has become possible. Not only can this approach overcome the limitations of bacterial culture, but it can also accurately identify the types and relative abundance of pathogens. In this study, we used high-throughput sequencing of 16S rRNA to identify the pathogens in purulent fluid samples. Our results not only confirmed the presence of the main pathogen reported by previous researchers, Trueperella pyogenes, but also other obligate anaerobes, Fusobacterium necrophorum and Bacteroides fragilis as the dominant pathogens causing abscess diseases for the first time. Therefore, our findings suggest that high-throughput sequencing technology has the potential to replace traditional bacterial culture and targeted PCR methods.

Exploring the role of antimicrobials in the selective growth of purple phototrophic bacteria through genome mining and agar spot assays

Purple non-sulphur bacteria (PNSB) are an emerging group of microbes attractive for applied microbiology applications such as wastewater treatment, plant biostimulants, microbial protein, polyhydroxyalkanoates and H₂ production. These photoorganoheterotrophic microbes have the unique ability to grow selectively on organic carbon in anaerobic photobioreactors. This so-called selectivity implies that the microbial community will have a low diversity and a high abundance of a particular PNSB species. Recently, it has been shown that certain PNSB strains can produce antimicrobials, yet it remains unclear whether these contribute to competitive inhibition. This research aimed to understand which type of antimicrobial PNSB produce and identify whether these compounds contribute to their selective growth. Mining 166 publicly-available PNSB genomes using the computational tool BAGEL showed that 59% contained antimicrobial encoding regions, more specifically biosynthetic clusters of bacteriocins and non-ribosomal peptide synthetases. Inter- and intra-species inhibition was observed in agar spot assays for Rhodobacter blasticus EBR2 and Rhodopseudomonas palustris EBE1 with inhibition zones of, respectively, 5.1 and 1.5-5.7 mm. Peptidomic analysis detected a peptide fragment in the supernatant (SVLQLLR) that had a 100% percentage identity match with a known non-ribosomal peptide synthetase with antimicrobial activity.

Evaluation of Immunoprotective Effects of Fusobacterium necrophorum Outer Membrane Proteins 43K OMP, Leukotoxin and Hemolysin Multi-Component Recombinant Subunit Vaccine in Mice

We evaluated the efficacy of three vaccine formulations containing different combinations of proteins (43K OMP, leukotoxin recombinant protein PL4 and hemolysin recombinant protein H2) and killed whole cell Fusobacterium necrophorum in preventing liver abscess. Four subcutaneous vaccines were formulated: vaccine 1 (43K OMP), vaccine 2 (PL4 and H2), vaccine 3 (43K OMP, PL4 and H2), and vaccine 4 (killed whole bacterial cell). 43K OMP, PL4, and H2 proteins were produced by using recombinant protein expression. To evaluate vaccine efficacy, we randomly allocated 50 BALB/c female mice to one of five different treatment groups: PBS control group, vaccine 1, vaccine 2, vaccine 3, and vaccine 4. Mice were vaccinated three times, with 14 days between each immunization. After immunization, the mice were challenged with F. necrophorum. The three key findings of this study are as follows: (1) Vaccine 3 has enabled mice to produce higher antibody titer following bacterial challenge, (2) in the liver pathology of mice, the vaccine 3 liver showed the least pathology, and (3) all four vaccines produced high levels of antibodies and cytokines in mice, but the level of vaccine 3 was the highest. Based on our results, it has been demonstrated that a mixture of F. necrophorum 43K OMP, PL4, and H2 proteins inoculated with mice can achieve protection against liver abscess in mice. Our research may therefore provide the basis for the development of a vaccine against F. necrophorum bovine infections.

Biological and molecular characterization of a sheep pathogen isolate of Mannheimia haemolytica and leukotoxin production kinetics

Background and Aim

Mannheimia haemolytica (Mha) is a common agent of pneumonia in ruminants globally, causing economic losses by morbidity, mortality, and treatment costs. Infection by Mha is often associated with or promoted by respiratory viral pathogens and environmental conditions. Infections due to Mha have rarely been described in small ruminants. This study reports the biological and molecular characteristics of a new Moroccan Mha isolate from small ruminants presenting typical respiratory symptoms. We also studied the cultural parameters, growth kinetics, and Lkt excretion of the isolate and its pathogenicity on laboratory animals and small ruminants.

Materials and Methods

Suspected pasteurellosis cases in sheep and goat flocks in Morocco were investigated. A local strain of Mha was isolated and identified using biochemical and molecular methods. Polymerase chain reaction-targeting specific genes were used for serotyping and phylogenetic analyses; further, leukotoxin production, cytotoxicity, and pathogenicity of the isolate in mice, goats, and sheep were investigated.

Results

Phylogeny analysis revealed 98.76% sequence identity with the USA isolate of 2013; the strain growth with a cycle of 9-10 h with leukotoxin secretion was detected by NETosis and quantified by cytotoxicity and mortality of mice. Goat and sheep infections cause hyperthermia, with characteristic postmortem lesions in the trachea and lung.

Conclusion

A local isolate of Mha from sheep that died of pneumonia was characterized for the 1^st time in North Africa using biological and molecular methods. Although growth on appropriate culture media is accompanied by intense leukotoxin secretion, experimental infections of sheep and goats cause hyperthermia and typical lesions of pneumonia.

Genome Sequence of Mannheimia haemolytica MHA.Sh.MOR19 Serotype 1, a Moroccan Sheep Isolate

Mannheimia haemolytica is the principle bacterial pathogen in ruminants associated with respiratory disease. Here, we report the draft genome sequence of the Mannheimia haemolytica MHA.Sh.MOR19 strain that was recently isolated in the northwest of Morocco from the lung of a lamb that died from pneumonia. The genome size is 2,434,458 bp.

Leukotoxin production by Fusobacterium necrophorum strains in relation to severity of liver abscesses in cattle

Fusobacterium necrophorum, a Gram-negative anaerobe, is the primary etiologic agent of liver abscesses of beef cattle. The bacterium, a member of the microbial community of the rumen, travels to the liver via portal circulation to cause abscesses. The severity of liver abscesses vary from mild with one or two small abscesses to severe with medium to large multiple abscesses. Leukotoxin, a secreted protein, is the critical virulence factor involved in the infection. Our objective was to compare leukotoxin production between strains of F. necrophorum isolated from mild and severe liver abscesses collected from slaughtered cattle. The quantification of leukotoxin was based on assays to measure cytotoxicity and protein antigen concentration. One-hundred strains, 50 from mild and 50 from severe abscesses, were utilized in the study. Cell-free supernatants were prepared from cultures grown in anaerobic broth at 9 and 24 h incubations. The leukotoxic activity was quantified by measuring cytotoxicity based on the release of lactic dehydrogenase from bovine lymphocyte cells, BL3, treated with the culture supernatant. Leukotoxin protein concentration was quantified by a sandwich ELISA assay with a leukotoxin-specific monoclonal antibody as the capture antibody. The leukotoxin activity and concentration were highly variable among the strains within each severity of liver abscesses. Although the leukotoxic activity was unaffected by incubation time, leukotoxin protein concentration was consistently higher at 24 h compared to 9 h incubation. Strains from severe liver abscesses had significantly higher leukotoxic activity and higher protein concentration compared to strains from mild liver abscesses (P < 0.0001) at both 9 and 24 h culture supernatants. Across all strains, the correlation coefficients between leukotoxic activity and leukotoxin concentration at 9 and 24 h were 0.14 (P = 0.17) and 0.47 (P < 0.0001), respectively. In conclusion, strains isolated from severe liver abscesses had significantly higher leukotoxic activities and leukotoxin protein concentrations compared to strains isolated from mild liver abscesses.

Understanding the pathogenesis of Flavobacterium psychrophilum using the rainbow trout monocyte/macrophage-like cell line, RTS11, as an infection model

The life cycle of Flavobacterium psychrophilum (Fp), the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), appears to involve interactions with spleen and head kidney macrophages. To develop an in vitro model for studying this, F. psychrophilum was incubated with a rainbow trout splenic monocyte/macrophage-like cell line (RTS11) and fundamental macrophage functions evaluated. The animal cell basal medium, L15, supplemented with bovine serum (FBS) supports RTS11 maintenance, and surprisingly, L15 with 2% FBS (L15/FBS) also supported F. psychrophilum growth. L15/FBS in which the bacteria had been grown is referred to as F. psychrophilum conditioned medium (FpCM). Adding FpCM to RTS11 cultures caused a small, yet significant, percentage of cells to die, many cells to become more diffuse, and phagocytosis to be temporarily reduced. FpCM also significantly stimulated transcript expression for pro-inflammatory cytokines (IL-1β, TNFα and IL-6) and the anti-inflammatory cytokine (IL-10) after one day of exposure but this upregulation rapidly declined over time. Adding live F. psychrophilum to RTS11 cultures also altered the cellular morphology and stimulated cytokine expression more profoundly than FpCM. Additionally, the phagocytic activity of RTS11 was also significantly impaired by live F. psychrophilum, but not to the same extent as when exposed to FpCM. Adding heat-killed bacteria to RTS11 cultures elicited few changes. These bacteria/RTS11 co-cultures should be useful for gaining a deeper understanding of the pathogenesis of F. psychrophilum and may aid in the development of effective measures to prevent infection and spread of this troublesome disease.

Mannheimia haemolytica in bovine respiratory disease: immunogens, potential immunogens, and vaccines

Mannheimia haemolytica is the major cause of severe pneumonia in bovine respiratory disease (BRD). Early M. haemolytica bacterins were either ineffective or even enhanced disease in vaccinated cattle, which led to studies of the bacterium's virulence factors and potential immunogens to determine ways to improve vaccines. Studies have focused on the capsule, lipopolysaccharide, various adhesins, extracellular enzymes, outer membrane proteins, and leukotoxin (LKT) resulting in a strong database for understanding immune responses to the bacterium and production of more efficacious vaccines. The importance of immunity to LKT and to surface antigens in stimulating immunity led to studies of individual native or recombinant antigens, bacterial extracts, live-attenuated or mutant organisms, culture supernatants, combined bacterin-toxoids, outer membrane vesicles, and bacterial ghosts. Efficacy of several of these potential vaccines can be shown following experimental M. haemolytica challenge; however, efficacy in field trials is harder to determine due to the complexity of factors and etiologic agents involved in naturally occurring BRD. Studies of potential vaccines have led current commercial vaccines, which are composed primarily of culture supernatant, bacterin-toxoid, or live mutant bacteria. Several of those can be augmented experimentally by addition of recombinant LKT or outer membrane proteins.

Prevalence of Mannheimia haemolytica in Syrian Awassi sheep

The aim of the present study was to evaluate the prevalence of Mannheimia haemolytica in Syrian Awassi sheep. Between 2008 and 2012, 1520 nasal swabs from pneumonic and apparently healthy sheep, and 110 pneumonic lungs samples were collected and subjected to bacteriological, biochemical and PCR assays. A total of 191 isolates (11.7%) were identified as M. haemolytica, 44 (2.7%) were M. ruminalis and 18 (1.1%) were M. glucosida. All 191 isolates of M. haemolytica gave an amplified product of 1146 bp size by PCR when lktaA primer was applied. The highest rate of M. haemolytica isolation was from pneumonic lungs tissue (21.8%) and pneumonic sheep (14.1%), and the lowest was from apparently healthy sheep (8.5%). Geographical and seasonal distribution of M. haemolytica showed that the majority of the isolates originated from sheep reared in the Syrian Desert (30%) and Euphrates basin (26.7%), and a significant increase (P≤0.05) in the rate of positive isolates in summer and winter as compared to spring. These findings indicate that M. haemolytica may play an important role in development of pneumonia in Syrian Awassi sheep, especially in eastern parts of Syria, where drought and shortage of rain hit these zones periodically.

Solving the riddle: Unraveling the mechanisms of blocking the binding of leukotoxin by therapeutic antagonists in periodontal diseases

Aggregatibacter actinomycetemcomitans is a Gram-negative bacteria that has gained wide recognition for its causative role in the development of various immune diseases, which includes localized aggressive periodontitis. Its ability to evade host defense mechanisms is mediated by the secretion of leukotoxin (LtxA), which induces death of white blood cells (leukocytes) by specific binding to their surface-expressed leukocyte function-associated receptor (LFA-1) in its active state. Therapeutic compounds that interfere with this pathogenic process and abrogate A. actinomycetemcomitans virulence have been reported in literature. These include doxycycline, and more recently phytochemical compounds such as hamamelitanin, resveratrol, naringin, and quercetin. However, the question remains how do they work? Therefore, with the aid of computational tools, we explore the molecular mechanisms by which they possibly elicit their therapeutic functions. Molecular mechanics Poisson/Boltzmann surface area analyses revealed that these compounds bind favorably to active LFA-1 with high affinity and considerable stability, indicative of their ability to occupy the LtxA binding site (LBS) and prevent LtxA binding. The conformational transition of open LFA-1 to its closed state further describe the mechanistic activity of these compounds. In addition to notable reductions in structural mobility and flexibility, the burial of surface-exposed interactive side chains at the LBS was observed, an occurrence that could alter the complementary binding of LtxA. It is also important to mention that these occurrences were induced more prominently by the phytochemicals. We believe that these findings will enhance the scope of drug design and discovery for potent LtxA antagonists with improved activities and therapeutic efficacies in the treatment of virulent A. actinomycetemcomitans diseases.

Pathogenesis and Treatment of Bovine Foot Rot

Bovine foot rot (BFR) is an infectious disease of the interdigital skin and subcutaneous tissues of beef and dairy cattle that occurs under a variety of management and environmental settings. The anaerobic, gram-negative bacteria Fusobacterium necrophorum, Porphyromonas levii, and Prevotella intermedia are commonly isolated from lesions. A multitude of host, agent, and environmental factors contribute to the development of BFR. Initiation of systemic antimicrobial therapy early in the course of disease commonly leads to resolution. Delays in treatment may result in extension of infection into deeper bone, synovial structures, or ligamentous structures, and the prognosis for recovery is reduced.

Bronchopneumonia associated with Mannheimia granulomatis infection in a Belgian hare (Lepus europaeus)

Mannheimia granulomatis was first isolated from pneumonic European hares in the 1980s and has since been reported sporadically in pneumonic Swedish roe deer and Australian cattle. Although the pneumonic lesions caused by M. haemolytica in livestock have been extensively studied and reported, little is published with regard to the pneumonic lesions associated with M. granulomatis infection in any species. We describe the histopathology of purulent bronchopneumonia associated with M. granulomatis in a Belgian hare ( Lepus europaeus) resident in British Columbia, Canada, and compare the lesions with those caused by M. haemolytica in livestock.

Identification of Leukotoxin and other vaccine candidate proteins in a Mannheimia haemolytica commercial antigen

Bovine Respiratory Disease is the most costly disease that affects beef and dairy cattle industry. Its etiology is multifactorial, arising from predisposing environmental stress conditions as well as the action of several different respiratory pathogens. This situation has hindered the development of effective control strategies. Although different type of vaccines are available, many currently marketed vaccines are based on inactivated cultures of the main viral and bacterial agents involved in this pathology. The molecular composition of commercial veterinary vaccines is a critical issue. The present work aims to define at the proteomic level the most relevant valence of a line of commercial respiratory vaccines widely used in Central and South America. Since Mannheimia haemolytica is responsible for most of the disease associated morbid-mortality, we focused on the main proteins secreted by this pathogen, in particular Leukotoxin A, its main virulence factor. By Western blot analysis and mass spectrometry, Leukotoxin A was identified as a major component of M. haemolytica culture supernatants. We also identified other ten M. haemolytica proteins, including outer membrane proteins, periplasmic transmembrane solute transporters and iron binding proteins, which are relevant to achieve protective immunity against the pathogen. This work allowed a detailed molecular characterization of this vaccine component, providing evidence of its quality and efficacy. Furthermore, our results contributed to the identification of several proteins of interest as subunit vaccine candidates.

Interaction between periodontitis and liver diseases

Periodontitis is an oral disease that is highly prevalent worldwide, with a prevalence of 30–50% of the population in developed countries, but only ~10% present with severe forms. It is also estimated that periodontitis results in worldwide productivity losses amounting to ~54 billion USD yearly. In addition to the damage it causes to oral health, periodontitis also affects other types of disease. Numerous studies have confirmed the association between periodontitis and systemic diseases, such as diabetes, respiratory disease, osteoporosis and cardiovascular disease. Increasing evidence also indicated that periodontitis may participate in the progression of liver diseases, such as non-alcoholic fatty liver disease, cirrhosis and hepatocellular carcinoma, as well as affecting liver transplantation. However, to the best of our knowledge, there are currently no reviews elaborating upon the possible links between periodontitis and liver diseases. Therefore, the current review summarizes the human trials and animal experiments that have been conducted to investigate the correlation between periodontitis and liver diseases. Furthermore, in the present review, certain mechanisms that have been postulated to be responsible for the role of periodontitis in liver diseases (such as bacteria, pro-inflammatory mediators and oxidative stress) are considered. The aim of the review is to introduce the hypothesis that periodontitis may be important in the progression of liver disease, thus providing dentists and physicians with an improved understanding of this issue.

Aggregatibacter actinomycetemcomitans - a tooth killer?

Strong evidence is available on Aggregatibacter actinomycetemcomitans (A.a) on its role as the causative agent of localised juvenile periodontitis (LJP), a disease characterised by rapid destruction of the tooth-supporting tissues. This organism possesses a large number of virulence factors with a wide range of activities which enable it to colonise the oral cavity, invade periodontal tissues, evade host defences, initiate connective tissue destruction and interfere with tissue repair. Adhesion to epithelial and tooth surfaces is dependent on the presence of surface proteins and structures such as microvesicles and fimbriae. Invasion has been demonstrated in vivo and in vitro. The organism has a number of means of evading host defences which include: (i) production of leukotoxin; (ii) producing immunosuppressive factors; (iv) secreting proteases capable of cleaving IgG; and (v) producing Fc-binding.

Establishment of a new murine model of liver abscess induced by Fusobacterium necrophorum injected into the caudal vein

Anaerobic bacterial infection is often accompanied by abscess formation; however, few in vivo studies have been published with descriptive data specifically evaluating antimicrobial activity in the presence of abscesses. The aim of this study was to establish a murine model of anaerobic infection with abscess formation and to verify the utility of this model for evaluating the in vivo efficacy of an antimicrobial agent. A clinical isolate of Fusobacterium necrophorum was inoculated into the caudal vein of immunocompetent BALB/c mice at 108 c.f.u. per mouse. Changes in body weight, bacterial load and histopathology of key organs were evaluated. After inoculation, bacterial counts in the liver increased from 104 to 108 c.f.u. after 1–3 days, and liver abscess formation was observed on the day following infection. Abscess formation and bacterial growth were not observed in other organs. In this model, 3 days of treatment with 5 mg metronidazole kg−1 eradicated F. necrophorum in the liver; however, a reduction in bacterial load was not observed with 0.05 mg metronidazole kg−1. In this study, we established a novel murine model of F. necrophorum liver abscess via haematogenous infection that may be useful for investigating in vivo antimicrobial activity against anaerobic abscesses and understanding the pathogenesis of F. necrophorum infection.

Bronchopneumonia in two dairy calves associated with Mannheimia species cluster V infection

The pathological, bacteriological, and molecular findings of two 3-week-old Holstein calves with bronchopneumonia are presented. Heavy pure growth of a Mannheimia species most closely aligned with the unnamed cluster V strains on the basis of 16S ribosomal RNA sequencing was detected in the lungs of both calves in association with Bovine respiratory syncytial virus infection. While Mannheimia species closely related to cluster V strains have occasionally been reported in association with pneumonia, meningitis, and abortion in cattle, the current report provides a description of the gross and histopathological lesions produced by a cluster V strain of Mannheimia species. Lesions in the lung were found to be typical of those described for Mannheimia haemolytica with the absence of areas of coagulation necrosis rimmed by leukocytes and more pronounced intra-alveolar hemorrhage. Lesions were linked to the presence of leukotoxin A based on phenotypic hemolysis and molecular demonstration of the leukotoxin A gene.

Effects of drotaverine hydrochloride on viability of rat cultured cerebellar granulocytes

The neurocytotoxic effect of drotaverine hydrochloride was studied in culture of rat cerebellar granulocytes. Incubation of cells with 100 and 250 μM drotaverine reduced neuronal survival to 60 and 4%, respectively.

Adenosine-5'-triphosphate release by Mannheimia haemolytica, lipopolysaccharide, and interleukin-1 stimulated bovine pulmonary epithelial cells

Mannheimia haemolytica, one of the agents associated with bovine respiratory disease complex, can cause severe lung pathology including the leakage of vascular products into the airways and alveoli. Previous work by this laboratory has demonstrated that bovine lung endothelial and epithelial cells undergo dramatic permeability increases when exposed to adenosine-5'-triphosphate (ATP). Therefore, we wanted to determine if ATP levels were elevated in bronchoalveolar lavage (BAL) samples from calves experimentally infected with M. haemolytica. In addition, cultured bovine pulmonary epithelial (BPE) cells were stimulated with heat-killed and live M. haemolytica bacteria, lipopolysaccharide (LPS), lipoteichoic acid (LTA), interleukin-1 (IL-1), and zymosan activated plasma (ZAP) to determine whether they might release extracellular ATP during in vitro infection. Calves experimentally exposed to M. haemolytica had an approximately 2-fold higher level of ATP in their BAL samples compared to control. BPE cells exposed to increasing numbers of heat-killed or live M. haemolytica had significantly increased levels of ATP release as compared to time-matched controls. Finally, BPE cells treated with several concentrations of LPS and IL-1 had increases in ATP release as compared to time-matched controls. This increase appeared to be a result of active ATP secretion by the cells, as cell viability was similar between treated and non-treated cells. Neither ZAP nor LTA induced any ATP release by the cells. In conclusion, ATP levels are elevated in lung secretions from calves infected with M. haemolytica. In addition, lung epithelial cells can actively release ATP when exposed to heat-killed or live M. haemolytica, LPS or IL-1.

Smoking and periodontal disease: discrimination of antibody responses to pathogenic and commensal oral bacteria

Smoking is an independent risk factor for the initiation, extent and severity of periodontal disease. This study examined the ability of the host immune system to discriminate commensal oral bacteria from pathogens at mucosal surfaces, i.e. oral cavity. Serum immunoglobulin (Ig)G antibody reactive with three pathogenic and five commensal oral bacteria in 301 current smokers (age range 21-66 years) were examined by enzyme-linked immunosorbent assay. Clinical features of periodontal health were used as measures of periodontitis. Antibody to the pathogens and salivary cotinine levels were related positively to disease severity; however, the antibody levels were best described by the clinical disease unrelated to the amount of smoking. The data showed a greater immune response to pathogens than commensals that was related specifically to disease extent, and most noted in black males. Significant correlations in individual patient responses to the pathogens and commensals were lost with an increasing extent of periodontitis and serum antibody to the pathogens. Antibody to Porphyromonas gingivalis was particularly distinct with respect to the discriminatory nature of the immune responses in recognizing the pathogens. Antibody responses to selected pathogenic and commensal oral microorganisms differed among racial groups and genders. The antibody response to the pathogens was related to disease severity. The level of antibody to the pathogens, and in particular P. gingivalis, was correlated with disease severity in black and male subsets of patients. The amount of smoking did not appear to impact directly serum antibody levels to these oral bacteria.

Cardiovascular disease and the role of oral bacteria

In terms of the pathogenesis of cardiovascular disease (CVD) the focus has traditionally been on dyslipidemia. Over the decades our understanding of the pathogenesis of CVD has increased, and infections, including those caused by oral bacteria, are more likely involved in CVD progression than previously thought. While many studies have now shown an association between periodontal disease and CVD, the mechanisms underpinning this relationship remain unclear. This review gives a brief overview of the host-bacterial interactions in periodontal disease and virulence factors of oral bacteria before discussing the proposed mechanisms by which oral bacterial may facilitate the progression of CVD.

Preparation of rat gingival mitochondria with an improved isolation method

In order to establish a method of obtaining rat gingival mitochondria (Mt), Mt fractions were prepared in various combinations of homogenizing time with collagenase concentration. Rat gingival tissues were excised, minced, treated with collagenase, homogenized, and subjected to differential centrifugation rates. Both the respiratory control ratio (RCR) and adenosine diphosphate/oxygen (ADP/O) ratio of the Mt fraction prepared in a combination of 40, 50, or 60 sec homogenization with collagenase in a concentration range of 0.115%–0.130% (w/v) were measured. The values for the RCR and ADP/O ratio of the Mt fraction obtained in an optimal condition was 1.80 ± 0.05 and 1.65 ± 0.03, respectively. These results suggest that Mt of fairly high quality can be obtained through this refined combination of the homogenizing time and collagenase concentration.

Mannheimia haemolytica: bacterial-host interactions in bovine pneumonia

Mannheimia haemolytica serotype S1 is considered the predominant cause of bovine pneumonic pasteurellosis, or shipping fever. Various virulence factors allow M haemolytica to colonize the lungs and establish infection. These virulence factors include leukotoxin (LKT), lipopolysaccharide, adhesins, capsule, outer membrane proteins, and various proteases. The effects of LKT are species specific for ruminants, which stem from its unique interaction with the bovine β2 integrin receptor present on leukocytes. At low concentration, LKT can activate bovine leukocytes to undergo respiratory burst and degranulation and stimulate cytokine release from macrophages and histamine release from mast cells. At higher concentration, LKT induces formation of transmembrane pores and subsequent oncotic cell necrosis. The interaction of LKT with leukocytes is followed by activation of these leukocytes to undergo oxidative burst and release proinflammatory cytokines such as interleukins 1, 6, and 8 and tumor necrosis factor α. Tumor necrosis factor α and other proinflammatory cytokines contribute to the accumulation of leukocytes in the lung. Formation of transmembrane pores and subsequent cytolysis of activated leukocytes possibly cause leakage of products of respiratory burst and other inflammatory mediators into the surrounding pulmonary parenchyma and so give rise to fibrinous and necrotizing lobar pneumonia. The effects of LKT are enhanced by lipopolysaccharide, which is associated with the release of proinflammatory cytokines from the leukocytes, activation of complement and coagulation cascade, and cell cytolysis. Similarly, adhesins, capsule, outer membrane proteins, and proteases assist in pulmonary colonization, evasion of immune response, and establishment of the infection. This review focuses on the roles of these virulence factors in the pathogenesis of shipping fever.

Bacteria-induced phagocyte secondary necrosis as a pathogenicity mechanism

Triggering of phagocyte apoptosis is a major virulence mechanism used by some successful bacterial pathogens. A central issue in the apoptotic death context is that fully developed apoptosis results in necrotic cell autolysis (secondary necrosis) with release of harmful cell components. In multicellular animals, this occurs when apoptosing cells are not removed by scavengers, mainly macrophages. Secondary necrotic lysis of neutrophils and macrophages may occur in infection when extensive phagocyte apoptosis is induced by bacterial cytotoxins and removal of apoptosing phagocytes is defective because the apoptotic process exceeds the available scavenging capacity or targets macrophages directly. Induction of phagocyte secondary necrosis is an important pathogenic mechanism, as it combines the pathogen evasion from phagocyte antimicrobial activities and the release of highly cytotoxic molecules, particularly of neutrophil origin, such as neutrophil elastase. This pathogenicity mechanism therefore promotes the unrestricted multiplication of the pathogen and contributes directly to the pathology of several necrotizing infections, where extensive apoptosis and necrosis of macrophages and neutrophils are present. Here, examples of necrotizing infectious diseases, where phagocyte secondary necrosis is implicated, are reviewed.

AIP56: a novel bacterial apoptogenic toxin

Photobacterium damselae subsp. piscicida (Phdp) is a Gram-negative pathogen agent of an important fish septicemia. The key virulence factor of Phdp is the plasmid-encoded exotoxin AIP56, which is secreted by exponentially growing pathogenic strains. AIP56 has 520 amino acids including an N-terminal cleavable signal peptide of 23 amino acid residues, two cysteine residues and a zinc-binding region signature HEXXH that is typical of most zinc metallopeptidases. AIP56 induces in vitro and in vivo selective apoptosis of fish macrophages and neutrophils through a caspase-3 dependent mechanism that also involves caspase-8 and -9. In vivo, the AIP56-induced phagocyte apoptosis progresses to secondary necrosis with release of cytotoxic phagocyte molecules including neutrophil elastase. Fish injected with recombinant AIP56 die with a pathology similar to that seen in the natural infection.

When two is better than one: macrophages and neutrophils work in concert in innate immunity as complementary and cooperative partners of a myeloid phagocyte system

The antimicrobial effector activity of phagocytes is crucial in the host innate defense against infection, and the classic view is that the phagocytes operating against intracellular and extracellular microbial pathogens are,respectively, macrophages and neutrophils. As a result of the common origin of the two phagocytes, they share several functionalities, including avid phagocytosis,similar kinetic behavior under inflammatory/infectious conditions, and antimicrobial and immunomodulatory activities. However, consequent to specialization during their differentiation, macrophages and neutrophils acquire distinctive, complementary features that originate different levels of antimicrobial capacities and cytotoxicity and different tissue localization and lifespan.This review highlights data suggesting the perspective that the combination of overlapping and complementary characteristics of the two professional phagocytes promotes their cooperative participation as effectors and modulators in innate immunity against infection and as orchestrators of adaptive immunity. In the concerted activities operating in antimicrobial innate immunity, macrophages and neutrophils are not able to replace each other. The common and complementary developmental,kinetic, and functional properties of neutrophils and macrophages make them the effector arms of a myeloid phagocyte system that groups neutrophils with members of the old mononuclear phagocyte system. The use by mammals of a system with two dedicated phagocytic cells working cooperatively represents an advantageous innate immune attack strategy that allows the efficient and safe use of powerful but dangerous microbicidal molecules.This crucial strategy is a target of key virulence mechanisms of successful pathogens.

Aggregatibacter actinomycetemcomitans LtxC is required for leukotoxin activity and initial interaction between toxin and host cells

Aggregatibacter actinomycetemcomitans is a human pathogen that produces the RTX toxin (repeats in toxin), leukotoxin (LtxA). Based on other RTX toxin systems, the product of ltxC, the first gene of the ltx operon, is predicted to be involved in fatty acid modification of LtxA. To determine the function of ltxC in A. actinomycetemcomitans, we generated an ltxC mutation in the highly leukotoxic strain JP2N using random mutagenesis. The toxin from the ltxC mutant (LtxA(ltxC)) was expressed and secreted into the cell culture supernatant but could not lyse human leukocytes or erythrocytes. Mass spectrometric analysis of LtxA(ltxC) and LtxA from strain JP2N (LtxA(wt)) revealed two peptides that differed and this data suggests that two internal lysine residues of LtxA from the wild-type strain are modified. In blocking experiments, pre-treatment of cells with LtxA(ltxC) was unable to prevent LtxA(wt) from killing cells. Furthermore, in contrast to LtxA(wt), LtxA(ltxC) did not cause an increase in intracellular calcium levels in human leukocytes. Taken together, our data show that ltxC is required for full activity and modification of LtxA in A. actinomycetemcomitans and that modification is important for initial binding of toxin to host cells, as defined by an increase in intracellular calcium levels.

Dynamin-2-dependent targeting of mannheimia haemolytica leukotoxin to mitochondrial cyclophilin D in bovine lymphoblastoid cells

Exotoxins which belong to the family containing the RTX toxins (repeats in toxin) contribute to a variety of important human and animal diseases. One example of such a toxin is the potent leukotoxin (LKT) produced by the bovine respiratory pathogen Mannheimia haemolytica. LKT binds to CD18, resulting in the death of bovine leukocytes. In this study, we showed that internalized LKT binds to the outer mitochondrial membrane, which results in the release of cytochrome c and collapse of the mitochondrial membrane potential (psi(m)). Incubation of bovine lymphoblastoid cells (BL-3 cells) with the mitochondrial membrane-stabilizing agent cyclosporine (CSA) reduced LKT-mediated cytotoxicity, cytochrome c release, and collapse of the psi(m). Coimmunoprecipitation and intracellular binding studies suggested that LKT binds to the mitochondrial matrix protein cyclophilin D. We also demonstrated that LKT mobilizes the vesicle scission protein dynamin-2 from mitochondria to the cell membrane. Incubation with CSA depleted mitochondrial dynamin-2 in BL-3 cells, making it unavailable for vesicle scission and LKT internalization. The results of this study show that LKT trafficking and LKT-mediated cell death involve dynamin-2 and cyclophilin D, in a process that can be prevented by the mitochondrial membrane-protecting function of CSA.

Screen for leukotoxin mutants in Aggregatibacter actinomycetemcomitans: genes of the phosphotransferase system are required for leukotoxin biosynthesis

Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans is a pathogen that causes localized aggressive periodontitis and extraoral infections including infective endocarditis. Recently, we reported that A. actinomycetemcomitans is beta-hemolytic on certain growth media due to the production of leukotoxin (LtxA). Based on this observation and our ability to generate random transposon insertions in A. actinomycetemcomitans, we developed and carried out a rapid screen for LtxA mutants. Using PCR, we mapped several of the mutations to genes that are known or predicted to be required for LtxA production, including ltxA, ltxB, ltxD, and tdeA. In addition, we identified an insertion in a gene previously not recognized to be involved in LtxA biosynthesis, ptsH. ptsH encodes the protein HPr, a phosphocarrier protein that is part of the sugar phosphotransferase system. HPr results in the phosphorylation of other proteins and ultimately in the activation of adenylate cyclase and cyclic AMP (cAMP) production. The ptsH mutant showed only partial hemolysis on blood agar and did not produce LtxA. The phenotype was complemented by supplying wild-type ptsH in trans, and real-time PCR analysis showed that the ptsH mutant produced approximately 10-fold less ltxA mRNA than the wild-type strain. The levels of cAMP in the ptsH mutant were significantly lower than in the wild-type strain, and LtxA production could be restored by adding exogenous cAMP to the culture.

Human Fusobacterium necrophorum strains have a leukotoxin gene and exhibit leukotoxic activity

Fusobacterium necrophorum, a Gram-negative anaerobe, causes a variety of necrotic infections in humans and animals. There are two subspecies: subsp. necrophorum and subsp. funduliforme. In cattle, subsp. necrophorum is more prevalent and production of leukotoxin is a major virulence factor. The leukotoxin operon (lktBAC) consists of three genes, lktB, lktA and lktC, of which lktA is the structural toxin gene. The subspecies identity of human F. necrophorum is less certain and it is not known whether human strains possess the leukotoxin gene or leukotoxin activity. Therefore, the objective of this study was to identify the subspecies status of four human clinical strains of F. necrophorum and determine whether they have the leukotoxin gene or leukotoxin activity. Phenotypic and genotypic characteristics suggested that the four strains belonged to subsp. funduliforme, which was confirmed by sequencing the 16S rRNA gene. Analysis of the four strains by PCR revealed the presence of the leukotoxin operon. Partial DNA sequencing identified one human strain with full-length lktA, whereas the others exhibited considerable heterogeneity in size. All strains had a leukotoxin operon promoter-containing intergenic region similar to that of bovine subsp. funduliforme strains, which was confirmed by DNA sequencing and Southern blotting. Despite variations in the lktA gene, all strains secreted leukotoxin as demonstrated by Western blotting. Flow cytometry assays revealed that the leukotoxin was toxic to human white blood cells. In conclusion, the human strains examined contained a leukotoxin gene whose gene product was biologically active. The importance of leukotoxin as a virulence factor in human fusobacterial infections needs further evaluation.

Secondary necrosis in multicellular animals: an outcome of apoptosis with pathogenic implications

In metazoans apoptosis is a major physiological process of cell elimination during development and in tissue homeostasis and can be involved in pathological situations. In vitro, apoptosis proceeds through an execution phase during which cell dismantling is initiated, with or without fragmentation into apoptotic bodies, but with maintenance of a near-to-intact cytoplasmic membrane, followed by a transition to a necrotic cell elimination traditionally called “secondary necrosis”. Secondary necrosis involves activation of self-hydrolytic enzymes, and swelling of the cell or of the apoptotic bodies, generalized and irreparable damage to the cytoplasmic membrane, and culminates with cell disruption. In vivo, under normal conditions, the elimination of apoptosing cells or apoptotic bodies is by removal through engulfment by scavengers prompted by the exposure of engulfment signals during the execution phase of apoptosis; if this removal fails progression to secondary necrosis ensues as in the in vitro situation. In vivo secondary necrosis occurs when massive apoptosis overwhelms the available scavenging capacity, or when the scavenger mechanism is directly impaired, and may result in leakage of the cell contents with induction of tissue injury and inflammatory and autoimmune responses. Several disorders where secondary necrosis has been implicated as a pathogenic mechanism will be reviewed.

A multiplex polymerase chain reaction assay for the identification of Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis

The objective of this study was to design a multiplex PCR assay to identify Mannheimia haemolytica, Mannheimia glucosida and Mannheimia ruminalis. The multiplex PCR included primer sets HP, amplifying a DNA region from an unknown hypothetical protein, Lkt and Lkt2, amplifying different regions of the leukotoxinD gene, and 16S to amplify universal bacterial sequences of the 16S rRNA gene. Based on positive amplification, isolates were delineated as M. haemolytica (HP, Lkt, 16S), M. glucosida (HP, Lkt, Lkt2, 16S), or M. ruminalis (HP, 16S). The validity of the assay was examined against 22 reference strains within the family Pasteurellaceae and 17 field isolates (nasal) that had been collected previously from feedlot cattle and tentatively identified as M. haemolytica based on morphology and substrate utilization. Additionally, 200 feedlot cattle were screened for M. haemolytica using multiplex PCR. Forty-four isolates from 25 animals were identified as M. haemolytica. The PCR assay positively identified all M. haemolytica, as confirmed by phenotypic tests and clustering based upon cellular fatty acid methyl ester (FAME) profiles. Selected nasal isolates that exhibited evidence of haemolysis, but were M. haemolytica-negative based on PCR, were also confirmed negative by phenotypic and FAME analyses. The multiplex PCR assay required no additional phenotypic tests for confirmation of M. haemolytica, within the group of bacteria tested.

Liver abscesses in feedlot cattle

Liver abscesses in feedlot cattle result from aggressive grain-feeding programs and are influenced by a number of dietary and management factors. They have a major economic impact on the feedlot industry because of liver condemnation and reduced animal performance and carcass yield. Ruminal lesions resulting from acidosis usually are accepted as the predisposing factors. Generally, control of liver abscesses in feedlot cattle has depended on the use of tylosin, which reduces abscess incidence by 40% to 70%. However, new methods and products for liver abscess control are needed. Corn milling by-products that are less fermentable may aide in the quest for cattle production techniques that lead to lower usage of antimicrobials. A vaccine is also commercially available.

IL-1beta secretion induced by Aggregatibacter (Actinobacillus) actinomycetemcomitans is mainly caused by the leukotoxin

Aggregatibacter (Actinobacillus) actinomycetemcomitans forms a leukotoxin that selectively lyses primate neutrophils, monocytes and triggers apoptosis in promyeloic cells and degranulation of human neutrophils. Recently, we showed that the leukotoxin causes activation of caspase-1 and abundant secretion of bio-active IL-1beta from human macrophages. In this study, we show that high levels of IL-beta correlated with a high proportion of A. actinomycetemcomitans in clinical samples from a patient with aggressive periodontitis. To determine the relative contribution of leukotoxin to the overall bacteria-induced IL-1beta secretion, macrophages were isolated from peripheral blood and exposed to different concentrations of live A. actinomycetemcomitans strains with either no, low or high production of leukotoxin. Cell lysis and levels of IL-1beta, IL-6, TNF-alpha and caspase-1 were measured by ELISA and flow cytometry. Leukotoxin was the predominant cause of IL-1beta secretion from macrophages, even in the A. actinomycetemcomitans strain with low leukotoxin production. Macrophages exposed to non-leukotoxic bacteria accumulated cytosolic pro-IL-1beta, which was secreted by a secondary exposure to leukotoxic bacteria. In conclusion, the present study shows for the first time that A. actinomycetemcomitans-induced IL-1beta secretion from human macrophages in vitro is mainly caused by leukotoxin.

Characterisation of Mannheimia spp. strains isolated from bovine nasal exudate and factors associated to isolates, in dairy farms in the Central Valley of Mexico

Mannheimia spp. strains obtained from bovine nasal exudates of either clinically healthy or clinically affected by respiratory tract disease animals were isolated and characterised to estimate the prevalence of isolated serotypes in dairy farms in Mexico, by means of a trans-sectional descriptive study. Strains were isolated and typified through biochemical and immunological tests. chi(2) or Fisher statistical tests were applied, as well as odds ratio calculation and logistic regression analysis to evaluate the association and effect of some variables on Mannheimia spp. isolation. The apparent prevalence rates of Mannheimia haemolytica was significantly higher in diseased bovines (OR = 1.94; p < 0.05), as well as in bovines younger than 1 year of age (OR = 23.98; p < 0.05), and in bovines not vaccinated against bovine pasteurellosis (OR = 1.52; p < 0.05). Age was the variable that remained in the logistic regression model. Serotype A1 showed the highest prevalence, even when most isolates were not-typable. Bovines younger than one year of age and those with disease were the groups with the highest frequency of M. haemolytica and Mannheimia glucosida isolates.

tfoX (sxy)-dependent transformation of Aggregatibacter (Actinobacillus) actinomycetemcomitans

tfoX (sxy) is a regulatory gene needed to turn on competence genes. Aggregatibacter (Actinobacillus) actinomycetemcomitans has a tfoX gene that is important for transformation. We cloned this gene on an IncQ plasmid downstream of the inducible tac promoter. When this plasmid was resident in cells of A. actinomycetemcomitans and tfoX was induced, the cells became competent for transformation. Several strains of A. actinomycetemcomitans, including different serotypes, as well as rough (adherent) and isogenic smooth (nonadherent) forms were tested. Only our two serotype f strains failed to be transformed. With the other strains, we could easily get transformants with extrachromosomal plasmid DNA when closed circular, replicative plasmid carrying an uptake signal sequence (USS) was used. When a replicative plasmid carrying a USS and cloned DNA from the chromosome of A. actinomycetemcomitans was linearized by digestion with a restriction endonuclease or when genomic DNA was used directly, the outcome was allelic exchange. To facilitate allelic exchange, we constructed a suicide plasmid (pMB78) that does not replicate in A. actinomycetemcomitans and carries a region with two inverted copies of a USS. This vector gave allelic exchange in the presence of cloned and induced tfoX easily and without digestion. Using transposon insertions in cloned katA DNA, we found that as little as 78 bp of homology at one of the ends was sufficient for that end to participate in allelic exchange. The cloning and induction of tfoX makes it possible to transform nearly any strain of A. actinomycetemcomitans, and allelic exchange has proven to be important for site-directed mutagenesis.

Systemic macrophage and neutrophil destruction by secondary necrosis induced by a bacterial exotoxin in a Gram-negative septicaemia

Bacterial modulation of phagocyte cell death is an emerging theme in pathogenesis. Here we describe the systemic destruction of macrophages and neutrophils by the Gram-negative Photobacterium damselae ssp. piscicida (Phdp) in fish pasteurellosis, a deadly systemic infection. Following experimental inoculation, Phdp spreads by bacteraemia and colonizes the organs, producing a septicaemic infection, and secretes the apoptogenic exotoxin AIP56 which is systemically disseminated. In experimental and natural pasteurellosis, destruction of macrophages and neutrophils by secondary necrosis following caspase-3-associated apoptosis was seen predominantly in the spleen, head kidney and gut lamina propria. Identical phagocyte destruction occurred after injection of rAIP56, but not of heat-inactivated rAIP56, or AIP56-negative Phdp strains, indicating that AIP56 is responsible for phagocyte destruction occurring in pasteurellosis. Active caspase-3 and active neutrophil elastase are present in the blood in advanced infection, indicating that phagocyte lysis by secondary necrosis is accompanied by release of tissue-damaging molecules. The AIP56-induced lysis of phagocytes represents a very efficient, self-amplifying etiopathogenic mechanism, because it results in two effects that operate in concert against the host, namely, evasion of the pathogen from a crucial defence mechanism through the destruction of both professional phagocytes, and release of tissue-damaging molecules. The induction by a bacterial exotoxin of in vivo systemic lysis of both professional phagocytes by secondary necrosis, now described for the first time, may represent an overlooked etiopathogenic mechanism operating in other infections of vertebrates.

Bacterial pathogenesis and mediators in apical periodontitis

Apical periodontitis is a group of inflammatory diseases caused by microorganisms (mainly bacteria) infecting the necrotic root canal system. The pathogenesis of different types of apical periodontitis and even the same type in different individuals is unlikely to follow a stereotyped fashion with regard to the involved bacterial mediators. Disease pathogenesis is rather complex and involves a multitude of bacteria- and host-related factors. This review article discusses the bacterial pathogenesis of acute and chronic apical periodontitis, with the main focus on the bacterial mediators conceivably involved in the different stages of the infectious process, including secreted products (enzymes, exotoxins, N-formyl-methionyl-leucyl-phenylalanine peptides, heat-shock proteins and metabolic end-products) and structural components (lipopolysaccharide, peptidoglycan, lipoteichoic acid, lipoproteins, fimbriae, flagella, outer membrane proteins and vesicles, DNA, and exopolysaccharides). Knowledge of the bacterial factors involved in the pathogenesis of apical periodontitis is important to the understanding of the disease process and to help establishing proper therapeutic measures to inactivate this bacterial "artillery".