. BALB/c mice given a live vaccine of an A mutant of by intraperitoneal (i.p.) injection were not protected against i.p. challenge with its virulent parental strain but were protected against i.p. challenge with either of two virulent strains of (O [1],4,[5],12). Vaccination with a live vaccine of A protected against challenge with but not with . Intraperitoneal administration of either strain evoked high levels of serum antibody against the homologous lipopolysacharide (LPS) as determined by an enzyme immunoassay. Sera from vaccinated mice also reacted with heterologous LPS but at dilutions at least seven-fold lower than homologous endpoint titres. The vaccination schedule employed with either live-vaccine strain conferred an equal degree of resistance to challenge with . After mixed infection of mice with equal numbers of virulent and by the i.p. route, the former was isolated in numbers at least 50 000 times greater than the latter from the liver and spleen between days 1 and 5. When mice were vaccinated i.p. with , or before mixed infection, neither serotype showed more than a slight predominance in the liver and spleen during the same period. Thus, in relative terms, vaccination with or inoculation with unrelated bacteria suppressed the growth of virulent in mice but allowed virulent to multiply. These results clearly show that 38 can multiply to kill immunised BALB/c mice.


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