Development of Bacterial Therapeutics against the Bovine Respiratory Pathogen Mannheimia haemolytica

Bovine respiratory disease (BRD) is a major cause of morbidity and mortality in beef cattle. Recent evidence suggests that commensal bacteria of the bovine nasopharynx have an important role in maintaining respiratory health by providing colonization resistance against pathogens. The objective of this study was to screen and select bacterial therapeutic candidates from the nasopharynxes of feedlot cattle to mitigate the BRD pathogen Mannheimia haemolytica In a stepwise approach, bacteria (n = 300) isolated from the nasopharynxes of 100 healthy feedlot cattle were identified and initially screened (n = 178 isolates from 12 different genera) for growth inhibition of M. haemolytica Subsequently, selected isolates were evaluated for the ability to adhere to bovine turbinate (BT) cells (n = 47), compete against M. haemolytica for BT cell adherence (n = 15), and modulate gene expression in BT cells (n = 10). Lactobacillus strains had the strongest inhibition of M. haemolytica, with 88% of the isolates (n =33) having inhibition zones ranging from 17 to 23 mm. Adherence to BT cells ranged from 3.4 to 8.0 log10 CFU per 105 BT cells. All the isolates tested in competition assays reduced M. haemolytica adherence to BT cells (32% to 78%). Among 84 bovine genes evaluated, selected isolates upregulated expression of interleukin 8 (IL-8) and IL-6 (P < 0.05). After ranking isolates for greatest inhibition, adhesion, competition, and immunomodulation properties, 6 Lactobacillus strains from 4 different species were selected as the best candidates for further development as intranasal bacterial therapeutics to mitigate M. haemolytica infection in feedlot cattle.IMPORTANCE Bovine respiratory disease (BRD) is a significant animal health issue impacting the beef industry. Current BRD prevention strategies rely mainly on metaphylactic use of antimicrobials when cattle enter feedlots. However, a recent increase in BRD-associated bacterial pathogens that are resistant to metaphylactic antimicrobials highlights a pressing need for the development of novel mitigation strategies. Based upon previous research showing the importance of respiratory commensal bacteria in protecting against bronchopneumonia, this study aimed to develop bacterial therapeutics that could be used to mitigate the BRD pathogen Mannheimia haemolytica Bacteria isolated from the respiratory tracts of healthy cattle were characterized for their inhibitory, adhesive, and immunomodulatory properties. In total, 6 strains were identified as having the best properties for use as intranasal therapeutics to inhibit M. haemolytica If successful in vivo, these strains offer an alternative to metaphylactic antimicrobial use in feedlot cattle for mitigating BRD.

Genomic signatures of Mannheimia haemolytica that associate with the lungs of cattle with respiratory disease, an integrative conjugative element, and antibiotic resistance genes

BACKGROUND

Mannheimia haemolytica typically resides in cattle as a commensal member of the upper respiratory tract microbiome. However, some strains can invade their lungs and cause respiratory disease and death, including those with multi-drug resistance. A nucleotide polymorphism typing system was developed for M. haemolytica from the genome sequences of 1133 North American isolates, and used to identify genetic differences between isolates from the lungs and upper respiratory tract of cattle with and without clinical signs of respiratory disease.

RESULTS

A total of 26,081 nucleotide polymorphisms were characterized after quality control filtering of 48,403 putative polymorphisms. Phylogenetic analyses of nucleotide polymorphism genotypes split M. haemolytica into two major genotypes (1 and 2) that each were further divided into multiple subtypes. Multiple polymorphisms were identified with alleles that tagged genotypes 1 or 2, and their respective subtypes. Only genotype 2 M. haemolytica associated with the lungs of diseased cattle and the sequence of a particular integrative and conjugative element (ICE). Additionally, isolates belonging to one subtype of genotype 2 (2b), had the majority of antibiotic resistance genes detected in this study, which were assorted into seven combinations that ranged from 1 to 12 resistance genes.

CONCLUSIONS

Typing of diverse M. haemolytica by nucleotide polymorphism genotypes successfully identified associations with diseased cattle lungs, ICE sequence, and antibiotic resistance genes. Management of cattle by their carriage of M. haemolytica could be an effective intervention strategy to reduce the prevalence of respiratory disease and supplemental needs for antibiotic treatments in North American herds.

Bactericidal effects of various concentrations of enrofloxacin, florfenicol, tilmicosin phosphate, and tulathromycin on clinical isolates of Mannheimia haemolytica

OBJECTIVE

To determine bactericidal effects of enrofloxacin, florfenicol, tilmicosin, and tulathromycin on clinical isolates of Mannheimia haemolytica at various bacterial densities and drug concentrations.

SAMPLE

4 unique isolates of M haemolytica recovered from clinically infected cattle.

PROCEDURES

Minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) were determined for each drug and isolate. Mannheimia haemolytica suspensions (10(6) to 10(9) CFUs/mL) were exposed to the determined MIC and MPC and preestablished maximum serum and tissue concentrations of each drug. Log10 reduction in viable cells (percentage of cells killed) was measured at various points.

RESULTS

Bacterial killing at the MIC was slow and incomplete. After 2 hours of isolate exposure to the MPC and maximum serum and tissue concentrations of the tested drugs, 91% to almost 100% cell killing was achieved with enrofloxacin, compared with 8% growth to 93% cell killing with florfenicol, 199% growth to 63% cell killing with tilmicosin, and 128% growth to 43% cell killing with tulathromycin over the range of inoculum tested. For all drugs, killing of viable organisms was evident at all bacterial densities tested; however, killing was more substantial at the MPC and maximum serum and tissue drug concentrations than at the MIC and increased with duration of drug exposure. Rank order of drugs by killing potency was enrofloxacin, florfenicol, tilmicosin, and tulathromycin.

CONCLUSIONS AND CLINICAL RELEVANCE

Findings suggested that antimicrobial doses that equaled or exceeded the MPC provided rapid killing of M haemolytica by the tested drugs, decreasing opportunities for antimicrobial-resistant subpopulations of bacteria to develop during drug exposure.

Comparison of Mannheimia haemolytica isolates from an outbreak of bovine respiratory disease

The objective of this study was to determine the clonal relatedness of Mannheimia haemolytica isolates responsible for an outbreak of bovine respiratory disease in a commercial feedlot. The isolates were obtained from the lungs of 21 calves with fatal pneumonia that were part of a group of 206 total calves. All isolates were serotyped and analyzed by pulsed-field gel electrophoresis (PFGE) and for antibiotic sensitivity patterns. ELISA and immunoblotting assays were performed to compare serum antibody levels to M. haemolytica antigens in calves with fatal pneumonia to those calves that survived the outbreak. Isolates were categorized into 14 different PFGE groups based on 90% similarity. Two Group D isolates (1 and 6), and 3 Group H isolates (14, 15, and 16) were characterized as 100% similar. Antimicrobial susceptibility profiles defined 8 groups based on differences in patterns of resistance between isolates. The two 100% similar isolates from PFGE Group D were both in susceptibility Group 1. All but isolate 14 from PFGE Group H (3, 15, 16, and 19) were in susceptibility Group 4a. Serum antibody levels to M. haemolytica antigens in the dead calves were not different than the antibody levels in the 185 calves that survived the outbreak. Immunoblots of selected isolates from each of the PFGE groups demonstrated only minimal differences in antigenic profiles between strains when reacted with serum from calves that either died from or survived the outbreak. Based on the characteristics of these isolates, multiple strains of M. haemolytica were responsible for fatal pneumonia during this outbreak.

Immunohistochemical expression of nuclear factor erythroid-2-related factor 2 and heme oxygenase 1 in normal bovine lung and bovine lung infected with Mannheimia haemolytica

Mannheimia haemolytica is an important cause of pneumonia in feedlot cattle. Nuclear factor erythroid-2-related factor 2 (Nrf2) is a redox-sensitive transcription factor responsible for the induction of antioxidant enzymes, such as heme oxygenase 1 (HO-1), within the lung. The expression of Nrf2 and HO-1 was immunohistochemically evaluated in 4 calves 24 h after experimental infection with M. haemolytica. Calves receiving normal saline served as controls. In the infected lungs, cytoplasmic Nrf2 expression was high in macrophages and bronchioles and low in alveolar epithelium, whereas nuclear expression was high in endothelial cells, macrophages, and bronchioles and lowest in alveolar epithelium. Normal lung samples displayed only faint Nrf2 cytoplasmic staining within bronchiolar epithelium. Expression of HO-1 was detected within the cytoplasm of macrophages and bronchiolar epithelial cells in all infected lung samples, whereas normal lungs displayed only weak cytoplasmic staining in bronchiolar epithelial cells. These findings suggest that bronchiolar epithelial cells and macrophages up-regulate Nrf2 expression early in the course of infection, which results in increased expression of HO-1 within these cells.

Serotyping of Mannheimia haemolytica isolates from bovine pneumonia: 1987–2006

Over a period of 20 years, a total of 207 Mannheimia haemolytica samples were isolated from calves affected with pneumonic pasteurellosis and serotyped by the indirect haemagglutination test. Serotypes A1 (102 isolates), A2 (47 isolates) and A6 (42 isolates) were most common; in addition, 16 isolates were serotypes A7, A13, A14 or untypable. The relative prevalence of serotype A6 has increased recently in Japan, as has been reported from other countries. The results of this study provide useful information towards the design of efficient vaccines for the prevention of M. haemolytica infection in Japan.

Exact one-sided confidence bounds for the risk ratio in 2 x 2 tables with structural zero

This paper examines exact one-sided confidence limits for the risk ratio in a 2 x 2 table with structural zero. Starting with four approximate lower and upper limits, we adjust each using the algorithm of Buehler (1957) to arrive at lower (upper) limits that have exact coverage properties and are as large (small) as possible subject to coverage, as well as an ordering, constraint. Different Buehler limits are compared by their mean size, since all are exact in their coverage. Buehler limits based on the signed root likelihood ratio statistic are found to have the best performance and recommended for practical use.

The response of Mannheimia haemolytica to iron limitation: implications for the acquisition of iron in the bovine lung

Mannheimia haemolytica is the major causative agent of shipping fever, a severe pneumonia in cattle causing high morbidity and mortality. A prerequisite of successful lung colonization by M. haemolytica is the necessity to adapt to the paucity of iron. The lack of genome information has precluded an assessment of the genetic repertoire available to M. haemolytica to adapt to low iron environments. To close this knowledge-gap, we have determined 90% of a virulent M. haemolytica serotype A1 genome sequence and produced a microarray in order to study gene expression under iron-limiting growth for 15, 30 and 60 min. M. haemolytica responded to iron limitation by the up-regulation of transcripts coding for receptors and ABC-type transporters of transferrin, haemoglobin, haem and siderophores. Real time PCR analysis of lung tissue from Mannheimia-infected calves demonstrated the in vivo transcription of two potential haemoglobin receptors, hmbR1 and hmbR2. The relative hmbR1 and hmbR2 transcript levels in the infected lung tissue were comparable to the induced levels observed under iron-limiting growth, demonstrating in vivo induction of receptor transcription in the context of an infection. When the iron response of M. haemolytica was compared to the iron response of Pasteurella multocida, another pathogen colonizing the bovine lung, only few homologous genes were induced in both organisms. These included the haemoglobin receptor hmbR2 and the periplasmic transport systems yfeABCD and fbpABC. The comparative analysis suggests that the two pathogens use different strategies to adapt to the iron-limiting environment in the bovine host.

Calf Respiratory Disease and Pen Microenvironments in Naturally Ventilated Calf Barns in Winter

Relationships between air quality, a variety of environmental risk factors, and calf respiratory health were studied in 13 naturally ventilated calf barns during winter. A minimum of 12 preweaned calves were randomly selected and scored for the presence of respiratory disease in each barn. An air sampling device was used to determine airborne bacteria colony-forming units per cubic meter (cfu/m3) of air in calf pens and central alleys within the barns. Airborne bacteria samples were collected on sheep blood agar (BAP) and eosin methylene blue (EMB) agar plates. Temperature and relative humidity were recorded in each calf pen, the barn alley, and outside the barn. Samples of bedding were collected in each pen and DM was measured. Pen bedding type and a calf nesting score (degree to which the calves could nestle into the bedding) was assigned to each barn. Calf numbers, barn and pen dimensions, ridge, eave, and curtain openings, and exterior wind speed and direction were determined and used to estimate building ventilation rates. Factors that were significantly associated with a reduced prevalence of respiratory disease were reduced pen bacterial counts (log10 cfu/m3) on BAP, presence of a solid barrier between each calf pen, and increased ability to nest. Individual calf pen bacterial counts were significantly different from barn alley bacterial counts on both BAP and EMB. Significant factors associated with reduced calf pen bacterial counts on BAP were increasing pen area, increasing number of open planes of the calf pen, decreasing pen temperature, and wood-particle bedding. Significant factors associated with reduced alley bacterial counts on BAP were increased ventilation changes per hour, increased barn volume per kilogram of calf, reduced pen bacterial counts, and barn type.

Maternally and naturally acquired antibodies to Mannheimia haemolytica and Pasteurella multocida in beef calves

The dynamics and duration of maternally derived antibodies as well as the onset of acquired immunity against Mannheimia haemolytica and Pasteurella multocida in range-pastured beef calves were investigated. Two groups of unvaccinated cattle were used in this study. Serum antibody responses were measured by enzyme-linked immunoassay for antibodies of the IgG1, IgG2 and IgM isotypes binding M. haemolytica whole cells (WC) or leukotoxin (LKT) and P. multocida outer membrane proteins (OMPs). Comparisons of mean antibody responses to M. haemolytica LKT and WC and P. multocida OMPs were made within each group. Maternally derived antibodies against M. haemolytica and P. multocida reached lowest levels at 30-90 days after birth. Calves began production of antibodies against M. haemolytica and P. multocida between 60 and 90 days of age in both groups. Based on the results of this study, in beef herds vaccinated against M. haemolytica and/or P. multocida, it may be best to vaccinate calves around 3 months of age. In contrast, beef calves from unvaccinated herds might benefit from vaccination at 4 months of age.