Female chickens known to be heterozygous for resistance to subgroups A and B of the avian leukosis-sarcoma viruses were mated to males known to be homozygously resistant to both. The progeny were assayed both on the chorioallantoic membrane (CAM) and in tissue culture for resistance to representative viruses of the A, B, and tentatively defined C subgroups. Segregation ratios of resistance to A and B subgroup viruses agreed with the previously suggested hypothesis of single-autosomal-recessive genes controlling resistance to each subgroup. Mixed infection on the CAM and replicate plate infection in tissue culture with subgroup A and B viruses showed that resistance to the A and B subgroups was inherited independently. Assays with viruses tentatively classified as subgroup C indicated that they were largely composed of a mixture of subgroup A and B viruses or of particles possessing the host range specificity of both. However, virus stocks of the subgroup C category, as well as some stocks classified as subgroup B, produced small numbers of pocks or foci on individuals known to be resistant to subgroup A and B viruses. It is suggested that these Rous sarcoma virus stocks carry between 1 and 10% of a true subgroup C virus.

A simplified procedure is described for screening meat and poultry tissues for the presence of antibiotic residues. The method involved inserting a cotton swab directly into meat or poultry tissues, allowing it to absorb tissue fluids. The swab is then removed and placed on a test plate using Antibiotic Medium No.5 (BBL) and a seed layer of Bacillus subtilis ATCC 6633 spores. The plate is incubated overnight at 29 C, then observed for evidence of inhibition around the swab. The method was compared with the conventional bioassay procedures using routing meat and poultry tissues submitted for analysis. Of a total 1,780 tissues tested, the screening procedure was either in agreement with or detected inhibition was not found by the conventional procedures in 99.4% of the samples. The test was shown to have equal sensitivity to conventional procedures for detection of chlortetracycline, oxytetracycline, tetracycline, erythromycin, neomycin, penicillin, streptomycin and tylosin.

A culture medium and testing procedure were developed to detect and differentiate Salmonella in pure culture study and for presumptive detection of Salmonella in dairy products. Most pure cultures of Salmonella were easily differtiated from other members of the Enterobacteriaceae group after 18 hr incubation at 37 C in a neutral red-lysine-iron-cystine broth. Salmonella changed neutral red in the medium from red to yellow and most strains turned the medium black through formation of a massive black precipitate. Species of Enterobacter, Citrobacter, Proteus, Shigella, and Pseudomonas intensified the red color of the medium and failed to blacken it. Species of Klebsiella and Escherichia usually changed the medium from red to yellow in 18 hr but none developed a black precipitate. The few nonhydrogen sulfide producing strains (no medium blackening) of salmonellae were differentiated from Escherichia and Klebsiella sp. by continued incubation to a total of 42 hr followed by the use of a second indicator, brom thymol blue. Only salmonellae gave an alkaline reaction and converted the medium from yellow to green or blue. The medium provided for rapid detection of most salmonellae after 18 hr incubation, as characterized by medium blackening or color change from red to yellow. Related enteric bacteria, other than Arizona strains were readily differentiated. For dairy products a slight modification in medium formula and use of novobiocin and trypsin were required. Novobiocin selectively inhibits growth of most interfering spore formers and gram-positive bacteria and also certain strains of Escherichia coli and Proteus. Trypsin was used to digest casein added with the dairy product sample. A positive presumptive test for Salmonella in dairy products was indicated in the medium by a color change from red to yellow and/or production of a massive black precipitate of iron sulfide after 24 hr incubation. Absence of salmonellae was indicated by no color change or no medium blackening. Results from testing several dairy products indicated that the procedure may be of value in the rapid screening of these foods for salmonellae. Although confirmation and serological identification are still essential, the test eliminates preenrichment and gives presumptive evidence of salmonellae contamination after 24 hr.

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