Using a Co60 source, repeated irradiation of survivors of bacterial cultures at a level initially destroying a high proportion of cells gave rise to strains which were more resistant to irradiation than the original cultures. Development of resistance was shown in single strains of Escherichia coli, Streptococcus faecalis, and Clostridium botulinum type A, and in one of three strains of Staphylococcus aureus, but not in two strains of C. botulinum type E nor in Salmonella gallinarum. Changes were noted in phage patterns of the staphylococci, biochemical characteristics of E. coli, and toxin production of C. botulinum in response to repeated irradiation.Irradiation of parent and resistant cultures at −78 °C did not change their comparative resistance, though each was afforded some protection at this temperature. These findings suggest that the induced resistance is an expression of resistance to the primary effect of irradiation and not to possible toxic substances formed by free-radical interaction.

SUMMARY From comparative study of four methods of isolation of Salmonella from egg products, of six enrichment broths and five selective media, and of several secondary technical operations, a recommended scheme for isolation of salmonellae from foods was devised and shown to be effective for diverse foods containing as few as 0.15 salmonellae per g, even in the presence of extreme coliform and other bacterial contamination.

Changes in host cell permeabilitf induced by fungus parasites selectecl from several distinctive typcs of disease relationship were studied by plasmolytic nrethotls.

When Escherichia coli was grown in a synthetic medium and fixed with osmium, sections of the cells revealed clearly defined mesosomes. These mesosomes appeared to develop, in dividing cells, as coiled infoldings of the cytoplasmic membrane. Mature mesosomes formed a link between the cytoplasmic membrane and the nucleus of the cell. The arrangement of the mesosomes in dividing cells led to the hypothesis that division of the nucleus in these cells is accomplished by two separate polar mesosomes. One mesosome is derived from the parent cell and is present at one pole of the daughter cell. The other is freshly synthesized at or near the newly forming pole of the daughter cell. While the old mesosome remains attached to the chromosome received from the parent cell, the newly synthesized mesosome becomes attached to and initiates replication of the new chromosome. As the cell grows and elongates, the two mesosomes, attached to their respective chromosomes move apart, thus effecting nuclear division.

Summary The development of botulinum toxin in smoked fish and of staphylococcal enterotoxin in packed bacon has been demonstrated. It is to be expected that these toxins may be produced in contaminated commercial products stored under conditions which permit substantial growth of the organisms, though formation of enterotoxin is not a predictable sequel to multiplication of staphylococci.