Insertion mutants of a strain of serotype Gallinarum, the cause of fowl typhoid, were produced with transposon Tn Eight mutants were identified as being less invasive in cell culture than the parent strain. Neither the parent strain nor the mutants showed mannose-sensitive haemagglutination of various red blood cell species. Although two mutants gave mannose-resistant (MR) haemagglutination of red cells of different animal species these MR activities could not be correlated with other characteristics or The mutant strains were divisible into three classes by their patterns of invasiveness and adhesiveness and by changes in membrane proteins. Strains of classes 1 and 2 had single transposon insertions, detected by Southern hybridisation, of 8•9 and 2•4 kb RV-digested chromosomal-fragments, respectively, and were slightly less invasive than the parent strain. They were no less virulent for chickens by the oral route than a mutant strain shown to be as invasive as the parent strain. The class-2 mutant was also less adhesive than the other strains. The single mutant strain of class 3 which contained insertions in several chromosomal fragments, was non-invasive in Vero cells (<log 1•0 cfu/ml recovered compared with log 2•88 for the parent strain) and showed reduced virulence by both oral and intramuscular routes. The mutant strains of all classes were taken up equally rapidly from the blood by the spleen. Intramuscular immunisation of chickens with the class-3 mutant strain gave complete protection against oral challenge 3 weeks later with 1000 oral LD50 doses of the virulent parent strain.


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