Pasteurella multocida possesses a characteristically gram-negative ultrastructure, yet its inability to grow in the presence of hydrophobic compounds and the general penicillin susceptibility of genera making up the family Pasteurellaceae suggest a cell envelope having atypical permeability properties. The cell surface hydrophobicity properties of strains representing 15 of the 16 somatic serotypes of P. miltocida and three strains of ActinobaciUlus lignieresii were assessed with hydrocarbon adherence and hydrophobic interaction chromatographic assays. These methods revealed surface hydrophobicity to vary dramatically among strains in both species. No direct correlation was observed with species, growth rate, or susceptibility to the antibiotics oxytetracycline (polar), polymyxin B (amphiphilic), or novobiocin (nonpolar) as measured with MIC determinations. All strains were susceptible to the antibiotics, although A. lignieresii was significantly less susceptible than P. multocida to novobiocin. These data suggest that cell surface hydrophobicity in P. multocida may be influenced by the type of lipopolysaccharide present but is not directly related to permeability of the antibiotics examined. The wide diversity of hydrophobic properties exhibited by strains of both P. multocida and A. lignieresii precludes the use of this parameter as a taxonomic aid.

Thirty independently derived reference strains and clinical isolates of Pasteurella multocida were tested to determine their potential for acquiring adaptive resistance to oxytetracycline in an effort to better understand the prolonged high efficacy of the antibiotic for pasteurellosis in poultry. All reference strains and clinical isolates exhibited uniform susceptibility as measured with the broth dilution method. None of the strains or isolates readily acquired significant resistance when grown in subinhibitory oxytetracycline levels under the conditions employed. These data support the conclusions that spontaneous variation in P. multocida resulting in oxytetracycline resistance is uncommon in the field and that the organism possesses a very low propensity for acquiring adaptive resistance in response to growth in the presence of the antibiotic.