- Volume 8, Issue 1, 1975

A virulent variant of the normally avirulent Kunz strain of influenza virus was obtained by serial passage in mice, and the pathogenesis of the infections caused by the two strains was studied.

The virulence of the passaged variant did not appear to result from increased growth in lungs, from acquisition of resistance to non-specific inhibitors or from inadequate immunogenicity, but from its greater ability to replicate in alveolar cells.

The apparent adequacy of defence mechanisms to protect against mucosal infection but their failure to protect against infection of alveolar cells is discussed.

The possession of any of eight different plasmids by Staphylococcus aurew strain 649-either singly or simultaneously (in no. 649MR)-caused changes in growth kinetics. Six of the plasmids caused an increase in exponential doubling time (by 8.25%), and most also altered the duration of the lag period. Strain 649MR was significantly less virulent for 10-day chick embryos than the corresponding plasmid-negative culture (no. 649N). The avirulence persisted even after loss of the plasmids from no. 649MR. The presence of a single plasmid specifying tetracycline resistance produced a moderate reduction in virulence, but chromosomal tetracycline resistance had an insignificant effect on it. The decrease in virulence could not be attributed to reduced formation of soluble products. It probably resulted from alterations in the cell surface, but membrane-polypeptide profiles of virulent and avirulent cells lacking plasmids were similar. Survival of strains 649MR and 649N on glass was identical. Therefore, reduction in the incidence of staphylococcal sepsis may be due in part to loss of virulence that has resulted from plasmid carriage.

A new, two-tier system for biotyping Salmonella typhimurium gives a fined and more reliable differentiation of strains than the Kristensen scheme and is. capable of future extension by the addition of new types and new tests. Strains are allocated to a primary type (1-32) by their reactions in five primary tests with Bitter’s xylose medium, meso-inositol, L-rhamnose, d-tartrate and mtartrate. Subtypes are distinguished within the primary types by reactions in ten secondary tests, which include observations for flagella and type-1 (haemagglutinating) fimbriae. Full biotypes are designated by letters indicating the subtype reactions appended to the primary-type numbers.

A series of 2030 strains of S. typhimurium collected from many different sources and countries during 53 years was classified into 19 of the 32 potential primary biotypes and into 144 full biotypes. Of the series, 14% (275) were non-fimbriate inositol-nonfermenting rhamnose-nonfermenting (FIRN) strain in primary biotypes 29-32. Most other strains were fimbriate and rhamnose fermenting.

Observations on several series of cultures isolated from different human or animal sources in the same epidemic showed that the biotype characters of a strain were generally stable during its growth in the natural environment and in the unselective media used for isolation and storage. Most non-fermenting strains gave rise to fermenting mutants on prolonged incubation in the substratecontaining- and therefore selective-test medium, and false-positive results from this cause were avoided by making the definitive readings of tests after a short, carefully chosen period of incubation.

A genealogical tree has been drawn to show how eighteen observed primary biotypes may have evolved from a presumed archetypal ancestor of biotype 1.