SUMMALRYThe mechanism causing viable Francisella tularensis to lose virulence in aerosols has been investigated. Fully virulent organisms were found to be encapsulated and avirulent organisms from aged aerosols, decapsulated. Capsules were also removed by suspension of F. tularensis in hypertonic sodium chloride. The resulting naked, but viable, organisms were predominantly avirulent for guinea-pigs challenged intraperitoneally. Capsular material and cell walls were found to contain large amounts of lipid, about 50 and 70 % (w/w) respectively, and to differ in lipid and sugar composition.

Eighty-two chickens purchased at 11 retailers (supplied by 12 wholesalers) in the south of England were cultured for Campylobacter jejuni by a method involving total immersion. The organism was isolated from 22 (48%) of 46 fresh birds, 12 of 12 uneviscerated (New York dressed) birds, but only 1 of 24 frozen birds. Viable counts of up to 1.5 x 10(6)/chicken were obtained from fresh birds and 2.4 x 10(7)/chicken from uneviscerated birds. Surface swabbing of breasts, thighs, wings and vents of fresh chickens showed that contamination was generally distributed over the carcasses. Salmonellas were found in only 2 of 69 of the fresh chickens. The prevalence of several Lior and Penner C. jejuni serogroups was similar in chickens and sporadic human cases of enteritis. Penner serogroup 4 (mostly of Lior serogroup 1) was found in 26% of human isolates and 14% of chicken isolates. The rising incidence of campylobacter enteritis during the last 6 years could well be a reflection of the increasing proportion of fresh chickens consumed over that period (32% higher in 1986 than in 1981).

The concentration of viable cells in an aerosol of micro-organisms is reduced by physical loss and biological decay. To study biological decay it is necessary to distinguish between the two processes. The preferred way of doing this is to mix the test aerosol with a tracer which is subject only to physical loss: the tracer and test cells should be mixed before dissemination.The recommended tracers for bacterial clouds are bacteria made radioactive by growing on a medium containing 32P, then killed with formaldehyde and washed. To ensure closest identity of physical behaviour it is necessary that the tracer should be of the same species as the cells under test.We are grateful to Mr E. O. Powell for helpful comment during the preparation of this paper.

The PR8 strain of influenza virus can remain viable in ageing aerosols for considerable periods (Harper, 1961). However, viability of micro-organisms determined by their growthin vitromay not indicate their ability to infect animals via the respiratory route (Schlamm, 1960; Hood, 1961). If influenza is normally transmitted via this route it is epidemiologically important to establish whether any correlation exists between viability as measured byin vitroorin ovomethods and respiratory infectivity for a susceptible animal host. Previous reports on respiratory infectivity of aged influenza virus aerosols by Edward, Elford & Laidlaw (1943), Loosli, Lemon, Robertson & Appel (1943a) and Loosli, Robertson & Puck (1943b) lack such a comparison.

SuMMaRY: To test the capacity of different staphylococci to YqUi re resistance to bacteriophage in &TO, twenty phages were grown on solid medium and in fluid medium with their propagating strains of Staphylococcus aureus, different phage types and patterns being represented.Resistant secondary growth never appeared among staphylococci of the 3 A group and often appeared among members of the 6/47 group. Some members of the 29/52 group acquired resistance readily, and others never became resistant, Cross-resistance to other phages was oornmonly acquired. Secondary growth was shown to be resistant or sensitive to the phage, or to yield a mixture of resistant and sensitive colonies.With the exception of propagating strain PS69, all staphylococci which became resistant to phage acquired lysogenicity for the sensitive parent strain. All but one of the staphylococci which had acquired resistance'to a phage appeared to be capable of absorbing that phage. A substance which caused non-specific inhibition of phage lysis on agar medium was present in phage lysates of a staphylococcus that had shown sensitive secondary growth. These results am discussed with reference to phage typing and to the possibilities of therapy by phage.