Survival of avian strains of Pasteurella multocida in chicken serum

Host-like carbohydrates promote bloodstream survival of Vibrio vulnificus in vivo

Sialic acids are found on all vertebrate cell surfaces and are part of a larger class of molecules known as nonulosonic acids. Many bacterial pathogens synthesize related nine-carbon backbone sugars; however, the role(s) of these non-sialic acid molecules in host-pathogen interactions is poorly understood. Vibrio vulnificus is the leading cause of seafood-related death in the United States due to its ability to quickly access the host bloodstream, which it can accomplish through gastrointestinal or wound infection. However, little is known about how this organism persists systemically. Here we demonstrate that sialic acid-like molecules are present on the lipopolysaccharide of V. vulnificus, are required for full motility and biofilm formation, and also contribute to the organism's natural resistance to polymyxin B. Further experiments in a murine model of intravenous V. vulnificus infection demonstrated that expression of nonulosonic acids had a striking benefit for bacterial survival during bloodstream infection and dissemination to other tissues in vivo. In fact, levels of bacterial persistence in the blood corresponded to the overall levels of these molecules expressed by V. vulnificus isolates. Taken together, these results suggest that molecules similar to sialic acids evolved to facilitate the aquatic lifestyle of V. vulnificus but that their emergence also resulted in a gain of function with life-threatening potential in the human host.

Pharmacokinetic/pharmacodynamic relationship of marbofloxacin against Pasteurella multocida in a tissue-cage model in yellow cattle

The fluoroquinolone antimicrobial drug marbofloxacin was administered to yellow cattle intravenously and intramuscularly at a dose of 2 mg/kg of body weight in a two-period crossover study. The pharmacokinetic properties of marbofloxacin in serum, inflamed tissue-cage fluid (exudate), and noninflamed tissue-cage fluid (transudate) were studied by using a tissue-cage model. The in vitro and ex vivo activities of marbofloxacin in serum, exudate, and transudate against a pathogenic strain of Pasteurella multocida (P. multocida) were determined. Integration of in vivo pharmacokinetic data with the in vitro MIC provided mean values for the area under the curve (AUC)/MIC for serum, exudate, and transudate of 155.75, 153.00, and 138.88, respectively, after intravenous dosing and 160.50, 151.00, and 137.63, respectively, after intramuscular dosing. After intramuscular dosing, the maximum concentration/MIC ratios for serum, exudate, and transudate were 21.13, 9.13, and 8.38, respectively. The ex vivo growth inhibition data after intramuscular dosing were fitted to the inhibitory sigmoid Emax equation to provide the values of AUC/MIC required to produce bacteriostasis, bactericidal activity, and elimination of bacteria. The respective values for serum were 17.25, 31.29, and 109.62, and slightly lower values were obtained for transudate and exudate. It is proposed that these findings might be used with MIC50 or MIC90 data to provide a rational approach to the design of dosage schedules which optimize efficacy in respect of bacteriological as well as clinical cures.

Serum-resistance in Haemophilus parasuis is associated with systemic disease in swine

Haemophilus parasuis can cause pneumonia and systemic disease in swine but it is also a coloniser of the upper respiratory tract of healthy pigs. These differences in pathogenicity are probably the result of diverse mechanisms of virulence in different strains. Since serum-resistance is a feature frequently found in systemic pathogens, 31 H. parasuis strains of different clinical origin were tested and a variety of serum susceptibility levels detected. Nasal strains from healthy piglets were sensitive to the bactericidal effect of the serum, while systemic strains were mainly resistant. The pulmonary strains included both serum-sensitive and serum-resistant strains. Interestingly, the serum-resistant pulmonary strains were isolated from animals with systemic lesions. Heat-treatment of the sera abolished the bactericidal activity, indicating that complement is a key factor in this effect. Equivalent susceptibility was observed with rabbit and porcine sera, and the presence of H. parasuis specific antibodies did not increase the killing of the strains by serum. In an attempt to associate serum-resistance to a surface determinant of the bacteria, agglutination in acriflavine was tested but no direct link with serum susceptibility was found. The results indicate that serum-resistance is a virulence mechanism in H. parasuis.

Virulence characterization ofAvibacterium paragallinarumisolates from Uganda

Avibacterium paragallinarum isolates from Uganda were characterized for their virulence by comparison of their pathogenicity and their resistance to serum. Pathogenicity was evaluated using commercial Hisex Brown layer chickens, local indigenous chickens, local turkeys and local guineafowls inoculated with 108 colony-forming units of Av. paragallinarum and comparing their overall mean disease scores over a period of 20 days. Persistence of the bacteria in the host and water was also investigated for a 60-day period by culture and polymerase chain reaction as well as use of sentinel chickens. Serum resistance was measured by comparison of the growth kinetics and survival indices at 3 and 6 h. There was no difference in the virulence of the isolates. Commercial layer chickens and local indigenous chickens were equally susceptible to challenge, while turkeys and guineafowls only showed transient mild signs and did not transmit infection. Turkeys and guineafowls did not acquire the infection when placed in contact with infected chickens. The isolates were resistant in normal chicken serum at both 3 and 6 h of incubation but were resistant at 3 h and sensitive at 6 h in turkey and guineafowl sera. The resistance of the isolates to serum correlated with their pathogenicity in the different hosts. No carrier status was demonstrated in this study using polymerase chain reaction and culture. The present study demonstrates that Ugandan Av. paragallinarum isolates are pathogenic to chickens with only transient signs in turkeys and guineafowls, and that serum resistance could be a subject for further investigation as a predictor of virulence of these bacteria. The role of turkeys and guineafowls in transmission of Av. paragallinarum was not demonstrated in the present investigation.

A heptosyltransferase mutant of Pasteurella multocida produces a truncated lipopolysaccharide structure and is attenuated in virulence

Pasteurella multocida is the causative agent of fowl cholera in birds. In a previous study using signature-tagged mutagenesis, we identified a mutant, AL251, which was attenuated for virulence in mice and in the natural chicken host. Sequence analysis indicated that AL251 had an insertional inactivation of the gene waaQ(PM), encoding a putative heptosyl transferase, required for the addition of heptose to lipopolysaccharide (LPS) (M. Harper, J. D. Boyce, I. W. Wilkie, and B. Adler, Infect. Immun. 71:5440-5446, 2003). In the present study, using mass spectrometry and nuclear magnetic resonance, we have confirmed the identity of the enzyme encoded by waaQ(PM) as a heptosyl transferase III and demonstrated that the predominant LPS glycoforms isolated from this mutant are severely truncated. Complementation experiments demonstrated that providing a functional waaQ(PM) gene in trans can restore both the LPS to its full length and growth in mice to wild-type levels. Furthermore, we have shown that mutant AL251 is unable to cause fowl cholera in chickens and that the attenuation observed is not due to increased serum sensitivity.