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Pneumonic pasteurellosis: examination of typable and untypable Pasteurella haemolytica strains for leukotoxin
Abstract:
Plasmid DNA screening experiments were conducted to determine whether a relationship existed between the presence of plasmids and antibiotic resistance in Pasteurella haemolytica or the capability to produce hemolysin or leukotoxin (cytotoxin). Regardless of plasmid content, all P haemolytica isolates produced characteristic hemolysis on blood agar plates. Similarly, standardized suspensions of living bacteria and sterile concentrated (approx 200:1) culture supernatant from strains representing each of the 15 recognized P haemolytica serotypes and 7 field strains of P haemolytica (biotype A, serotype 1) produced leukotoxin, which was detected by their capability to cause inhibition of the luminol-dependent chemiluminescence response of bovine neutrophils. However, neither living bacterial suspensions nor concentrated culture supernatant from 4 untypable P haemolytica strains or a P multocida strain caused an inhibition of the luminol-dependent chemiluminescence response. The production of neither hemolysin nor leukotoxin by P haemolytica seemed to be plasmid mediated. Leukotoxin production is apparently a stable phenotypic characteristic of pathogenic P haemolytica strains, and the gene(s) coding for this activity is probably located on the bacterial host chromosome. Antibiotic susceptibility profiles were determined for the different bacterial strains. Studies of ampicillin and penicillin resistance in 8 P haemolytica (biotype A, serotype 1) strains provided evidence that the plasmid, with size of approximately 5,200 base pairs, may code for their resistance to these compounds.
Insights
Pasteurella haemolytica produces leukotoxin regardless of plasmid DNA. However, a specific plasmid may confer antibiotic resistance in some strains, indicating separate genetic control for these traits.
Area of Science:
- Veterinary Microbiology
- Bacterial Genetics
Background:
- Pasteurella haemolytica is an important animal pathogen.
- Understanding the genetic basis of virulence factors like hemolysin and leukotoxin, and antibiotic resistance is crucial.
Purpose of the Study:
- To investigate the role of plasmids in Pasteurella haemolytica hemolysin and leukotoxin production.
- To determine the association between plasmid presence and antibiotic resistance in P. haemolytica.
Main Methods:
- Plasmid DNA screening in P. haemolytica isolates.
- Assessing hemolysin production via hemolysis on blood agar.
- Detecting leukotoxin activity using bovine neutrophils and luminol-dependent chemiluminescence.
- Determining antibiotic susceptibility profiles.
Main Results:
- All P. haemolytica isolates produced hemolysin and leukotoxin, irrespective of plasmid content.
- Leukotoxin production was identified as a stable characteristic, likely chromosomally encoded.
- A ~5,200 bp plasmid was associated with ampicillin and penicillin resistance in specific P. haemolytica strains.
Conclusions:
- Hemolysin and leukotoxin production in P. haemolytica are not plasmid-mediated.
- Leukotoxin production is a stable phenotypic trait of pathogenic strains.
- Plasmids may play a role in antibiotic resistance in certain P. haemolytica strains.