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The ability of the aerobic bacterial flora from the normal bovine respiratory and reproductive tracts to enhance or inhibit the growth of Pasteurella haemolytica, P. multocida, and Haemophilus somnus was tested in vitro. Six strains of each of these pathogens were cross streaked with each isolate of bovine normal flora. Flora which enhanced the growth of these pathogenic bacteria outnumbered inhibitors four to one. An intermediate number of isolates produced no effect on pathogen growth. Most enhancers were gram positive (Micrococcus, Staphylococcus, Corynebacterium, or Rhodococcus isolates), although several isolates of Moraxella and Actinobacter were also good enhancers. For H. somnus, there were proportionally more organisms which produced marked enhancement among the preputial flora than among the nasal flora, which may account for the greater number of genital carriers than nasal carriers. Bacillus isolates were the most significant inhibitors among the nasal flora, whereas no genus or species from the reproductive tract was noted to produce appreciable inhibition. It is proposed that changes in ratios of inhibitors to enhancers may determine, in part, whether a carrier state or disease occurs. Also, suggestions are made for in vitro use of this phenomenon for diagnostic tests.

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