Nasal epithelial changes induced in piglets by acetic acid and by Bordetella bronchiseptica

Pasteurella multocida: from zoonosis to cellular microbiology

In a world where most emerging and reemerging infectious diseases are zoonotic in nature and our contacts with both domestic and wild animals abound, there is growing awareness of the potential for human acquisition of animal diseases. Like other Pasteurellaceae, Pasteurella species are highly prevalent among animal populations, where they are often found as part of the normal microbiota of the oral, nasopharyngeal, and upper respiratory tracts. Many Pasteurella species are opportunistic pathogens that can cause endemic disease and are associated increasingly with epizootic outbreaks. Zoonotic transmission to humans usually occurs through animal bites or contact with nasal secretions, with P. multocida being the most prevalent isolate observed in human infections. Here we review recent comparative genomics and molecular pathogenesis studies that have advanced our understanding of the multiple virulence mechanisms employed by Pasteurella species to establish acute and chronic infections. We also summarize efforts being explored to enhance our ability to rapidly and accurately identify and distinguish among clinical isolates and to control pasteurellosis by improved development of new vaccines and treatment regimens.

Bacterial colonization and invasion in pigs experimentally exposed to Streptococcus suis serotype 2 in aerosol

A recently developed porcine model for aerogenous infection with Streptococcus suis serotype 2 was applied in a study of the phases of bacterial colonization and initial invasion. Eighteen pigs were exposed to aerosolized S. suis serotype 2 after pre-exposure to mild acetic acid in aerosol. The animals were killed consecutively within the first six days after challenge. After death, all animals were necropsied and examined by bacteriology, histopathology, and immunohistochemistry. Systemic infection was established in four out of 18 animals exposed to S. suis serotype 2. All systemically infected animals developed clinical signs and lesions typical of the infection. In four additional animals, subclinical infection was demonstrated by re-isolation of S. suis from the palatine tonsil. However, in all 18 challenged animals, immunohistochemistry demonstrated S. suis serotype 2 antigen in the palatine and/or nasopharyngeal tonsils. In all four systemically infected animals, S. suis serotype 2 antigen was also found in the mandibular lymph node. These observations point towards the tonsils as possible portals of entry for S. suis serotype 2 with subsequent lymphogenous spread. Thus, the present findings parallel the proposed pathogenesis for S. suis serotype 1 infection in pigs.

Ultrastructural pathology of nasal and tracheal mucosa of rabbits experimentally infected with Pasteurella multocida serotype D:1

Sixteen 8- to 9-week-old Pasteurella multocida-free New Zealand White rabbits were divided into two equal groups. The first group was inoculated intranasally with P multocida serotype D:1 strain and the second group that was inoculated with phosphate-buffered saline (PBS) only was used as a control group. Pasteurella multocida was isolated from the nasal cavity of all infected rabbits in group 1 and from tracheal swabs of seven rabbits in this group. Four rabbits in group 1 died with clinical signs of septicaemia, two rabbits had mucopurulent nasal discharge and pneumonic lesions and the other two did not show any clinical signs or gross lesions. The ultrastructural changes detected were deciliation or clumping of cilia of ciliated epithelium, cellular swelling, vacuolation and sloughing. The subepithelial capillaries showed congestion, intravascular fibrin deposition, platelets aggregation and endothelial injury. Pasteurella multocida was observed attached to the injured endothelial cells. Heterophils, mast cells, vacuolated monocytes and macrophages infiltrated the lamina propria and between the degenerated epithelial cells.

Enhancement of histamine--induced vascular permeability in guinea pigs infected with Bordetella bronchiseptica

In the nasal mucosa, histamine induces vascular permeability, stimulates nociceptive nerves, and recruits parasympathetic reflexes that regulate glandular exocytosis. Unilateral histamine nasal provocations were performed in a group of guinea pigs in the prodromal stage of undiagnosed Bordetella bronchiseptica infection. Vascular permeability in the histamine challenged nostrils was increased approximately 2- to 4-fold compared to healthy animals (p < 0.001). The duration of significant vascular leak was prolonged from 10 to 30 minutes. In the contralateral, nonchallenged nostrils, secretion of total protein and albumin, but not exudation of intravenously infected 125I-bovine serum albumin, was increased, suggesting an augmentation of parasympathetic reflexes without changes in contralateral vascular leak. These observations suggest that Bordetella bronchiseptica infection leads to hyperresponsiveness to histamine in the nasal mucosa with increased vascular permeability and recruitment of nociceptive nerve-parasympathetic reflexes.

A highly adherent phenotype associated with virulent Bvg+-phase swine isolates of Bordetella bronchiseptica grown under modulating conditions

The ability of Bvg(-)-phase and Bvg(+)-phase Bordetella bronchiseptica swine isolates, grown under modulating or nonmodulating conditions, to adhere to swine ciliated nasal epithelial cells was determined. When virulent strains were cultivated at 37 degrees C in the Bvg+ phase, numerous adherent bacteria (approximately eight per cell, depending on the strain used) were observed. However, when such strains were grown under modulating conditions (23 degrees C), a significant increase in the level of attachment was seen, suggesting that B. bronchiseptica produces a Bvg-repressed adhesin under these conditions. bvg mutant strains, including an isogenic bvgS mutant, adhered minimally. Western blots indicated that two putative B. bronchiseptica adhesins, filamentous hemagglutinin and pertactin, were not detectable in cultures displaying the highly adherent phenotype. Several proteins apparent in Western blots obtained by using bacterial extracts enriched in outer membrane proteins derived from B. bronchiseptica grown at 23 degrees C were not present in similar extracts prepared from an isogenic bvgS mutant grown at 23 degrees C or from the parent strain grown at 37 degrees C. Adherence of bacteria cultivated at 23 degrees C was almost completely abolished by pretreatment of organisms at 60 degrees C; adherence was reduced by 57% when bacteria were pretreated with pronase E. Temperature shift experiments revealed that the heightened level of adhesion that occurs following growth at 23 degrees C was maintained for up to 18 h when bacteria were subsequently incubated at 37 degrees C. We propose that a Bvg-repressed adhesin, expressed only by modulated bvg+ strains of B. bronchiseptica, may play a key role in the initial colonization of naturally infected swine.

Virulence mechanisms of avian fimbriated Escherichia coli in experimentally inoculated chickens

Virulence mechanisms of avian pathogenic Escherichia coli were investigated by inoculating commercial broiler chickens via the left caudal thoracic air sac with three highly pathogenic and three less pathogenic E. coli isolates. At 6 h postinoculation, all isolates had colonized the respiratory tract (trachea, lungs, and air sacs) and internal organs (liver, spleen, and kidney) of inoculated birds, but bacteria were recovered from pericardial fluid and blood only of birds inoculated with the more pathogenic isolates. F1 fimbriae were expressed on a high proportion of bacteria colonizing the trachea and to a lesser extent on bacteria in the lungs of birds inoculated with each of the isolates. F1 fimbriae were also expressed on bacteria in air sacs only for the less pathogenic isolates. P(F11) fimbriae were expressed on bacteria present in air sacs, lungs, kidney, blood, and pericardial fluid of birds inoculated with one of the more virulent isolates. On electron microscopy, bacteria of the more pathogenic isolates but not of the less pathogenic isolates were observed often associated with or within macrophages, which appeared to be viable, in the air sacs and lungs. In in vitro assays, the more pathogenic but not the less pathogenic isolates, were resistant to opsonization and phagocytosis in the absence of F1 fimbriae, whereas bacteria of all isolates were rapidly killed by avian macrophages when they expressed F1 fimbriae. These results suggest that resistance to phagocytosis may be an important mechanism in avian colisepticemia. They also suggest that F1 fimbrial phase variation to the nonfimbriated phase is favored in the avian lower respiratory tract, is more marked for the more pathogenic-isolates, and may be a virulence mechanism.

Localization of the in vivo expression of P and F1 fimbriae in chickens experimentally inoculated with pathogenic Escherichia coli

Escherichia coli causing septicemia in poultry often possess F1 (type 1) and/or P fimbriae which may be involved in bacterial colonization and infection. To investigate the expression of these fimbriae in vivo, two pathogenic E. coli strains with different fimbrial profiles, TK3 (fim+/pap+) and MT78 (fim+/pap-), were administered to 2-week-old chickens by either the intratracheal or caudal thoracic air sac inoculation route. Antibodies specific for native F1 fimbriae were detected by ELISA and immunodot in the serum of chickens inoculated with either strain MT78 or strain TK3, irrespective of the route of inoculation. Antibodies specific for P fimbriae of serotype F11 were detected by ELISA and immunoblotting in the serum of chickens inoculated by either route with strain TK3. F1, but not P fimbriae, were expressed by bacteria colonizing the trachea of chickens inoculated by the air sac route with strain MT78 or TK3, as demonstrated by examination of frozen tissue sections using immunofluorescence. F1 fimbriae were also expressed by bacteria colonizing the air sacs and lungs, but not by bacteria in the blood or other internal organs, of chickens inoculated with either strain. P fimbriae were expressed by bacteria colonizing the air sacs, lungs, kidney, blood, and pericardial fluid, but not by bacteria colonizing the trachea, of chickens inoculated with strain TK3. Fimbriae-like structures were observed by electron microscopy on bacteria adhering to the epithelial cells of the air sacs of chickens inoculated with strain TK3. These results demonstrate that both strains MT78 and TK3 undergo in vivo phase variation with respect to their fimbrial profiles and site of bacterial colonization in different organs of infected chickens and suggest that F1 fimbriae are important for initial bacterial colonization of the upper respiratory tract whereas P fimbriae are important for later stages of the infection.

A porcine model for the evaluation of virulence of Bordetella bronchiseptica

Studies of virulence factors of Bordetella bronchiseptica require a suitable system. Such a system was devised in colostrum-deprived, caesarean-derived pigs, aged 7 d. In two different experiments, pigs (n = 11) were inoculated intranasally with 10(6) colony-forming units of the virulent strain 4609. In the same way, further pigs (n = 11) were inoculated with a strain (B133) of unknown virulence. No significant differences between 4609 and B133 colonization were seen. However, colonization of the turbinates was significantly higher than that of the trachea, lung and tonsil, and a significantly higher degree of colonization was present at 11 d post-inoculation (PI) than at 15 days. Moderate turbinate atrophy was present by 11 d PI, and peribronchiolar fibrosis was present at 15 days. Immunocytochemical methods showed that all pigs had bacterial antigen in the ciliated cells of the turbinates and trachea, and in the lung; some pigs also had antigen in the bronchi. Bacterial antigen was present in some bronchioles and within the cytoplasm of pulmonary macrophages and neutrophils. This model should prove useful for comparing strains of B. bronchiseptica and isogenic mutants deficient in putative virulence factors.

Ultrastructural characterization of the nasal respiratory epithelium in the piglet

BACKGROUND

Very little information is available on the ultrastructure of the nasal cavity epithelium of the piglet. However, the nasal respiratory epithelium plays an important role in the pathology of atrophic rhintis of the piglet. Indeed, ciliated cells and mucus play a co-ordinate role in the colonization of the nasal cavity by the etiological agents of the disease.

METHODS

In the present study, samples of the ventral nasal turbinates of germ-free piglets were processed for observation in the transmission electron microcope to describe the ultrastructure of their covering respiratory epithelium.

RESULTS

Five morphologically distinct cell types were observed. Ciliated cells and basal cells were similar to that described in the nasal cavity of other species. On the basis of their secretory granule morphology, five forms of goblet cells were observed. Nonciliated, nonsecretory columnar cells with short, thick, regularly and densely spaced apical microvilli were identified as brush cells. A distinct type of secretory cells was found. Their apical surface protruded above the adjacent cells and had a few microvilli covered with thin hairlike projections. They were rich in smooth endoplasmic reticulum and had an apocrinelike type of secretion.

CONCLUSIONS

These findings indicate the complexity of cell types of the piglet nasal respiratory epithelium.